Oceanic lithosphere exposed at the sea floor undergoes seawater-rock alteration reactions involving the oxidation and hydration of glassy basalt. Basalt alteration reactions are theoretically capable of supplying sufficient energy for chemolithoautotrophic growth. Such reactions have been shown to generate microbial biomass in the laboratory, but field-based support for the existence of microbes that are supported by basalt alteration is lacking. Here, using quantitative polymerase chain reaction, in situ hybridization and microscopy, we demonstrate that prokaryotic cell abundances on seafloor-exposed basalts are 3-4 orders of magnitude greater than in overlying deep sea water. Phylogenetic analyses of basaltic lavas from the East Pacific Rise (9°N) and around Hawaii reveal that the basalt-hosted biosphere harbours high bacterial community richness and that community membership is shared between these sites. We hypothesize that alteration reactions fuel chemolithoautotrophic microorganisms, which constitute a trophic base of the basalt habitat, with important implications for deep-sea carbon cycling and chemical exchange between basalt and sea water.
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Acknowledgements The authors thank D. Fornari, M. Tivey and H. Schouten for allowing C.M.S. and W.B. to participate in their research cruise AT11-7 to collect samples at the EPR, D. Rogers for samples collected on the research cruise KOK 02-24 at Hawaii, L. Kerr for instruction and guidance on the SEM and confocal microscope, P. Schloss for support with the DOTUR and SONS programs, S. Simmons for support and instruction on qPCR, and E. Leadbetter for advice on the manuscript. This research was supported by a Ridge2K grant awarded to K.J.E. and W.B., in part by a NAI CAN awarded to M.L.S. and K.J.E., and also in part by a Project Development Award from WWU’s Office of Research and Sponsored Programs to C.L.M.