Laura L. Lapham

Evidence for Microbial Carbon and Sulfur Cycling in Deeply Buried Ridge Flank Basalt

Under the Sea Floor Microorganisms living in basaltic sea floor buried beneath sediments derive energy from inorganic components from the host rocks that interact with infiltrating seawater, which brings dissolved oxygen and other trace nutrients with it. Lever et al. (p. 1305) directly sampled the subseafloor community off the eastern flank of the Juan de Fuca Ridge in the Pacific Ocean and found evidence for ongoing microbial sulfate reduction and methanogenesis. Multiyear incubation experiments with samples of host rock confirmed the microbial activities measured in situ. Active methane- and sulfur-cycling microbial communities exist in deep basaltic ocean crust. Sediment-covered basalt on the flanks of mid-ocean ridges constitutes most of Earths oceanic crust, but the composition and metabolic function of its microbial ecosystem are largely unknown. By drilling into 3.5-million-year-old subseafloor basalt, we demonstrated the presence of methane- and sulfur-cycling microbes on the eastern flank of the Juan de Fuca Ridge. Depth horizons with functional genes indicative of methane-cycling and sulfate-reducing microorganisms are enriched in solid-phase sulfur and total organic carbon, host δ13C- and δ34S-isotopic values with a biological imprint, and show clear signs of microbial activity when incubated in the laboratory. Downcore changes in carbon and sulfur cycling show discrete geochemical intervals with chemoautotrophic δ13C signatures locally attenuated by heterotrophic metabolism.