Microbial sulfur cycling in marine sediments often occurs in environments characterized by transient chemical gradients that affect both the availability of nutrients and the activity of microbes. High turnover rates of intermediate valence sulfur compounds and the intermittent availability of oxygen in these systems greatly impact the activity of sulfur‐oxidizing micro‐organisms in particular. In this study, the thiosulfate‐oxidizing hydrothermal vent bacterium Thiomicrospira thermophila strain EPR85 was grown in continuous culture at a range of dissolved oxygen concentrations (0.04–1.9 mM) and high pressure (5–10 MPa) in medium buffered at pH 8. Thiosulfate oxidation under these conditions produced tetrathionate, sulfate, and elemental sulfur, in contrast to previous closed‐system experiments at ambient pressure during which thiosulfate was quantitatively oxidized to sulfate. The maximum observed specific growth rate at 5 MPa pressure under unlimited O2 was 0.25 hr−1. This is comparable to the μmax (0.28 hr−1) observed at low pH (<6) at ambient pressure when T. thermophila produces the same mix of sulfur species. The half‐saturation constant for O2 (KO2) estimated from this study was 0.2 mM (at a cell density of 105 cells/ml) and was robust at all pressures tested (0.4–10 MPa), consistent with piezotolerant behavior of this strain. The cell‐specific KO2 was determined to be 1.5 pmol O2/cell. The concentrations of products formed were correlated with oxygen availability, with tetrathionate production in excess of sulfate production at all pressure conditions tested. This study provides evidence for transient sulfur storage during times when substrate concentration exceeds cell‐specific KO2 and subsequent consumption when oxygen dropped below that threshold. These results may be common among sulfur oxidizers in a variety of environments (e.g., deep marine sediments to photosynthetic microbial mats).
Shallow-sea (5 m depth) hydrothermal venting off Milos Island provides an ideal opportunity to target transitions between igneous abiogenic sulfide inputs and biogenic sulfide production during microbial sulfate reduction. Seafloor vent features include large (>1 m2) white patches containing hydrothermal minerals (elemental sulfur and orange/yellow patches of arsenic-sulfides) and cells of sulfur oxidizing and reducing microorganisms. Sulfide-sensitive film deployed in the vent and non-vent sediments captured strong geochemical spatial patterns that varied from advective to diffusive sulfide transport from the subsurface. Despite clear visual evidence for the close association of vent organisms and hydrothermalism, the sulfur and oxygen isotope composition of pore fluids did not permit delineation of a biotic signal separate from an abiotic signal. Hydrogen sulfide (H2S) in the free gas had uniform δ34S values (2.5±0.28‰, n=4) that were nearly identical to pore water H2S (2.7±0.36‰, n=21). In pore water sulfate, there were no paired increases in δ34SSO4 and δ18OSO4 as expected of microbial sulfate reduction. Instead, pore water δ34SSO4 values decreased (from approximately 21° to 17°) as temperature increased (up to 97.4°C) across each hydrothermal feature. We interpret the inverse relationship between temperature and δ34SSO4 as a mixing process between oxic seawater and 34S-depleted hydrothermal inputs that are oxidized during seawater entrainment. An isotope mass balance model suggests secondary sulfate from sulfide oxidation provides at least 15% of the bulk sulfate pool. Coincident with this trend in δ34SSO4, the oxygen isotope composition of sulfate tended to be 18O-enriched in low pH (<5), high temperature (>75°C) pore waters. The shift toward high δ18OSO4 is consistent with equilibrium isotope exchange under acidic and high temperature conditions. The source of H2S contained in hydrothermal fluids could not be determined with the present dataset; however, the end-member δ34S value of H2S discharged to the seafloor is consistent with equilibrium isotope exchange with subsurface anhydrite veins at a temperature of ~300°C. Any biological sulfur cycling within these hydrothermal systems is masked by abiotic chemical reactions driven by mixing between low-sulfate, H2S-rich hydrothermal fluids and oxic, sulfate-rich seawater.
Scientific outreach efforts tend to be unidirectional, with information moving from the scientist to the public. We have created an interactive interface that allows the public to directly participate in the process of scientific discovery. The interface is based on a 2D solute transport model that incorporates microbial kinetics to simulate biogeochemical processes in subsurface marine environments. Users can populate the subsurface with microbes chosen from our database that provides the kinetic parameters used in the model. Each step in the simulation is accompanied by imbedded in-depth information, visual displays of 2D change in parameters in real-time, and the ability to interrogate results in 2D. Access to the microbial database can be granted to other PIs to interrogate and sort the information available. The game will initially be tested in 7-12 grade biology, chemistry, environmental science, and statistics classes. Example lesson plans will be posted on an ongoing blog/wiki resource that can also inform undergraduate teaching strategies. A reciprocal benefit to scientists will be an infusion of creative, testable scenarios ranging from novel consortium interactions to user-created hypothetical microbes that lead to the discovery of new species.
Seafloor hydrothermal activity dominates heat and chemical exchange between seawater and ocean rocks, however the extent to which diffuse venting contributes to these processes is poorly constrained. The low-temperature, ultra-diffuse venting and extensive iron mats at the FeMO Deep (~0.2°C anomaly) and Shinkai Deep (~6°C anomaly) sites on the flanks of Loihi Seamount provides a unique opportunity to study Fe, C, N, and S cycling in modern analogues to ancient seafloor hematitic formations that are preserved in the rock record. While aerobic microbial activity near the mat surface is responsible for much of the iron deposition, these microaerophiles are limited to the uppermost suboxic zone. Most of the mat thickness is characterized by anoxic conditions, and we hypothesize that anaerobic metabolisms dominate in these zones. Here we propose preliminary studies to characterize this cryptic anaerobic community. We will use isotope-tracer incubation experiments, functional gene analyses and a silver sulfide capture technique to quantify S-, N-, and C-based microbial metabolisms in the anoxic mat interior. Coupled to our ongoing geochemical and bioenergetic efforts, this work will lead to a coupled microbial/geochemical model at FeMO Deep style mats that elucidates the role these systems play in global biogeochemical cycles.