Marine sediments represent one of the greatest but least understood microbial ecosystems, supporting ~50–85% of the microbial biomass on Earth and playing an important role in the biogeochemical cycling of carbon and other elements. In comparison to terrestrial ecosystems, the oceans are unique in their geochemical composition, with sulfate serving as the major oxidant for microbial respiration, accounting for up to half of the organic carbon turnover in some systems. Much of this sulfur-driven carbon cycling in marine sediments occurs through complex microbially-driven geochemical and biological interactions that are poorly understood, with the metabolic conversion of sulfur and carbon occurring at spatial scales that are difficult to measure with current techniques. A greater understanding of the interactions and metabolic versatility of microbial communities in sedimentary environments requires new, creative methodological approaches that are compatible with the spatial scale of microbial metabolic interactions and specific to the responsible microorganisms. We are developing and testing a suite of sensitive sulfur stable isotope methods that are applicable across a range of scales (microns to meters) to expand the geochemical toolkit available to microbiologists and biogeochemists for detailed study of microbial interactions associated with sulfur cycling in the environment.
The coupling of stable isotope tracers with molecular biological methods has been shown to be an effective strategy for unpacking microbial ecosystem ‘black boxes’. In this project we are expanding the geochemical toolkit available for diagnosing microbial transformation of sulfur in the environment and linking these processes to the associated microorganisms by developing new analytical methods for measuring sulfur and carbon stable isotopes in microbial macromolecules and metabolites. Specifically these techniques include the isotopic analysis of inorganic and organic sulfur species by MC-ICP-MS, in situ capture and high spatial resolution analysis of 34S and 33S of pore water sulfate and sulfide by SIMS, and clumped isotope analysis of gaseous microbial metabolites by high-resolution gas source isotope ratio mass spectrometry. Natural abundance isotopic geochemical measurements and sulfur 33 and sulfur 34-enriched isotope incubation experiments will be conducted in tandem with proteomic and FISH-nanoSIMS analysis of sulfur-metabolizing microbial communities to directly characterize the microorganisms and interspecies interactions driving the cycling of sulfur at redox active interfaces in organic-rich sediments.
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