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Bacterial incorporation of relict carbon in the hydrothermal environment of Guaymas Basin
Authors:A Pearson  JS Seewald
Institution:1 Department of Earth and Planetary Sciences, Harvard University, Cambridge, MA 02138, USA
2 Department of Marine Chemistry and Geochemistry, Woods Hole Oceanographic Institution, Woods Hole, MA 02543, USA
Abstract:Radiocarbon analyses of bulk carbon and individual organic compounds are presented for the hydrothermal environment of the Rebecca’s Roost vent in the southern trough of the Guaymas Basin hydrothermal field. The Δ14C values of CO2 and CH4in the hottest hydrothermal fluids (317°C) are nearly “radiocarbon dead” (−944‰ and −923‰, respectively). In contrast, the Δ14C values of sediments and individual fatty acids (−418‰ to −227‰) obtained from a bacterial mat located south of the vent site are similar to values previously reported for hydrothermal petroleum in this environment and are more depleted in 14C than overlying waters. Hydrothermal fluids moving through the sediments appear to supply 14C of intermediate age to the bacteria. This carbon may take the form of, or may be supplied by processes similar to, the generation of hydrothermal petroleum. Although the bacterial mat visibly was dominated by Beggiatoa spp., such mats are known to include numerous other species. Individual compound data show that preaged carbon is being consumed by the integrated bacterial assemblage. Values of δ13C and Δ14C indicate that petroleum-derived carbon is incorporated directly into fresh bacterial biomass. Subsequently, some of this newly synthesized material also is consumed by heterotrophs, as eukaryotic sterols from the same sample also have 14C-depleted values (Δ14C = −136‰ to −110‰). Therefore, the entire system may operate as a complex consortium to transform relict carbon back into biomass. Bacterial consumption of relict carbon occurs despite the ample supply of fresh carbon delivered from the productive, overlying water column.
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