Implications of Microbial Thiosulfate Utilization in Red Clay Sediments of the Central Indian Basin: The Martian Analogy

Microbial thiosulfate utilization and S-disproportionation could be important mechanisms of sulfate-formations on Earth and Mars. Sulfates on Mars date back to late-Noachian to Hesperian period. In contrast, the large sulfur/sulfate formations on Earth evolved under different chronological sequences. The S-cycle was provoked intermittently, permitting multiple appearances of the S-oxidizers on an evolutionary timescale. Hydrothermally altered deep-oceanic red clay sediments of the Central Indian Basin were examined as potential analogue for sulfur (S) oxidation on Mars. The basin sediments supported an active microbial S-metabolism that exhibited S-disproportionation coupled to microbial carbon-fixation through intermediate processes like thiosulfate utilization. Sulfur-oxidizers/thiotrophic denitrifiers were isolated in large numbers at circum-neutral pH, from these cold and dark abyssal Fe-oxide dominated organic-C starved clay. Experimental simulations under psychrophilic and thermo-tolerant conditions revealed the coexistence of an anaerobic, thermal component under the predominantly oxic, circum-neutral seafloor conditions. Multiple causative factors like hydrothermal seafloor circulation, in situ volcanism, and fracture zone reactivation could drive the widespread S-cycle activity in the Central Indian Basin, albeit at a low scale. It is postulated that these conditions are analogous to Great Oxidation Event situations on Earth, when S-oxidizers evolved and flourished. Experimental studies on microbial thiosulfate flux are few in spite of intense scientific interest in microbial S-disproportionation. To the best of our knowledge, this is a new report on regression model development for microbial thiosulfate flux. These clay-systems and their component microbes could serve as analogue to the ancient well-hydrated Noachian Mars and throw light on planetary hydration and desiccation mechanisms.