1 Introduction
sulfate-reducing bacteria (SRB), as a widely distributed group of anaerobic bacteria, are extensively found in ecosystems such as soil, ocean, oil wells, drainage, and animal intestines, as shown in
Figure 1, playing a significant role in the global sulfur, carbon, and nitrogen cycles.
SRB possesses the ability to utilize a variety of electron donors and electron acceptors, which gives it unique advantages in wastewater treatment, leading to its widespread application in different types of wastewater treatments. For example, using a UASB reactor under 35 °C anaerobic conditions can effectively treat conventional black water from toilets. Research has found that the dominant bacterial community in the reactor is sulfate-reducing bacteria, while the activity of hydrogenotrophic methanogens is relatively low in this system. Utilizing SRB for the removal and degradation of azo dyes (procion red HE-7B) in dyeing wastewater, under alkaline conditions (pH=11) and at high COD, the removal rate of HE-7B reaches over 90%. In the treatment of heavy metal wastewater, SRB promotes the formation of metal sulfides through sulfate reduction, thereby effectively removing heavy metals from the wastewater.
SRB may primarily reduce sulfate through two pathways
[1]. One is assimilatory sulfate reduction, where SRB use the products of sulfate reduction to directly synthesize cellular materials. The other pathway is dissimilatory sulfate reduction, in which SRB can completely oxidize and decompose organic carbon sources into carbon dioxide, or partially oxidize and decompose them into acetate under anaerobic conditions. Meanwhile, substrate-level phosphorylation generates a small amount of adenosine triphosphate (ATP) and high-energy electrons; the high-energy electrons released by phosphorylation are transferred stepwise to SRB via the electron transport chain (flavoproteins, cytochrome c, etc.). SRB then produce a large amount of ATP through "electron transfer phosphorylation," providing energy for biochemical processes. Finally, sulfur oxides are reduced to S
2− by consuming ATP to provide energy.
As shown in
Figure 2, the sulfate reduction process typically requires organic matter or H
2 as an electron donor. However, most sulfate-containing wastewaters suffer from a lack of sufficient organic carbon sources, which necessitates the addition of extra electron donors to the wastewater to ensure the reduction and removal of sulfates. Relevant studies have confirmed that when organic matter is used as the electron donor, methanogenic bacteria (MB) have a more competitive advantage
[2]; whereas autotrophic SRB are more competitive when H
2 is used as the electron donor.
Therefore, using autotrophic SRB for sulfate reduction not only reduces the addition of organic matter but also decreases the competitive reactions of MB. However, the use of autotrophic SRB for sulfate treatment is not suitable for all types of wastewater: in most sulfate-containing wastewaters, the common method to balance SRB and MB is to control environmental factors to provide advantageous growth conditions for SRB, but for wastewater with large fluctuations in water quality and sewage containing complex components, this balancing method also has limitations.
To seek a balance between carbon source utilization and microbial community competition, electrocoupling technology has been introduced. In recent years, electrocoupled sulfate removal technology has found application in the treatment of sulfate-containing wastewater. Facing the limited electron donors in the environment, sulfate-reducing bacteria (SRB) inevitably compete or coexist with other microbial communities in the environment. According to changes in sulfate concentration, the primary pathways of electron transfer conducted by SRB will also change, along with the patterns of electron donor competition. The introduction of an electric field into the reaction system significantly impacts the natural competitive outcomes. An externally applied electric field intervenes in the electron transfer processes within the microbial community, utilizing the regulation of the competition process to achieve the goal of enhancing sulfate removal efficiency. For instance, microbial electrochemical system (MES) technology can produce H
2 in situ through the construction of a biocathode, thereby influencing the competition process between SRB and methanogens (MB) via an electro-mediated process. Additionally, in the typical MES process, the microbial fuel cell (MFC), organic compounds or elemental sulfur are oxidized in the anode chamber, with electrons being transferred through external conductors to the cathode, where they are consumed by microorganisms as electron donors
[3]. Given that microorganisms have the ability to directly or indirectly utilize electrons from the cathode as electron donors for reducing pollutants (such as sulfate, nitrate, Cr
6+, and aniline, etc.)
[4⇓~6], setting up a microbial electrochemical system in wastewater containing multiple types of pollutants makes it possible to simultaneously remove two or more thermodynamically non-adjacent pollutants.
SRB plays a crucial role in wastewater treatment, with unique advantages in the treatment of both sulfate-containing and sulfide-containing wastewaters. In addition, the interactions between SRB and other microorganisms affect the structure and function of the entire microbial community. By studying the status and role of SRB in microbial competition, one can better understand the succession patterns and stabilization mechanisms of microbial communities, which is more conducive to guiding the design and optimization of wastewater treatment processes. This paper mainly reviews research and achievements on the competitive status of sulfate-reducing bacteria (SRB) within microbial communities and their mechanisms of electron donor utilization, particularly summarizing and analyzing the activities of SRB communities under the influence of microbial electrochemical systems. It objectively proposes factors influencing the competition between SRB and other microbial groups, aiming to further explore control methods and provide relevant assistance for improving the biochemical treatment level of sulfur-containing wastewaters.
2 Bacterial Community Relationships of Sulfate-Reducing Bacteria in Aquatic Environments
In the wastewater from industries such as pulp, printing and dyeing, brewing, and in acid mine drainage, the concentration of sulfate can reach several hundred to over a thousand milligrams per liter, which has a serious impact on the traditional process of removing organic pollutants. In sulfate-rich environments, SRB use sulfate as the terminal electron acceptor during the oxidation of organics, thereby producing sulfides. During this process, SRB will compete with other microbial communities in the system. However, in environments lacking sulfate, SRB and MB are not in a competitive relationship but rather in a symbiotic one during the degradation of organics. It can be seen that by studying the interactions between SRB and different microbial communities in water treatment processes, it is possible to understand the factors influencing the symbiosis and competition of SRB with other microbial communities, revealing the regulatory mechanisms of SRB within microbial populations from a microbiome perspective, and providing a microbiological theoretical foundation for optimizing their application in water treatment processes.
2.1 Symbiotic Relationship
Reeburgh
[7] proposed in 1976 that the anaerobic oxidation of methane could be coupled with sulfate reduction. Subsequently, Bryan et al.
[8] discovered in an anaerobic methanogenic reactor that sulfate-reducing bacteria (SRB) could exist as acetone-producing bacteria. A large number of subsequent studies have also revealed the microbiological processes involved in sulfate-dependent methane oxidation. At the same time, in natural water bodies lacking sulfate, SRB have played a significant role in organic matter fermentation and anaerobic oxidation. Therefore, it can be considered that in most environments lacking sulfate, the degradation of organic matter by SRB and methanogens (MB) occurs under a symbiotic rather than competitive relationship. Symbiotic bacteria (syntrophobacter species) are a special type of sulfate-reducing bacteria that can grow in propionate and sulfate environments. When co-cultured with hydrogenotrophic methanogens, they can convert propionate into acetate, carbon dioxide, and hydrogen. For example,
Desulfovibrio and
Desulfomicroium can metabolize and grow by utilizing acetic acid, carbon dioxide, and hydrogen produced from the fermentation of pyruvate. After hydrogenotrophic methanogens have utilized all available hydrogen, SRB can oxidize lactate and ethanol to acetate as growth substrates
[8].
Desulfobulbus can also utilize propionate and sulfate for growth; however, unlike symbiotic bacterial communities, when co-cultured with methanogens, it cannot convert propionate into acetate. In the absence of sulfate,
Desulfobulbus can ferment lactate and ethanol (in the presence of carbon dioxide) into acetate and propionate.
In the sulfate environment, the enrichment of SRB can promote methane production by altering metabolic pathways and electron transfer. Zan et al
[9]in the co-digestion study of food waste and waste activated sludge, by adding sulfate to the system, SRB acted as acetogenic bacteria in co-digestion, converting propionate into acetate, which, from a thermodynamic perspective, provided an alternative metabolic pathway for MB, thereby improving the efficiency of methane production in anaerobic co-digestion through the regulation of sulfate concentration. Li et al
[10]used the acid-producing phase (acid production-sulfate reduction reaction) of a two-phase anaerobic reactor (COD/SO
42-: 3) for sulfate reduction, thus avoiding competition between SRB and MB, effectively improving the removal efficiency of organic wastewater containing sulfate. In this process, SRB bacteria use the liquid end metabolites of hydrolytic acidifying bacteria as electron donors and SO
42- as electron acceptors for sulfate reduction. The presence of SRB bacteria can enhance the metabolic rate of acid-producing bacteria. During natural competition, the presence and level of sulfate have a significant impact on the outcome of the competition, with the symbiotic relationship between SRB and MB more commonly found in environments where sulfate concentrations are not high.
In natural systems, SRB can coexist with various oxidizing bacteria and dehydrogenating bacteria, forming microbial communities that collectively complete the cycling processes within ecosystems. Oxidizing bacteria that coexist with SRB include iron-oxidizing bacteria, manganese-oxidizing bacteria, etc., which can utilize the hydrogen sulfide gas produced by SRB as an electron donor to complete their own metabolic processes. Meanwhile, SRB can also coexist with dehydrogenating bacteria, such as methanogens, which can use the methane gas produced by SRB as an electron donor. However, when sulfate is present in the environment and the concentration of organic substrates is low, sulfate-reducing bacteria will compete for resources with other microbial communities; this competition will inhibit the growth of other communities, thereby allowing SRB to gain a dominant position in the ecosystem.
2.2 Competitive Relationship
Sulfate-reducing bacteria (SRB), as a common group of heterotrophic microorganisms in anaerobic environments, exhibit competitive behaviors not only among themselves but also with other anaerobic microbial communities. The main resources competed for include sulfate and organic matter. In the competition, SRB will employ various methods to secure these resources. Firstly, SRB can secrete specific enzymes to degrade organic matter, thereby obtaining more nutrients. Secondly, SRB might produce compounds such as antibiotics to inhibit the growth of other bacterial groups, thus reducing competition. Additionally, some SRB can form biofilms that are conducive to their own attachment, allowing them to occupy more ecological advantages. Moreover, as a genus, different SRB exhibit different nutritional patterns, which affect each other's survival and metabolism. According to the carbon metabolic pathway, SRB can be divided into two major categories: one capable of completely degrading organic matter into CO
2, and the other performing incomplete oxidation to convert organic matter into acetate. Therefore, SRB not only compete with MB for H
2/CO
2 and acetate but also compete with synthetic bacteria for substrates, such as propionate, butyrate, and ethanol, which are key intermediates in the anaerobic digestion process. The competition for substrates is also reflected in the pathways of organic matter degradation, where the decomposition of organic matter in a sulfate-reducing environment (
Figure 3a) differs from that in a methanogenic environment (
Figure 3b)
[11]. Since the primary substrates directly utilized by MB are mainly CO
2, H
2, and acetate, when organic acids like lactate, propionate, and butyrate serve as growth substrates, SRB have a greater advantage, and SRB can utilize a wider variety of available substrates. Therefore, in an environment with the presence of sulfate, SRB can directly use more substrates for metabolic growth. In contrast, in a methanogenic reactor, compounds need to be first decomposed by the syntrophic community of acetogens and methanogens before they can become the nutritional substrate for methanogens (
Figure 3b).
In the anaerobic treatment process of sulfate-rich wastewater, there are usually problems such as sulfide toxicity, low methane production, generation of toxic gases like H
2S, and corrosion. Among these, the production of H
2S can severely harm the microbial community in the system, thereby inhibiting the improvement of reactor performance. By regulating the competition between SRB and MB, these phenomena can be alleviated and balanced. Studies have confirmed that with the increase in SRB biomass, the methane yield and biomass of methanogenic archaea (MA) gradually decrease, indicating that SRB can effectively compete with MA under low acetate concentrations
[12]. From a thermodynamic perspective, SRB are more competitive than MA and acetogenic bacteria (AcB). However, Jing et al.
[13] found that in a UASB treating ethanol and acetate-rich sulfate wastewater with a carbon-to-sulfur ratio (COD/SO
42-) of 1, operating for 180 days, the
Desulfovibrio species reducing sulfate using ethanol as a substrate (via SRB) did not significantly outperform the
Methanosaeta conilli GP6, which was primarily engaged in methanogenesis (via MA). Chen et al.
[14] to explore the long-term competitive dynamics of the microbial community in an anaerobic reactor, continuously operated a laboratory-scale UASB for 329 days, treating ethanol and acetate-rich sulfate wastewater with a constant COD/SO
42- ratio of 1. They extended the Anaerobic Digestion Model No. 1 (ADM1) by designing a comprehensive structured mathematical model to predict and investigate the long-term dynamic competition for electrons between sulfides and methanogens. Throughout the process, distinct phases of MA dominance, equilibrium, and SRB dominance were clearly observed. This indicates that the competitive behavior of SRB in the water treatment environment is a long-term and phased process. By studying the environmental conditions and microbial composition during this process, the competition outcomes can be interfered with and changed, directing the competition towards a favorable direction.
In the microbial electrochemical process, SRB and MB are two important microbial communities. The long-term competition between them has a significant impact on the efficiency and stability of wastewater treatment. Microbial electrochemical technology can utilize the interaction between microorganisms and solid electrodes to achieve pollutant removal in the treatment of sulfate-containing wastewater. In this process, SRB and other microorganisms compete on the electrode surface or participate in sulfate degradation through electron transfer processes and metabolic activities. On the other hand, by appropriately applying an electric field through electrochemical regulation, the growth of dominant bacterial groups can be promoted, thereby altering or accelerating the overall competitive process. Meanwhile, the main product of microbial electrochemical reduction of sulfate is sulfide, which is a pollutant with stricter emission requirements. Therefore, the complete treatment of sulfate wastewater should integrate the oxidation and recovery process of sulfide. The Pozo et al. team
[15] achieved a 74% sulfur recovery rate by treating the effluent from sulfate reduction using a bioanode enriched with sulfur-oxidizing bacteria. Thus, bioelectrochemistry can not only be applied to the reduction of sulfate in wastewater but also to the recovery of elemental sulfur from sulfate-containing wastewater.
2.3 Competitors of Sulfate-Reducing Bacteria in Different Environments
In environments lacking sulfate, SRB and MPB can undergo symbiotic growth; in environments where sulfate reduction is predominant, that is, in sulfate-rich conditions, sulfate-reducing bacteria and methanogens will compete for the substrates acetate and H
2, and also compete with synthetic methanogens
[11]. Thus, the concentration of sulfate in the system becomes a particularly important indicator influencing the microbial community relationships. For example,
Table 1 lists the sulfate concentrations that the system endures during the treatment process of some sulfate-containing wastewaters.
Due to their high affinity for H
2 and low threshold, hydrogenotrophic methanogens (MB) and homoacetogenic bacteria are easily outcompeted by hydrogenotrophic sulfate-reducing bacteria (SRB). As shown in
Table 2, it can be seen through thermodynamic calculations that the process of sulfate reduction is more favorable than methanogenesis and acetogenesis. However, most SRB use acetate as a carbon source; therefore, when acetate is not provided, SRB will coexist with homoacetogenic bacteria
[21]. Although acetate-utilizing SRB also have an advantage over acetate-utilizing MB in substrate utilization, this competition is not as pronounced or intense as that between hydrogenotrophic MB. The predominant SRB is
Desulfobacca acetoxans, a bacterium that primarily uses acetate as its metabolic substrate, and its growth kinetics are superior to those of
Methanosaeta spp. SRBs that degrade propionate and butyrate grow much faster than MB and acetate-utilizing SRB.
SRB can be divided into acetogenic SRB (ASRB) and methanogenic SRB (MSRB). When the sulfate concentration in the environment is not sufficient to support the complete oxidation of organic matter by SRB, ecologically, there will be competition for sulfate among SRB populations. However, there are relatively few studies that explain and explore the competition among these populations. Under sulfate-limited conditions, sulfate-reducing bacteria preferentially use H2, lactate, and ethanol as substrates, rather than propionate and acetate.
Although SRB are named for their ability to use sulfate as the terminal electron acceptor, they can also grow using other electron acceptors and can ferment substrates in the absence of organic electron acceptors. Therefore, the presence of a large number of SRB in an environment does not necessarily reflect sulfate reduction occurring in that environment. Thus, from a thermodynamic perspective, the process of sulfate reduction indirectly reflects the survival relationships of microbial communities in the environment. However, regulating microbial community relationships is a complex task with many uncontrollable factors. By introducing additional disturbances and artificially influencing the thermodynamic process, new ideas can be provided for this work. Microbial electrolysis cells (MECs) are a type of system that achieves net energy input through externally applied potential. In MECs, the reduction potential at the cathode surface is lower than the oxidation potential at the anode surface, resulting in a system potential difference less than 0, and the Gibbs free energy change (∆
Gº) greater than 0. From a thermodynamic standpoint, the system reaction will not occur spontaneously and requires an external potential to compensate for the potential difference in the system to initiate redox reactions at the electrode surfaces. The electron transfer mechanisms for sulfate reduction at the cathode mainly fall into two categories. One is where SRB use
H2 produced at the cathode to reduce sulfate. Liu et al.
[22] used carbon cloth as the cathode material and found that hydrogen could be produced to reduce sulfate when the cathode potential was -0.8 V (vs NHE). The other is where SRB directly accept electrons from the cathode to reduce sulfate. Yan et al.
[23] used
Desulfopila corrodens IS4 in a pure culture system with a graphite-germanium electrode as the electron donor and found that
Desulfopila corrodens IS4 could directly accept electrons from the cathode to reduce sulfate when the cathode potential was below -0.4 V (vs NHE). In mixed cultures, SRB can also directly accept electrons to reduce sulfate. Based on this, MECs can also be applied to accelerate some thermodynamically spontaneous but slow reactions
[15]. By constructing different functional bio-cathodes, the interrelationships among N, S, and C elements and the related microbial community relationships can be intervened and regulated under electrochemical drive.
Due to the complex and diverse relationships among different bacterial communities in SRB, it is currently impossible to fully control the interactions between these communities within a single system. More often, the existing microbial interactions are organized, summarized, and analyzed under different environmental influences across various systems, serving as references for similar treatment processes. The utilization of bioelectrochemistry provides new methods and means for regulating the growth processes of microbial communities, offering more possibilities for research development.
3 Electron Donor Utilization Pathways of Sulfate-Reducing Bacteria
3.1 Thermodynamic Utilization of Electron Donors by SRB
Sulfate-reducing microorganisms (SRM) are anaerobic prokaryotes that primarily use dissimilatory sulfate reduction for energy storage and metabolic growth. Because the energy produced from sulfide formed by sulfate accepting electrons is much less than that from other pathways, despite the energy disadvantage of the sulfur reduction pathway, SRM have been detected in various environments and have a significant impact on sulfur and carbon cycles
[24]. As shown in
Figure 4, different types of SRB constitute the sulfur cycle process in nature. This indicates that during water treatment, researchers often need to reduce and remove sulfates in the system but also hope to control the discharge of sulfides, processes that all depend on SRB, making the study of bacterial behavior necessary. According to the concept of "thermodynamic ladder" in geomicrobiology, SRM find it difficult to compete with microorganisms with higher energy output (such as denitrifying bacteria or iron-reducing bacteria) for the same substrate for growth and metabolism. From
Table 3, it can be seen that, thermodynamically, microorganisms cannot effectively reduce SO
42− in the presence of NO
3−. However, in the water treatment process, desulfurizing bacteria can not only use sulfides for redox reactions, converting them into sulfates to remove pollutants from water; they can also reduce nitrates to nitrogen gas, thereby reducing nitrogen content.
Many industries, such as dyeing, tanning, and pharmaceuticals, produce large amounts of wastewater containing both ammonia (NH
4+) and sulfate (SO
42-). To remove ammonia nitrogen from the wastewater, processes such as hydrogenotrophic denitrification and anaerobic ammonia oxidation have been developed. In the process of hydrogenotrophic denitrification, denitrifying bacteria use H
2 as an electron donor. When SO
42- coexists, autotrophic sulfate reduction may become a competitive reaction to hydrogenotrophic denitrification. For industrial wastewater containing both NH
4+ and SO
42-, sulfide autotrophic denitrification is a good solution, which uses S
2- as an electron donor to reduce NO
3-. Combining sulfide autotrophic denitrification with heterotrophic sulfate reduction can be used for treating municipal sewage containing SO
42- and NH
4+[25,26]. Using the SO
42-—S
2-—SO
42- cycle as an electron transfer pathway to promote denitrification, the NO
3- removal rate can reach over 90%, but this method is often limited by insufficient carbon-to-nitrogen ratios. Ren et al.
[25] combined bioelectrochemical sulfate reduction with heterotrophic sulfate reduction and sulfide autotrophic denitrification in the SANI process to remove nitrogen from industrial wastewater containing NH
4+ and SO
42-. In this coupled system, electrons are provided to the denitrification process through the sulfur cycle. Essentially, multi-chamber reactors or multiple reactors in series are used to create favorable conditions for different bacterial species, avoiding competition among microbial populations and achieving synergistic effects in wastewater treatment
[27].
Sulfate-reducing microorganisms (SRM) have a higher affinity for H
2 and acetate in anaerobic environments in the presence of sulfate, which are the main substrates for hydrogenotrophic methanogens and homoacetogenic bacteria, respectively. Compared to methanogenic populations (MPB), SRM exhibit a higher affinity and lower threshold for H
2 and acetate
[28,29]. Therefore, in wastewater treatment, it is more common to regulate the competition between SRB and MPB for substrates to achieve a stable equilibrium in the reactor, thus obtaining the most efficient treatment and energy recovery. Consequently, thermodynamics can only serve as a simple analysis of the competitive relationship among SRB communities. Although denitrifying bacteria have an advantage over SRM from a thermodynamic perspective, in practical applications, sulfate reduction processes and denitrification processes can complement each other through improvements in electron transfer pathways; on the other hand, although SRB are thermodynamically superior to methanogens, this does not mean that in environments where SRB dominate, methanogens will be completely inhibited. On the contrary, SRB can indirectly promote methanogenesis by utilizing long-chain fatty acids.
The introduction of bioelectrochemistry provides a new approach for the microbial community competition of SRB. Based on the coupling of MEC with traditional anaerobic reactors, the bioelectrocatalytic reactor treats high-concentration sulfate wastewater by oxidizing organics at the anode, achieving rapid decomposition of organic acids, thereby maintaining the acid-base balance within the system, which is conducive to improving the efficiency of anaerobic digestion and enhancing the overall reactor's resistance to shock. In addition, the cathode, acting as an electron donor, can strengthen the degradation capacity of sulfates. Therefore, combining MEC for pollutant degradation with anaerobic sulfate reduction for pollutant degradation theoretically has a higher pollutant degradation capacity, and it is expected to accelerate the degradation of organics in sulfate wastewater.
3.2 Effect of Electro-Mediation on SRB Communities
Based on the utilization of carbon sources, SRB can be differentiated into dissimilatory or assimilatory types, the former being heterotrophic SRB which use organic compounds as substrates, and the latter being autotrophic SRB which use CO
2 as a carbon source and H
2 as an electron donor. The treatment of sulfur-containing industrial wastewater often utilizes heterotrophic SRB, which not only compete with MB but also have a synergistic effect with acetogenic bacteria, although it is not clearly reported whether they compete with autotrophic SRB for CO
2. Autotrophic SRB can grow using H
2, CO
2, and CO as their sole electron donors
[33]. Houten et al.
[34] used H
2 and CO
2 as substrates and found that a mesophilic bioreactor could achieve a high sulfate reduction rate within 10 days. Studies show that using syngas (a mixture of H
2, CO
2, and CO, along with small amounts of methane and other components like nitrogen) as an electron donor, the sulfate conversion rate can reach 15 kg/(m
3·d) after 7 weeks, with an average sulfate removal rate of 88%
[35]. Using syngas as an electron donor not only has economic benefits but also ensures that there are no residual organics in the treated effluent. However, the current utilization of syngas is limited by the H
2 transfer rate, and also needs to consider the competition between SRB and other microorganisms and the low H
2 utilization efficiency due to methane production
[36]. In the treatment of ammonia nitrogen-containing wastewater, the synergy between heterotrophic and autotrophic bacteria is utilized to improve denitrification efficiency, where the CO
2 produced by heterotrophic metabolism can serve as a carbon source for autotrophic bacteria. However, to date, no significant synergistic effect has been observed between heterotrophic SRB and autotrophic SRB.
Based on the degree of substrate oxidation by the microbial community, SRB can be classified into complete oxidizers and incomplete oxidizers. The electron acceptors for SRB are not only sulfate, sulfite, thiosulfate, and elemental sulfur, but some specific SRB can also use nitrate, nitrite, iron, and other compounds as electron acceptors
[33]. Typically, during anaerobic degradation, incomplete oxidizing SRB often utilize substrates more effectively than complete oxidizing SRB, making incomplete oxidation the predominant metabolic pathway in most SRB reactors
[37]. Incomplete oxidative metabolism can lead to residual acetate in the effluent of bioreactors, which is one of the reasons for the non-compliance of SRB reactor effluent. In the future development of sulfate reduction technology, it is necessary to conduct research on the complete oxidation of SRB.
Whether based on the utilization of different carbon sources or the classification analysis according to the degree of substrate oxidation, these are related to the use of electron donors by SRB and different pathways of electron transfer. Under natural conditions, the results of microbial community competition are influenced by the environment and reaction thermodynamics, but after the introduction of bioelectrochemistry, it becomes possible to drive or accelerate a class of chemical reactions through the action of an externally applied electric field. Additionally, as shown in
Figure 5, by adding conductive media to the system, interspecies electron transfer can be promoted, thereby altering the metabolism of the microbial community, which offers more possibilities for practical wastewater treatment. Based on the symbiotic relationship of energy and electron transfer among multiple microbial species, the symbiosis between chemoheterotrophic bacteria and MA makes a significant contribution to CH
4 production under various anaerobic conditions and also has a great impact on the global carbon cycle
[38,39]. For example, in electrosynthetic methanogenesis, interspecies electron transfer from chemoheterotrophic bacteria to methanogenic archaea not only occurs via the diffusion of small molecules (such as H
2 and formate) acting as electron carriers
[40,41], but can also be mediated by current through conductive solid materials, a process known as electro-symbiosis or direct interspecies electron transfer
[42,43].
The addition of conductive media mainly affects the electrochemical actions, aggregation effects, and microbial activities in the environment. In methanogenic systems, to promote methane production, the supplementation of conductive particles has been proposed as a novel biotechnology for improving anaerobic wastewater treatment systems
[44⇓~46]. In recent studies, adding zero-valent iron (ZVI) to AD systems can facilitate the in situ conversion of CO
2 into CH
4, thereby achieving carbon capture through biotechnology. The simultaneous supplementation of hydrated iron and sulfate in the system promotes methane generation; the coexistence of sulfate and ferric oxide can promote methane production through the biomineralization of (semi)conductive iron sulfides, generating methane via electrosynthesis. From the perspective of microbial communities, when hydrated iron and sulfate are added simultaneously, the enriched DIRB and SRB communities alter the metabolic pathways and electron flow within the system, thus contributing to the enrichment of MB and increasing the rate of CH
4 production
[47]. In the treatment of metal-containing wastewater, the addition of graphene oxide (GO), reduced graphene oxide (rGO), ZVI, etc., is commonly used, as shown in
Figure 6, where ZVI and SRB work synergistically to remove heavy metals from wastewater. The good conductivity of conductive particles in water allows them to participate in various electrochemical reactions. In electrolyte solutions, redox reactions occur on the surface of conductive particles, generating current. This electrochemical action promotes the oxidation, reduction, or precipitation of pollutants in water, thereby enhancing the efficiency of wastewater treatment.
The addition of ZVI can significantly enhance the activity of SRB and the removal rate of heavy metals in the SRB system. As an electron donor, ZVI reduces the competition for electrons between MB and SRB in sulfate-containing wastewater; SRB attached to ZVI can obtain electrons from the H
2 produced during the corrosion process of ZVI. This decreases the COD required for sulfate reduction
[48⇓⇓~51]. The removal of heavy metals by ZVI-SRB mainly relies on reductive precipitation, sulfide precipitation, co-precipitation, and biosorption
[52]. In addition, the introduction of ZVI alters the microbial composition in sulfate-rich environments, acting as a buffer in bacterial community competition, while also providing a good attachment surface for microorganisms, promoting biofilm formation. The presence of conductive media means that the conditions influencing microbial relationships are not limited to environmental factors; as shown in
Table 4, different conductive media are suitable for different water treatment environments. The introduction of electrochemistry not only affects the composition of microbial communities within the system but also changes the pathways of bacterial electron transfer, offering more abundant, flexible, and energy-friendly treatment methods when dealing with more complex and variable water environments.
3.3 Energy-Saving Hydrogen Production Pathways by SRB
SRB are the primary hydrogen-consuming microorganisms in anaerobic environments, but in environments lacking sulfate, SRB can function as hydrogen-producing microorganisms in the system, which provides a wide range of possibilities for developing related treatment technologies based on SRB. It is worth noting that SRB have extremely high hydrogenase activity and simultaneously possess the ability to utilize lactate, ethanol, formate, and butyrate
[54]. This means that in environments with limited sulfate, they can ferment and grow by producing H
2, CO
2, and acetate through symbiosis with other organisms
[55]. The intermediate products that may produce H
2 during the sulfate reduction process proposed by Odom and Peck
[56] also indicate that in the absence of sulfate, SRB can act as H
2 producers. Biohydrogen (BioH
2) production is a very interesting form of energy output, requiring only low energy input, and if waste or renewable biomass is used as substrates, this process will be sustainable
[57,58]. This demonstrates the significant potential of SRB in energy-saving hydrogen production.
Hydrogenase is an important electroactive enzyme in SRB, which can combine its activity with a solid electrode by catalyzing the evolution and oxidation of hydrogen
[59]. Research mainly focuses on using these bacteria as biocathodes in MECs. The electrocatalytic hydrogen production activity of SRB is greater than or equal to their H
2 oxidation activity, confirming their high potential for hydrogen production
[60]. MECs have recently become a novel and promising renewable hydrogen production technology, capable of providing the energy required to reduce H
+ to H
2 through microbial power sources under ambient pressure and temperature by applying an external voltage
[61].
Desulfovibrio sp. is the dominant genus in MEC biocathodes, and a hydrogen cycling mechanism has been proposed that uses electrons on the cathode to reduce protons. Hydrogenases present in the cytoplasmic membrane or cytoplasm reduce protons through enzymatic reactions. Once cytoplasmic protons are consumed, hydrogen diffuses through the cytoplasmic membrane into the periplasm, where it is oxidized, initiating proton motive force. This proton motive force can be maintained by the flow of electrons to the cytoplasm, reducing the consumption of protons for sulfate reduction
[62]. Studies have found that pure cultures of
Desulfovibrio with methyl viologen as a mediator can produce hydrogen
[63]. It has been shown that
Desulfovibrio sp. can conserve energy by participating in hydrogenases present on the cytoplasmic membrane or energy-conserving hydrogenases
[64]. In SRB hydrogen production systems, because SRB are more efficient as hydrogen-producing bacteria than MA (microbial aggregates in anaerobic fermentation processes that play a crucial role in hydrogen production), BES is typically used to inhibit methanogens and recover hydrogen from anaerobic fermentation residues. Objectively, MB can hardly use propionate as a metabolic substrate, and a decrease in propionate production in MECs indicates the presence of microbial electrolysis
[65]. However, a higher propionate/acetate ratio can lead to failure in the anaerobic digestion process, and thus, inhibiting the propionate-utilizing pathway may increase hydrogen production, which could also be beneficial for hydrogen recovery in mixed cultures of SRB
[66].
Among these, iron, as an important nutrient and a special electro-mediated material, is not only essential for stimulating the growth of hydrogenotrophic methanogens but also helps promote the utilization of other metals
[67]. In UASB reactors, adding 10 µmol/L Fe can double the methane-producing activity of sludge
[68]. Typically, Fe and Ni exist in the form of Ni-Fe-s clusters and Fe-Fe-s clusters, forming enzyme subunits, including hydrogenases and acetyl-CoA synthetase
[69]. The addition of Fe also affects the metabolic pathways of sulfate-reducing bacteria, thereby promoting the hydrogen production process of SRBs. Fe can act as an electron carrier participating in the intracellular electron transfer process, thus facilitating the hydrogen production reaction. Moreover, Fe can provide the building materials needed for enzymes and coenzymes, further enhancing the bacterial hydrogen production capacity. As mentioned earlier, by adding an appropriate amount of iron, the growth environment of the microbial community in the system can be optimized, alleviating competition for electron donors and thus promoting the processing balance within the system.
In natural environments, various electron donors can be utilized by SRB. In traditional methods, the degree of oxidation of different substrates by SRB in the system can be determined based on thermodynamic advantages, thus allowing for the artificial selection of SRB communities. Fundamentally, this is a study of the extent to which SRB utilize electron donors. Different from simply regulating microbial relationships through nutritional substrates, adding electron mediators or using electrodes as electron donors to alter the pathways of electron transfer can achieve good results in both water treatment processes and hydrogen production for energy conservation.
4 Regulatory Control Factors of Sulfate-Reducing Bacteria
The metabolic pathways of bacteria and the utilization pathways of SRB for electron donors play a crucial role in the biodiversity and microbial community relationships within a single reactor, while the diversity of SRB is directly or indirectly influenced by process parameters such as the applied electric field, conductive medium, type of carbon source and carbon-to-sulfur ratio, hydraulic retention time (HRT), pH, and temperature. Meanwhile, sulfur, as a common natural element, has close relationships with other elemental cycles, as shown in
Figure 7, and sulfate-reducing microorganisms also play an important role in the harmless removal of nitrogen and phosphorus.
4.1 The Influence of an Applied Electric Field
The introduction of an external electric field essentially introduces a microbial electrochemical reaction process. The application of voltage mainly enhances methane and sulfide production, effectively treating sulfate-rich wastewater. Appropriately applying current enhances extracellular secretion, which is not only beneficial for the formation of biofilms but also strengthens electron transfer to MA. In microbial electrochemical systems, SRB can trigger strong synergistic effects; microbial analysis shows that the presence of an electric field significantly increases the abundance of
Desulfovibrio in the system, which can directly transfer electrons to methanogenic archaea and bacteria through pili. Additionally, appropriate electrical stimulation helps promote functional enzymes and microbial metabolism. A study compared sulfate removal in reactors with (EV) and without applied electric fields, gradually decreasing the COD/SO
42- ratio (CSR) in both reactors. Compared to the control group, the EV reactor showed a 30% increase in methane production and a 40% increase in sulfate removal rate when CSR was 2.0
[70]. Furthermore, microbial electrochemical systems can maintain the ORP (oxidation-reduction potential) environment of SRB systems, and the presence of an electric field can reduce substrate consumption. Theoretically, when the cell membrane is charged, the inner membrane has a positive charge, while the outer membrane has a negative charge
[71]. The impact of an electric field on microorganisms may primarily affect two physiological processes: (1) activation of voltage-sensitive channels; (2) higher amplitude electroporation permeating the cell membrane
[72]. However, some studies indicate that applied current can affect Na
+ concentration, and the membrane might depolarize due to the opening of Na
+ channels under the influence of voltage, potentially having negative effects on cells
[73]. Therefore, microorganisms in microbial electrochemical systems will undergo two different changes. On one hand, current stimulates bacterial activity and pollutant degradation. On the other hand, changes in membrane permeability, especially the disordering of substance and energy channels, can inhibit microbial activity, and even lead to cell membrane rupture at high currents. Research has shown that the optimal current for sulfate-rich MECs is 1.5 mA, and applying voltage can increase sulfate removal efficiency by 14.9%. However, excessive current can cause cell membrane rupture, slowing down growth and metabolic activities, thereby reducing sulfate removal efficiency. Appropriate current application can promote the production of extracellular secretions, facilitating the enrichment of bacteria in the cathodic biofilm; the formation of a biofilm on the cathode can accelerate its oxidation
[74].
ORP is used to reflect the macroscopic redox properties of all substances in aqueous solutions and is an important factor affecting microbial activity. An increase in ORP can indicate that the competition for carbon sources among microorganisms has become more intense, and the result of this competition will lead to significant changes in the microbial community. Maintaining a negative redox potential in the system is conducive to maximizing the reduction effect of sulfate, but according to the redox potential, the ability of sulfate to accept electrons is much lower than that of oxygen and NO
X-[75]. When ORP is below -100 mV, SRB can maintain metabolism, while when ORP is above -100 mV, the metabolism of SRB is inhibited
[76]. Moreover, ORP can also reflect the amount of dissolved oxygen present in the system, so in the treatment of sulfate-containing wastewater, it is possible to take advantage of the characteristic of SRB remaining active in a micro-aerobic environment, allowing sulfate to be reduced and oxidized, precipitating as elemental sulfur and being removed
[77].
The application of electric fields simultaneously affects changes in redox potential, and the electrolysis caused by applying excessively high voltages can even alter the system's environment, thereby having a significant impact on the microbial communities within the system. Furthermore, during anaerobic wastewater treatment, some redox substances, such as NO
2-, NO
3-, and ZVI, etc.
[78], affect the ORP. This also confirms that the addition of conductive media not only influences the pathways of electron transfer but also that their own physicochemical properties have an impact on the system.
4.2 Conductive Medium
The application of conductive particles in wastewater treatment mainly includes electrochemical methods and biological treatment methods. In electrochemical methods, conductive particles can serve as electrode materials participating in the wastewater treatment process; whereas, in biological treatment methods, conductive particles can act as carriers to promote the attachment and metabolism of microorganisms. In bioelectrochemical methods, conductive particles such as nanocarbon powder and iron powder are typically used as electrode materials, participating in the wastewater treatment process. Adding conductive particles can enhance the conductivity and active surface area of electrodes, thus increasing the efficiency of electrochemical reactions. For example, adding carbon nanotubes to the surface of electrodes can significantly improve the electrochemical activity of the electrodes and promote the degradation of organic matter in wastewater
[79]. In biological treatment methods, conductive particles can provide a good attachment surface for carriers, promoting the adhesion and metabolic activity of microorganisms, thereby enhancing the biodegradation efficiency of organic matter in wastewater. Introducing conductive particles like carbon nanotubes into granular sludge can significantly increase the degradation rate and stability of organic matter by microorganisms in wastewater
[80]. Adding granular activated carbon (GAC) in UASB reactors increases the treatment efficiency of high-concentration sulfate-containing organic wastewater. Compared with no medium added, GAC enhances microbial activity, improves the direct interspecies electron transfer between potential electroactive microorganisms and MB, and also enhances the metabolic activity of microorganisms, including glycolysis, pyruvate metabolism, sulfate reduction, and methanogenesis. Therefore, the enrichment of key microorganisms, promotion of DIET methanogenesis, and enhancement of functional metabolic activity may be important reasons for the higher methanogenic activity of GAC reactor sludge
[81]. The addition of conductive particles can improve the effectiveness of wastewater treatment by enhancing the efficiency of electrochemical reactions or promoting microbial activity. Granular activated carbon (GAC) or magnetite can promote methane production from organic waste, where magnetite enriches iron-reducing bacteria in sludge, and granular carbon has high electrical conductivity and large specific surface area, promoting syntrophic metabolism between iron-reducing bacteria and MB. Simultaneously adding magnetite and GAC in anaerobic digesters can accelerate both the hydrolysis of sludge and methane production, leading to better sludge digestion performance
[82].
In practical applications, it is necessary to select suitable conductive particles and their addition methods based on the characteristics of wastewater and treatment processes. The addition of conductive particles may also affect parameters such as the required current density, potential range, and pH value in the wastewater treatment system. In BES, adjusting the redox potential can influence the efficiency of conductive particles participating in the wastewater treatment process. For example, in MFCs, adjusting the redox potential between the anode and cathode can affect the electron transfer rate and electrochemical activity of the conductive particles. Adding magnetite particles in a two-chamber MEC enhances the autotrophic sulfate-reducing biocathode in the system, increasing the sulfate reduction rate by 85%, and significantly improving the biomass, biofilm thickness, and SRB community abundance on the cathode compared to reactors without added magnetite particles
[83]. By adjusting the redox potential between the anode and cathode, the electron transfer efficiency of conductive particles can be optimized, thereby enhancing the efficiency of wastewater treatment
[84]. pH value has a significant impact on microbial metabolic activity and the progress of electrochemical reactions in wastewater treatment. In the biological treatment process with added conductive particles, adjusting the pH of the wastewater can influence the synergistic effect between microorganisms and conductive particles, thereby affecting the degradation efficiency of organic matter. Research has found that under different pH conditions, the degradation rate and pathways of organic matter in wastewater by conductive particles may vary
[85]. Temperature is an important factor affecting microbial metabolic activity and the rate of electrochemical reactions. In the wastewater treatment process with added conductive particles, adjusting the temperature can influence the growth rate, metabolic activity of microorganisms, and the rate of electrochemical reactions. Under different temperature conditions, the degradation efficiency and recycling effectiveness of organic matter in wastewater by conductive particles will differ
[86]. These operating conditions should be adjusted according to specific circumstances to achieve optimal treatment effects, and adjustments and optimizations should be made in combination with actual situations.
4.3 Effect of Carbon Sources (Types and Carbon-to-Sulfur Ratio) on SRB
Lens et al
[83] reported the diversity of SRB in terms of carbon source utilization and metabolic activity. The differences in various carbon source transformation pathways are the main factors affecting the sulfate reduction process, which directly influences the competition between complete-oxidizing SRB (CO-SRB) and incomplete-oxidizing SRB (IO-SRB) (also impacting the reactor startup time, as IO-SRB is easier to enrich, and simultaneously affecting the amount of carbon source available for SRB; however, in practical applications, SRB needs to compete with environmental microorganisms for carbon sources). Due to the presence of IO-SRB, sugars and alcohols will be rapidly incompletely oxidized to produce acetate, leading to a decrease in solution pH that inhibits SRB activity; conversely, the conversion of pH-stable small organic acids would generate more sulfides
[88]. Therefore, when using different carbon sources as substrate matrices, the dominant microbial community types in the system will also change accordingly. For example, SRM with NaHCO
3 and acetate as carbon sources are mainly
Desulobacter and
Desulfobulbus, while sulfate-reducing bacteria with ethanol as the carbon source are primarily
Desulfoicroum and
Desulfovibrio[89]. The conversion rate of SRM for different substrates also varies, with the substrate consumption rate depending on the concentration of electron donors and sulfate, which inevitably affects the competition between SRB and MB. As shown in
Table 5, there are thermodynamic differences in the use of different carbon sources by SRB. Different carbon sources can affect microbial sulfate reduction in two aspects: on one hand, SRB has different affinities for different carbon sources; on the other hand, the carbon source metabolism process of SRB may provide feedback on SRB activity through its impact on solution pH and ORP. Since multiple substrates exist in actual water environments, from a thermodynamic perspective, different microbial communities can optimize the use of energy sources differently, and maximizing energy utilization can also minimize competitive behavior within the system. In practical applications, the dominant microbial community type in the water treatment system can be determined based on the type of specific pollutants or required energy production, and regulated through the type and concentration of carbon source addition.
It is worth emphasizing that in water environments containing sulfate, COD/SO
42- is an important indicator, which determines the electron flow in the processes of sulfate reduction and methane generation, and directly affects the proportion of SRB in the microbial community. Theoretically, when the COD/SO
42- ratio is below 0.67, SRB can completely degrade sulfate by utilizing organic matter, under which circumstance, all electrons would flow to SO
42-; when the ratio exceeds 0.67, the competition for electron donors between SRB and other microorganisms becomes intense
[91]. That is, when the ratio is between 1.7 and 2.7, the competition between SRB and MA is very fierce. In practice, when the COD/SO
42- ratio is 0.5, the total amount of electrons reduced and utilized by SRB accounts for more than 50% of the total electron flow
[92,93]; whereas, when the COD/SO
42- ratio is 2.0 or higher, MA uses over 80% of the total electron flow. However, O’Reilly and Colleran
[89] found that, with influent COD/SO
42- ratios ranging from 2 to 16, SRB species cannot compete with MA species for acetate. These differences in results may be related to variations in carbon source composition, sulfate load, as well as environmental factors such as pH, temperature, and ORP.
Different exogenous carbon sources can alter the dominant bacterial communities in a system by influencing environmental factors, while different amounts of carbon source addition affect the direction of electron flow by changing the carbon-to-sulfur ratio. However, in most actual sulfate-containing wastewater, there is a phenomenon of low carbon source, which generally requires the additional input of organic carbon as an electron donor, but this leads to an increase in treatment costs. In recent water treatment, the main approach has been to use bioelectrochemical technologies for the degradation of sulfate-containing wastewater, with techniques such as using MFC and conductive media (such as iron oxide, ZVI, etc.) for efficient treatment under low or no carbon conditions being developed and applied; furthermore, the treatment of sulfur-containing wastewater using nanomaterials or photocatalytic technology has also received considerable attention.
4.4 OLR and HRT
The organic loading rate (OLR) in organic wastewater represents the balance between microorganisms and organic pollutants, and must be controlled to maintain the balance between methanogenesis and acidification
[94]. An imbalance in OLR often leads to imbalances in steps such as hydrolysis, acidogenesis, acetogenesis, and methanogenesis
[95]. The increase in OLR can be mitigated by shortening the HRT; however, while reducing HRT and increasing OLR, the microbial community will change, and slowly growing methanogens may be eliminated
[96⇓~98]. Additionally, when the substrate is abundant, the formation rate of volatile fatty acids (VFAs) as intermediate products will be higher. The additional accumulation of these intermediates significantly disrupts the microbial community, reduces the performance of anaerobic digestion, and negatively impacts the activity of methanogens
[99]. HRT also affects the relationship between SRB and other microorganisms. Under longer HRT conditions, H
2 production increases, which intensifies the competition between SRB and MA. For example, a shorter HRT reduces the efficiency of hydrolytic acidogenic bacteria in degrading macromolecular organic matter, leading to fewer degradation products available for use by SRB. In the treatment of acid mine drainage, hydraulic retention time has a significant impact on treatment efficiency. A shorter HRT results in the accumulation of acids within the reactor and prevents the timely precipitation and separation of heavy metals in the water; whereas, a longer HRT leads to the depletion of nutrients due to excessive microbial growth and over-precipitation of minerals, ultimately reducing treatment efficiency
[100]. The appropriate HRT needs to be determined based on the type of reactor and the characteristics of the influent. For example, in packed-bed bioreactors, the optimal HRT under neutral pH conditions is 6 h, while under acidic conditions, the optimal HRT increases to 20 h. In upflow anaerobic sludge blanket (UASB) and downflow fluidized bed (DFB) reactors, a lower HRT is beneficial for improving the sulfate reduction rate
[101].
4.5 pH Value
Based on the pH value required for microbial growth, SRB can be classified into neutrophils and acidophiles. The optimal reduction range for SRB neutrophils is a pH of 7.0 to 7.8, with the maximum sulfate reduction rate occurring at a pH of 7.0 to 7.5
[102,103]. Under alkaline conditions, hydrolytic acidifying bacteria can produce more short-chain fatty acids, which are effectively utilized by SRB
[104]; at this time, acetate-nutritive SRB outperform acetate-nutritive methanogens
[105]. pH not only affects the proportion of non-free sulfides in total sulfides but also influences the conversion between sulfides and sulfates by affecting the activity of SRB
[106]. It has been found
[107] that the chemical equilibrium of sulfides depends on the pH value. Sulfides exist in the form of HS
− at a pH of 8, and in the form of H
2S at a pH of 6. At a pH of 7.0 to 7.5, the growth rates of MB and SRB are similar; higher pH favors the growth of MB, while lower pH favors the growth of SRB. The toxic effects of sulfides on anaerobic bacteria vary greatly under different digestion conditions, especially with respect to pH
[106]. Increasing the pH within an appropriate range may be an effective way to alleviate the inhibitory effect of sulfur on the growth of MB
[108].
The optimal pH for the growth of dominant bacterial communities varies depending on the substrate and digestion technology
[23]. Research has found that in the process of anaerobic sludge digestion, the initial pH of the sludge significantly affects the yield of H
2S. When the initial pH increases from 6.5 to 8.0, the content of H
2S in biogas decreases by 44.7%, and the methane content increases by 48.6%. This is because the initial pH influences the competition between SRB and MB, and a higher initial pH inhibits the growth of SRB, thereby reducing the production of H
2S
[19]. Additionally, the pH of the sludge significantly impacts the formation of sulfide species. The effect of pH on SRB is multifaceted: on one hand, it directly affects the physiological metabolism of SRB, controlling intracellular homeostasis; on the other hand, it influences the living environment of SRB, such as the form of sulfides present in the environment and the types of fatty acids, which indirectly alters the metabolic pathways of microorganisms in the environment, thus affecting the symbiotic and competitive relationships of SRB.
4.6 Temperature
SRB can survive under a wide range of temperature conditions, but most of the currently known SRBs mainly exist in the mesophilic zone. The suitable temperature for moderately thermophilic SRBs is 40~60 ℃
[109]. The optimal growth temperature for thermophilic SRBs is 65~70 ℃, while thermophilic SRBs above 80 ℃ are found only in marine hydrothermal vents
[108]. Moderate temperature increase is beneficial to both SRBs and MAs, but their competitive effects differ under different temperature conditions. SRBs have an advantage over MAs in high-temperature wastewater treatment. Omil et al.
[108] found that when the temperature rises to 55~65 ℃, sulfate reduction occurs faster than methane formation. The reason for this may be due to differences in substrates; the degradation capabilities of MAs and SRBs towards substrates such as methanol, ethanol, formate, or acetate may vary. It is also possible that under the influence of temperature, changes in the form and concentration of sulfides have different effects on them, as temperature alters the solubility of H
2S in wastewater, with higher temperatures reducing the solubility of H
2S in wastewater, thereby decreasing the inhibitory effect of H
2S on environmental microorganisms
[109].
As mentioned earlier, process parameters such as substrates (carbon, sulfur), HRT, pH, and temperature have significant impacts on microbial communities,
Table 6 summarizes the main influencing factors. In terms of engineering design and operation, enhancing and optimizing bioreactor performance is a critical goal, and after selecting the optimal operating conditions based on the treatment system, the introduction of an electric field will bring new possibilities for the synergy of various factors. Additionally, the addition of electron mediators has a non-negligible impact on the metabolic types of microorganisms in the system, electron transfer pathways, and changes in environmental conditions. However, this does not mean that the importance of process parameters is secondary; for example, in the SANI biological phosphorus removal experiment
[22], although the phosphate release and uptake cycle associated with sulfur was confirmed, the development of this process was limited due to the long required operating time (48 h).
Among the various influencing factors, the sensitivity of most known SRB to weakly acidic environments (pH < 5)
[109] and their low growth rates limit the design and application of sulfate reduction systems. Although the pH can be increased by adding lime or using a sidestream SRB reactor to avoid direct contact between SRB and acidic wastewater, the cultivation and operation of SRB bioreactors at low pH values (pH < 5) still lack research and summary. Granular sludge provides a solution for slowly growing anaerobic biomass. Self-immobilized SRB granules can improve the efficiency of SRB systems as they increase biomass concentration, reduce reactor volume, and enhance the reactor's tolerance to fluctuations in pH, temperature, etc. Therefore, the coupling effect of the electric field actually alters the competitive outcome by regulating the electron transfer pathways of bacterial communities, including SRB, providing new possibilities for treating wastewater containing sulfate and ammonium nitrogen.
5 Conclusions and Future Prospects
SRB utilize sulfate as the terminal electron acceptor to degrade organic compounds. However, their energy metabolism is not limited to sulfate reduction, and SRB can widely use electron acceptors. An important physiological characteristic of SRB in microbial community relationships is that they can compete with methanogens or grow syntrophically with methanogens depending on the availability of sulfate, where the metabolic flexibility of SRB in co-metabolism with other microbial communities deserves further attention. This has significant implications for the application of SRB in water treatment and energy production.
In addition, in environments lacking sulfate, due to the effectiveness of hydrogenases, SRB mainly exist as hydrogen-producing bacteria and work synergistically with other bacterial communities to produce hydrogen. However, there are fewer studies on how the hydrogen production efficiency of SRB under conditions of insufficient sulfate is affected by bacterial competition, and the balance between the hydrogen production efficiency of SRB and the methane production efficiency in the system needs further investigation. Therefore, when SRB act as hydrogen-producing bacteria, it is necessary to consider the competitive behavior and regulatory benefits of SRB and SMB for electron donors, that is, the benefit balance between hydrogen and methane production. At the same time, it is also necessary to take into account the impact of SRB on hydrogenotrophic methanogenic bacteria and acetoclastic methanogenic bacteria. Currently, more research focuses on altering the direction of electron flow by cycling sulfur elements in the system under the action of SRB communities, thereby improving treatment efficiency. However, to truly achieve widespread application in actual wastewater fields containing sulfate and high concentrations of specific pollutants, practical models coupling S and C/N cycles still need to be established. With the introduction of microbial electrochemistry, the regulation of electric fields and the addition of different conductive media will offer various possibilities for wastewater treatment. Research on the distribution, diversity, and their interrelationships of methanogenic communities and sulfate-reducing bacteria in bioreactors with externally added electric fields is currently not sufficiently explored.
The carbon, nitrogen, and sulfur elements in the aquatic environment undergo continuous cycling changes in the utilization of electron donors by bacteria, which confirms the interaction between SRB and other bacterial communities in the system. Utilizing the basic theories of geochemistry to analyze the mechanism of bacterial competition is conducive to proposing effective measures for the treatment of mixed complex wastewater. Typically, water quality characteristics and environmental conditions determine the dominant bacterial community in the treatment system; therefore, adjusting relevant process parameters can effectively control the competition among bacterial communities. Combining molecular biology techniques to analyze and predict the efficiency or role of sulfate-reducing bacteria (SRB) in the environment.
National Natural Science Foundation of China(51878448)
Cangzhou Institute of Tiangong University(TGCYY-F-0103)
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