Sulfur-reducing bacteria are microorganisms able to reduce elemental sulfur (S0) to hydrogen sulfide (H2S). These microbes use inorganic sulfur compounds as electron acceptors to sustain several activities such as respiration, conserving energy and growth, in absence of oxygen. The final product of these processes, sulfide, has a considerable influence on the chemistry of the environment and, in addition, is used as electron donor for a large variety of microbial metabolisms. Several types of bacteria and many non-methanogenic archaea can reduce sulfur. Microbial sulfur reduction was already shown in early studies, which highlighted the first proof of S0 reduction in a vibrioid bacterium from mud, with sulfur as electron acceptor and as electron donor. The first pure cultured species of sulfur-reducing bacteria, Desulfuromonas acetoxidans, was discovered in 1976 and described by Pfennig Norbert and Biebel Hanno as an anaerobic sulfur-reducing and acetate-oxidizing bacterium, not able to reduce sulfate. Only few taxa are true sulfur-reducing bacteria, using sulfur reduction as the only or main catabolic reaction. Normally, they couple this reaction with the oxidation of acetate, succinate or other organic compounds. In general, sulfate-reducing bacteria are able to use both sulfate and elemental sulfur as electron acceptors. Thanks to its abundancy and thermodynamic stability, sulfate is the most studied electron acceptor for anaerobic respiration that involves sulfur compounds. Elemental sulfur, however, is very abundant and important, especially in deep-sea hydrothermal vents, hot springs and other extreme environments, making its isolation more difficult. Some bacteriasuch as Proteus, Campylobacter, Pseudomonas and Salmonellahave the ability to reduce sulfur, but can also use oxygen and other terminal electron acceptors.
Taxonomy
Sulfur reducers are known to cover about 74 genera within the Bacteria domain. Several types of sulfur-reducing bacteria have been discovered in different habitats like deep and shallow sea hydrothermal vents, freshwater, volcanic acidic hot springs and others. Many sulfur reducers belong to the phylum Thermodesulfobacteriota (Desulfuromonas, Pelobacter, Desulfurella, Geobacter), the class Gammaproteobacteria, and the phylum Campylobacterota according to GTDB classification. Other phyla that present sulfur-reducing bacteria are: Bacillota (Desulfitobacterium, Ammonifex, and Carboxydothermus), Aquificota (Desulfurobacterium and Aquifex), Synergistota (Dethiosulfovibrio), Deferribacterota (Geovibrio), Thermodesulfobacteriota, Spirochaetota, and Chrysiogenota.
Metabolism
Sulfur reduction metabolism is an ancient process, found in the deep branches of the phylogenetic tree. Sulfur reduction uses elemental sulfur (S0) and generates hydrogen sulfide (H2S) as the main end product. This metabolism is largely present in extreme environments where, especially in recent years, many microorganisms have been isolated, bringing new and important data on the subject.
Many sulfur-reducing bacteria are able to produce ATP through lithotrophic sulfur respiration, using zero-valence sulfur as electron acceptor, for instance the genera Wolinella, Ammonifex, Desulfuromonas and Desulfurobacterium. On the other side, there are obligate fermenters able to reduce elemental sulfur, for example Thermotoga, Thermosipho and Fervidobacterium. Among these fermenters there are species, such as Thermotoga maritina, that are not dependent on sulfur reduction, and utilize it as a supplementary electron sink. Some researches propose the hypothesis that polysulfide could be an intermediate of sulfur respiration, due to the conversion of elemental sulfur into polysulfide that occurs in sulfide solutions, performing this reaction:
Pseudomonadota
The Pseudomonadota are a major phylum of gram-negative bacteria. There is a wide range of metabolisms. Most members are facultative or obligately anaerobic, chemoautotrophs and heterotrophics. Many are able to move using flagella, others are nonmotile. They are currently divided into several classes, referred to by Greek letters, based on rRNA sequences: Alphaproteobacteria, Betaproteobacteria, Gammaproteobacteria, Zetaproteobacteria, etc.
Class Gammaproteobacteria
The Gammaproteobacteria class include several medically, ecologically and scientifically important groups of bacteria. They are major organisms in diverse marine ecosystems and even extreme environments. This class contains a huge variety of taxonomic and metabolic diversity, including aerobic and anaerobic species, chemolitoauthotrophic, chemoorganotrophic and phototrophic species and also free living, biofilms formers, commensal and symbionts.