Sulfate-Reducing Bacteria (SRB) in Wastewater Treatment: Mechanisms, Applications, and Optimization

Heterotrophic SRB utilize organic compounds as carbon sources and electron donors, while autotrophic SRB use CO2/HCO32- as the carbon source and obtain electrons through the oxidation of H2 or Fe2+. As a result, SRB can establish two different lifestyles: acetogenic or hydrogenogenic metabolism. The sulfate reduction process of SRB is not fully understood, but it is believed to occur through two proposed pathways. One potential pathway is assimilative sulfate reduction, which directly uses sulfate reduction products for the synthesis of cellular materials. The other pathway is dissimilatory sulfate reduction, in which SRB can decompose organic carbon sources to carbon dioxide through complete oxidation or to acetic acid through incomplete oxidation under anaerobic conditions. Substrate-level phosphorylation produces small amounts of adenosine triphosphate (ATP) and high-energy electrons. These high-energy electrons are transferred stepwise to SRB through the electron transport chain (flavin proteins, cytochrome C, etc.), resulting in the production of large amounts of ATP through 'electron transport phosphorylation' to provide energy for biochemical processes. Finally, ATP is consumed in the reduction of sulfur oxides to sulfide, providing energy for the process.

The sulfate reduction process involves three main steps: (i) sulfate activation to adenosine 5'-phosphosulfate (APS), (ii) APS reduction to sulfite, and (iii) sulfite reduction to sulfide. In the sulfate activation step, sulfate is activated by ATP sulfurylase to produce APS and pyrophosphate. Then, APS can be exergonically reduced to sulfite by APS reductase. The mechanism of the third step (sulfite reduction to sulfide) is still a topic of debate. One proposed pathway involves the intermediates trithionate and thiosulfate, enabling three-step, two-electron reduction (tri-thionate pathway), with the involvement of trithionate and thiosulfate reductases. However, the commonly reported intermediates in the sulfate-reducing pathway are neither thiosulfate nor trithionate. Alternatively, the possibility of a direct six-electron reduction step (direct pathway) cannot be fully excluded.

The diversity of species and unique metabolic modes of SRBs make them highly suitable for wastewater treatment under various environmental conditions. This review summarizes the mechanism of pollutant removal, the role of SRB in wastewater treatment, and the related influencing factors. Understanding and optimizing the sulfate-reducing process driven by SRBs is of great significance for the future application of SRB-driven biotechnology in wastewater treatment.