UNDERSTANDING THE ROLE OF MICROBES IN THE FRESHWATER SEDIMENT CRYPTIC SULFUR CYCLE USING METAGENOMICS AND METATRANSCRIPTOMICS

The sulfur cycle is a complex biogeochemical cycle that involves both inorganic and organic species. However, the role of organic sulfur to the sulfur cycle is under appreciated. Recent studies have suggested organic sulfur likely fuels sulfate reduction, potentially through a cryptic sulfur cycle, and these processes were likely important in Earth’s geologic past. In order to determine the role that organic sulfur can play in an oligotrophic low-sulfate environment, we searched metagenomic and metatranscriptomic libraries for the presence of and expression of sulfur cycling genes. In sediment metagenomes, pathways and genes for sulfur reduction, oxidation and organic sulfur degradation were found. Using Lake Superior sediments, we also conducted sediment incubations to measure the biotransformation capability of sulfur containing amino acids. Analysis of metabolic pathways present in the metagenomes show presence of not only organic sulfur utilization, but thiosulfate disproportionation, tetrathionate reduction, elemental sulfur reduction, as well as sulfate reduction and sulfide oxidation. Regarding organic sulfur utilization, dimethyl sulfide (DMS) and sulfoacetaldehyde are the two organosulfurs that have complete metabolic pathways within the metagenomic data. Oxidation of DMS is present in nearly 40% of microbial genomes. Sulfoacetaldehyde degradation, present in 92% of microbial genomes, generates sulfite that could then be oxidized to sulfate. This supports our sediment incubation biotransformation data that shows high generation of sulfate in presence of taurine, as taurine can be degraded into sulfoacetaldehyde. In order to determine which pathways are most commonly used by microbes present, metatranscriptomic data will assist in determination of gene expression. By combining biotransformation results, metagenomics, and metatranscriptomics, a robust frame work for sulfur cycling in sediments informs how pathways may have operated in the past.