UNDERSTANDING NATURAL SULFUR BIOSIGNATURES FROM SULFIDIC SPRING MICROBIAL MATS BASED ON XANES SPECTROSCOPY, MICROBIAL DIVERSITY, AND STABLE ISOTOPE SYSTEMATICS

Accumulations of S-containing organic matter and rocks, including those deposited at springs, record geochemical and ecological changes that have occurred throughout Earth's history. However, the environmental, metabolic, and phylogenetic factors that govern S speciation, and the chemical nature and turnover rates of inorganic and organic S compounds, are poorly understood and constrained. These issues make interpreting the ancient S record questionable. In modern springs, microbial oxidation of reduced inorganic S compounds is a prevalent pathway. For a vast majority of S-oxidizing bacteria (SOB), sulfate is the end product (and this can be incorporated into deposits as carbonate-associated sulfate). The formation of S globules and S assimilation into biomolecules also results in S storage within biomass, and could be preserved in the rock record. The goal of our work has been to evaluate inorganic and organic S speciation in modern microbial mats from a variety of photic and aphotic sulfidic springs using S K-edge XANES spectroscopy and to compare the speciation results to habitat geochemistry, microbial community composition, and S isotope systematics. Thus far, microbes associated with S metabolism dominate the mats that we have studied, including groups not previously examined by XANES (e.g., Thiothrix). All microbial samples consisted of elemental S, with greater amounts of cyclo-octasulfur (S₈) (~60%) compared to polymeric S (S_m) (30%). S speciation did not reflect the metabolism of dominant microbial groups, but instead indicated that inorganic and organic S signatures revealed competing metabolic processes. For instance, high S₈ content may suggest a predominance of S-globule-forming microbes, but a low S₈ (+ S_m) content does not necessarily correlate to lower SOB abundance because of enhanced S oxidation by non-globule-accumulating SOB or microbial reduction of S₈+S_m. Moreover, mats with high S₈ content had the lowest d³⁴S values, indicating a record of oxidized sulfide having notable ³⁴S depletion resulting from microbial sulfate reduction and not of spring water sulfide. The S species distribution and variety of metabolic pathways present complicate inferring isotopic biosignatures and future studies should identify S species before interpreting geochemical and ecological changes.