Microbial oxidation of sulfur refers to the process by which microorganisms oxidize reduced sulfur compounds to obtain energy, often supporting autotrophic carbon fixation. This process is primarily carried out by chemolithoautotrophic sulfur-oxidizing prokaryotes, which use compounds such as hydrogen sulfide (H2S), elemental sulfur (S0), thiosulfate (S2O32−), and sulfite (SO32−) as electron donors. The oxidation of these substrates is typically coupled to the reduction of oxygen (O2) or nitrate (NO3−) as terminal electron acceptors. Under anaerobic conditions, some sulfur-oxidizing bacteria can use alternative oxidants, and certain phototrophic sulfur oxidizers derive energy from light while using sulfide or elemental sulfur as electron sources.
Several key microbial groups involved in sulfur oxidation include genera such as Beggiatoa, Thiobacillus, Acidithiobacillus, and Sulfurimonas, each adapted to specific redox conditions and environmental niches. Metabolic pathways like the Sox (sulfur oxidation) system, reverse dissimilatory sulfite reductase (rDSR) pathway, and the SQR (sulfide:quinone oxidoreductase) pathway are discussed as central mechanisms through which these microbes mediate sulfur transformations.
Microbial sulfur oxidation plays a major role in the biogeochemical cycling of sulfur and contributes to nutrient dynamics in environments hosting both abundant reduced sulfur species and low concentrations of oxygen. These include marine sediments, hydrothermal vents, cold seeps, sulfidic caves, oxygen minimum zones (OMZs), and stratified water columns. Microbial communities are structured by local biogeochemical gradients and their sulfur-oxidizing activity links carbon and nitrogen cycling in suboxic or anoxic environments. Through their metabolic versatility and ecological distribution, sulfur-oxidizing microorganisms help maintain redox balance and influence the chemistry of their surrounding environments, supporting broader ecosystem functioning.
Ecology The oxidation of hydrogen sulfide is a significant environmental process, particularly in the context of Earth's history, during which oceanic conditions were often characterized by very low oxygen and high sulfidic concentrations. The modern analog ecosystems are deep marine basins, for instance in the Black Sea, near the Cariaco trench and the Santa Barbara basin. Other zones of the ocean that experience periodic anoxic and sulfidic conditions are the upwelling zones off the coasts of Chile and Namibia, and hydrothermal vents, which are a key source of H2S to the ocean. Sulfur oxidizing microorganisms (SOM) are thus restricted to upper sediment layers in these environments, where oxygen and nitrate are more readily available. The SOM may play an important yet unconsidered role in carbon sequestration, since some models and experiments with Gammaproteobacteria have suggested that sulfur-dependent carbon fixation in marine sediments could be responsible for almost half of total dark carbon fixation in the oceans. Further, they may have been critical to the evolution of eukaryotic organisms, given that sulfur metabolism is hypothesized to have driven the formation of the symbiotic associations that sustained eukaryotes (see below).
Although the biological oxidation of reduced sulfur compounds competes with abiotic chemical reactions (e.g. the iron-mediated oxidation of sulfide to iron sulfide (FeS) or pyrite (FeS2)), thermodynamic and kinetic considerations suggest that biological oxidation far exceeds the chemical oxidation of sulfide in most environments. Experimental data from the anaerobic phototroph Chlorobaculum tepidum indicates that microorganisms may enhance sulfide oxidation by three or more orders of magnitude. However, the general contribution of microorganisms to total sulfur oxidation in marine sediments is still unknown. The SOM of Alphaproteobacteria, Gammaproteobacteria and Campylobacterota account for average cell abundances of 108 cells/m3 in organic-rich marine sediments. Considering that these organisms have a very narrow range of habitats, as explained below, a major fraction of sulfur oxidation in many marine sediments may be accounted for by these groups.
Given that the maximal concentrations of oxygen, nitrate and sulfide are usually separated in depth profiles, many SOM cannot directly access their hydrogen or electron sources (reduced sulfur species) and energy sources (O2 or nitrate) simultaneously. This limitation has led SOM to develop different morphological adaptations. The large sulfur bacteria (LSB) of the family Beggiatoaceae (Gammaproteobacteria) have been used as model organisms for benthic sulfur oxidation. They are known as 'gradient organisms,' species that are indicative of hypoxic (low oxygen) and sulfidic (rich in reduced sulfur species) conditions. They internally store large amounts of nitrate and elemental sulfur to overcome the spatial gap between oxygen and sulfide. Some species of Beggiatoaceae are filamentous and can thus glide between oxic/suboxic and sulfidic environments, while the non-motile species rely on nutrient suspensions, fluxes, or attach themselves to larger particles. Some aquatic, non-motile LSB are the only known free-living bacteria that utilize two distinct carbon fixation pathways: the Calvin-Benson cycle (used by plants and other photosynthetic organisms) and the reverse tricarboxylic acid cycle.
Another evolutionary strategy of SOM is form mutualistic relationships with motile eukaryotic organisms. The symbiotic SOM provides carbon and, in some cases, bioavailable nitrogen to the host, and receives enhanced access to resources and shelter in return. This lifestyle has evolved independently in sediment-dwelling ciliates, oligochaetes, nematodes, flatworms and bivalves. Recently, a new mechanism for sulfur oxidation was discovered in filamentous bacteria. This mechanism, called electrogenic sulfur oxidation (e-SOx), involves the formation of multicellular bridges that connect the oxidation of sulfide in anoxic sediment layers with the reduction of oxygen or nitrate in oxic surface sediments, generating electric currents over centimeter-long distances. The so-called cable bacteria are widespread in shallow marine sediments, and are believed to conduct electrons through structures inside a common periplasm of the multicellular filament. This process may influence the cycling of elements at aquatic sediment surfaces, for instance, by altering iron speciation. The LSB and cable bacteria are hypothesized to be restricted to undisturbed sediments with stable hydrodynamic conditions, while symbiotic SOM and their hosts have mainly been identified in permeable coastal sediments.
Microbial diversity The oxidation of reduced sulfur compounds is performed exclusively by bacteria and archaea. Archaea involved in this process are aerobic and belong to the order Sulfolobales, characterized by acidophiles (extremophiles that require low pHs to grow) and thermophiles (extremophiles that require high temperatures to grow). The most studied have been the genera Sulfolobus, an aerobic archaea, and Acidianus, a facultative anaerobe (i.e. an organism that can obtain energy either by aerobic or anaerobic respiration).
Sulfur oxidizing bacteria (SOB) are aerobic, anaerobic or facultative, with most of them being obligate (capable of metabolizing only a specific compound) or facultative (capable of metabolizing a secondary compound when primary compound is absent) autotrophs that can utilize either carbon dioxide or organic compounds as a source of carbon (mixotrophs). The most abundant and studied SOB are in the family Thiobacilliaceae, found in terrestrial environments, and in the family Beggiatoaceae, found in aquatic environments. Aerobic sulfur oxidizing bacteria are mainly mesophilic, growing optimally at moderate ranges of temperature and pH, although some are thermophilic and/or acidophilic. Outside of these families, other SOB described belong to the genera Acidithiobacillus, Aquaspirillum, Aquifex, Bacillus, Methylobacterium, Paracoccus, Pseudomonas Starkeya, Thermithiobacillus, and Xanthobacter. On the other hand, the cable bacteria belong to the family Desulfobulbaceae of the Desulfobacterota and are currently represented by two candidate genera, "Candidatus Electronema" and "Candidatus Electrothrix." The phylum Desulfobacterota also includes several lineages of anaerobic SOB.
Anaerobic SOB (AnSOB) are mainly neutrophilic/mesophilic photosynthetic autotrophs, obtaining energy from sunlight but using reduced sulfur compounds instead of water as hydrogen or electron donors for photosynthesis. AnSOB include some purple sulfur bacteria (Chromatiaceae) such as Allochromatium, and green sulfur bacteria (Chlorobiaceae), as well as the purple non-sulfur bacteria (Rhodospirillaceae) and some Cyanobacteria. The AnSOB Cyanobacteria are only able to oxidize sulfide to elemental sulfur and have been identified as Oscillatoria, Lyngbya, Aphanotece, Microcoleus, and Phormidium. Some AnSOB, such as the facultative anaerobes Thiobacillus spp., and Thermothrix sp., are chemolithoautotrophs, meaning that they obtain energy from the oxidation of reduced sulfur species, which is then used to fix CO2. Others, such as some filamentous gliding green bacteria (Chloroflexaceae), are mixotrophs. From all of the SOB, the only group that directly oxidize sulfide to sulfate in an abundance of oxygen without accumulating elemental sulfur are the Thiobacilli. The other groups accumulate elemental sulfur, which they may oxidize to sulfate when sulfide is limited or depleted.
SOB have prospective use in environmental and industrial settings for detoxifying hydrogen sulfide, soil bioremediation, and wastewater treatment. In highly basic and ionic environments, Thiobacillus thiooxidans has been observed to increase the pH of soil from 1.5pH to a neutral 7.0pH. The use of SOB in the detoxification of hydrogen sulfide can circumvent detrimental effects from the conventional oxidation methods of hydrogen peroxide (H2O2), chlorine gas (Cl2), and hypochlorite (NaClO) usage. SOB of the Beggiotoa genera oxidize sulfur compounds in microaerophilic up-flow sludge beds during wastewater treatment, and can be combined with nitrogen-reducing bacteria to effectively remove chemical build-ups in industrial settings.
The chemolithotrophic subset of SOB are gram-negative, rod-shaped bacteria, which abide in a wide range of environments—from anoxic to oxic, 4 to 90°C, and 1 to 9pH. Chemolithotrophic SOB play a key role in agricultural ecosystems by oxidizing reduced sulfur fertilizers into available forms, such as sulfate, for plants. SOB are often present in agricultural ecosystems at low densities, creating the opportunity for inoculation to increase nutrient availability. Presence of Thiobacillus thiooxidans has been shown to increase phosphorus availability in addition to the oxidation of sulfur. Utilization of SOB in treating alkaline and low available-sulfur soils, such as those in Iran, could directly increase crop yields in many ecosystems around the world.
Certain SOB have the potential to serve as biotic pesticides and anti-infectious agents for the control of crops. The benefits of utilization have been demonstrated through the outcomes of sulfur-oxidation, including balancing sodium content as well as increasing sulfur and phosphorus availability in the soil. Increased levels of reduced sulfur compounds in acidic soil permits the growth of Streptomyces scabies and S. ipomea, both pathogens of potato plants. Presence of SOB such as Thiobacillus have decreased the growth of these bacteria, as well as root pathogens such as Rhizoctonia solani. An additional impact of SOB on crop protection includes a collateral effect of increased sulfur content in plants, resulting in resistance to Rhizoctonia.