Cryptic role of tetrathionate in the sulfur cycle: A study from Arabian Sea sediments

Abstract. To explore the potential role of tetrathionate in the sulfur cycle of marine sediments, population ecology of microorganisms capable of metabolizing this polythionate was revealed at 15–30 cm resolution along two, ~ 3-m-long, cores collected from 530 and 580 meters below the sea level, off India's west coast, within the oxygen minimum zone (OMZ) of the Arabian Sea. Metagenome analysis along the two sediment-cores revealed widespread occurrence of genes involved in microbial formation, oxidation, and reduction of tetrathionate; high diversity and relative-abundance was also detected for bacteria that are known to render these metabolisms in vitro. Results of slurry-incubation of the sediment-samples in thiosulfate- or tetrathionate-containing microbial growth media, data obtained via pure-culture isolation, and finally metatranscriptome analyses, corroborated the in situ functionality of tetrathionate-forming, oxidizing, and reducing microorganisms. Geochemical analyses revealed the presence of up to 11.1 µM thiosulfate along the two cores, except a few sample-sites near the sediment-surface. Thiosulfate oxidation by chemolithotrophic bacteria prevalent in situ is the apparent source of tetrathionate in this ecosystem. However, potential abiotic origin of the polythionate can neither be ruled out nor confirmed from the geochemical information currently available for this territory. Tetrathionate, in turn, can be either oxidized to sulfate (via oxidation by the chemolithotrophs present) or reduced back to thiosulfate (via respiration by native bacteria). Up to 2.01 mM sulfide present in the sediment-cores may also reduce tetrathionate abiotically to thiosulfate and elemental sulfur. As tetrathionate was not detected in situ, high microbiological and geochemical reactivity of this polythionate was hypothesized to be instrumental in its cryptic status as a central sulfur cycle intermediate.