TAR SEEPS AND SULFIDE SPRINGS: SULFUR CYCLING WITHIN AN EXTREME (STREAM) ENVIRONMENT, SANTA PAULA CREEK, VENTURA, CA

Sulfur is an essential nutrient and supports a complex biogeochemical cycle. While many aspects of the marine sulfur cycle are well known, the intricacies of terrestrial sulfur biogeochemistry remain understudied. Here, we use Santa Paula Creek (SPC, Ventura, CA, USA), which is rich in tar seeps and sulfide springs from the underlying Monterey Formation, as a test case to investigate the biogeochemistry of sulfur in a terrestrial ecosystem. We applied a geobiological approach to characterize sulfur species and microbial sulfur metabolism along a 150 m long transect of classic step-pool stream morphology at SPC. Variations between the main creek body and influxing sulfide springs may indicate differences in sulfur source. High sulfate concentrations (~2 mmol/L), along with unusual S isotope compositions (δ³⁴S_sulfide heavier than δ³⁴S_sulfate) characterize the main stream body. In contrast, the sulfide springs contain less sulfate (~0.5 mmol/L) with a δ³⁴S composition similar to typical seawater. In situ sampling of sulfide within a microbial mat via a silver disk and subsequent high resolution SIMS analysis captured sharp gradients in δ³⁴S_sulfide with depth in the mat, showing a trend of inversely correlated sulfide abundance and δ³⁴S. These isotope data are tentatively consistent with significant fractionation during sulfide oxidation in this environment. Lipid biomarker data indicate that macrophytic algae dominate the creek’s biomass, whereas 16S rRNA gene sequencing data reveals a diverse bacterial population of sulfur-metabolizing organisms most closely related to the genera Thiothrix, Desulfocapsa, and Sulfurovum. These communities are spatially resolved from each other and follow gradients of pH and dissolved oxygen within the stream. Santa Paula Creek has a rich assemblage of sulfide-oxidizing and sulfate-reducing microbes on a variety of spatial scales and is potentially a valuable study site to reveal novel features of the terrestrial sulfur cycle. In particular, reversal of the usual isotopic ratios of sulfide and sulfate allow us to deconvolve fractionations associated with sulfate reduction and aerobic (and/or phototrophic) sulfide oxidation.