Detangling the Web of Sulfur Metabolisms in Santa Barbara Basin with High-Resolution δ34S and Genomic Profiles

Abstract:

Sulfur metabolisms are major drivers of organic matter remineralization and microbial growth in marine sediments. Sulfur-isotope systematics are particularly powerful for interrogating metabolic processes in these systems due to the large sulfur-isotope fractionations associated with bacterial sulfate reduction (BSR) and some other metabolic reactions. Recent analytical advancements have made it possible to measure δ³⁴S values of very small samples (>50 nmol), including aqueous sulfate and sulfide as well as pyrite, elemental sulfur, and multiple fractions of sedimentary organic matter. We have generated comprehensive 2.5 cm-resolution depth profiles of these sulfur pools over a 2-m core from Santa Barbara Basin, a sub-oxic environment off the California coast. We find that the porewater sulfide δ³⁴S values appear to be strongly influenced by anaerobic sulfide oxidation and sulfur disproportionation in addition to BSR. These sulfur-isotope signals can be tracked over the course of several thousand years of sediment diagenesis, moving from the oxic-anoxic transition at the sediment-water interface to the sulfate-methane transition zone in deeper sediments. Shifts in δ³⁴S relationships among sulfur pools correlate with changes in microbial community composition as shown in TAG genomic data, which supports the existence of the metabolisms indicated by δ³⁴S profiles. Our results suggest that the existence and activity of multiple microbial communities and coexisting sulfur metabolisms have the potential to be recorded in sedimentary δ³⁴S records.