LIPID AND SULFUR ISOTOPE GEOCHEMISTRY OF TUBEWORM-ASSOCIATED SEDIMENTS IN THE GULF OF MEXICO

Vestimentiferan tubeworms in the Gulf of Mexico are a visible and unique component of the macrofaunal community. Tubeworm metabolism is fueled by chemoautotrophic oxidation of sulfide. Sulfide is absorbed by tubeworms from sediments through posterior extensions of their bodies, euphemistically called "roots." Diffusion of seawater sulfate into the sediments may, on its own, provide insufficient sulfate for sulfate-reducing bacteria to supply the sulfide required by a large, mature tubeworm aggregation. Tubeworms potentially overcome this shortfall by actively facilitating sulfate migration into anoxic sediments, possibly by excreting sulfate through their roots and stimulating sulfate-reducing bacteria. The isotopic composition of sulfate is expected to reflect inputs of ³⁴S-depleted sulfate from tubeworms. In the summer of 2000, ten push cores were collected in close proximity to tubeworm aggregations using the submersible Johnson Sea-Link. Pore waters were analyzed for sulfur stable isotopes, and fatty acids from residual sediments were used to characterize the sediment microbial community. Lipid analysis shows a quantitive correlation between known bacterial biomarkers and the general biomass markers, hexadecanoate (16:0) and tetradecanoate (14:0), suggesting that the extracted material is largely bacterial. Additionally, good correlations between biomarkers for sulfate-reducing bacteria, 13-methyltetradecanoate (ai-15:0) and 15-methylhexadecanoate (i-17:0), with hexadecanoate suggest microbial community composition does not change with respect to proximity of the tubeworm aggregation. Lipid distributions do not suggest tubeworms irrigate sediments with sulfate, at least on the scale sampled in this study. Sulfate concentrations remain high throughout sampled cores, and sulfate δ³⁴S values do not become enriched with depth as is expected in closed systems where microbial sulfate reduction occurs. Additionally, Δ_SO4-H2S between sulfide and sulfate pairs from the same depths are small (about 4 permil). Sulfide removal by sulfide oxidizing bacteria could explain the enriched sulfide and bioturbation could explain both the high sulfate concentrations and the presence of oxygen deep in the sediment.