Microbial oxidation of sulfur
The oxidation of inorganic compounds is the strategy primarily used by chemolithotrophic microorganisms to obtain energy to survive, grow and reproduce.
[5] Experimental data from the anaerobic phototroph Chlorobaculum tepidum indicate that microorganisms enhance sulfide oxidation by three or more orders of magnitude.
[12] 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.
They internally store large amounts of nitrate and elemental sulfur to overcome the spatial gap between oxygen and sulfide.
Some of the Beggiatoaceae are filamentous and can thus glide between oxic/suboxic and sulfidic environments, while the non-motile ones rely on nutrient suspensions, fluxes, or attach themselves to bigger particles.
The symbiotic SOM provides carbon and, in some cases, bioavailable nitrogen to the host, and gets enhanced access to resources and shelter in return.
[4] Aerobic sulfur oxidizing bacteria are mainly mesophilic, growing at moderate ranges of temperature and pH, although some are thermophilic and/or acidophilic.
Outside these families, other SOB described belong to the genera Acidithiobacillus,[22] Aquaspirillum,[23] Aquifex,[24] Bacillus,[25] Methylobacterium,[26] Paracoccus, Pseudomonas [23] Starkeya,[27] Thermithiobacillus,[22] and Xanthobacter.
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.
[4] The AnSOB Cyanobacteria are only able to oxidize sulfide to elemental sulfur and have been identified as Oscillatoria, Lyngbya, Aphanotece, Microcoleus, and Phormidium.
[32][33] 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.
From all of the SOB, the only group that directly oxidize sulfide to sulfate in abundance of oxygen without accumulating elemental sulfur are the Thiobacilli.
[50] Similarly, both mechanisms operate in the chemoautotroph Thiobacillus denitrificans,[51] which can oxidize sulfide to sulfate anaerobically using nitrate as terminal electron acceptor [52] which is reduced to dinitrogen (N2).
[55] Other organisms, such as the Bacteria Sphaerotilus natans [56] and the yeast Alternaria [57] are able to oxidize sulfide to elemental sulfur by means of the rDsr pathway.
[4] Acidithiobacillus ferrooxidans and Thiobacillus thioparus can oxidize sulfur to sulfite by means of an oxygenase enzyme, although it is thought that an oxidase could be used as well as an energy saving mechanism.
[59] For the anaerobic oxidation of elemental sulfur, it is thought that the Sox pathway plays an important role, although this is not yet completely understood.
[4] A direct oxidation reaction (T. versutus [61]), as well as others that involve sulfite (T. denitrificans) and tetrathionate (A. ferrooxidans, A. thiooxidans and Acidiphilum acidophilum [62]) as intermediate compounds, have been proposed.
Given the very small fractionation of 18O that usually accompanies MSO, the relatively higher depletions in 18O of the sulfate produced by MSO coupled to DNR (-1.8 to -8.5 ‰) suggest a kinetic isotope effect in the incorporation of oxygen from water to sulfate and the role of nitrate as a potential alternative source of light oxygen.
Sulfate depletion in 34S from MSO could be used to trace sulfide oxidation processes in the environment, although it does not allow a discrimination between the SQR and Sox pathways.
