Biosignatures present in a hydrothermal massive sulfide from the Mid-Atlantic Ridge

Mid-ocean spreading and accompanying hydrothermal activities result in huge areas with exposure of minerals rich in reduced chemicals-basaltic and peridotitic rocks as well as metal sulfide precipitates-to the oxygenated seawater. Oxidation of Fe and S present in these rocks provides an extensive long-term source of energy to lithotrophs. Investigation of lipid biomarkers and their carbon isotope ratios from a massive iron sulfide of an inactive sulfide mound or inactive chimney sampled at the western flank of the Turtle-Pits hydrothermal field (Mid-Atlantic Ridge, 5 S) revealed a unique lipid distribution. The bacterial fauna appears to be dominated by chemolithotrophs with a distinct lipid composition mainly comprising of iso-branched fatty acids and nonisoprenoidal dialkyl glycerol diethers partially including the very rare macrocyclic cores with 30-35 carbon atoms (including 13,16-dimethyloctacosane and 5,13,16-trimethyloctacosane). The Bacteria are accompanied by most likely hydrogen/CO2-dependent methanogenic Archaea (e. g. Methanococcus) as well as other Archaea with a different life style (e.g. Ferroplasma). Alike some of the bacterial lipids the archaeal lipids predominantly consist of macrocyclic diethers including one C 40 and one C 41 isoprenoid. Structural homologues of the latter are so far only reported from a methanogenic archaeum and a Pleistocene sulfur deposit. Compound-specific analyses of the stable isotope ratios revealed delta C-13 values for the majority of bacterial and archaeal lipid components of about 0 parts per thousand (vs. VPDB), indicative for chemolithoautotrophically fixed carbon which is, for distinct pathways, accompanied by only negligible fractionations. However, the presence of methanogenic Archaea is indicated by C-13-depleted isoprenoidal lipids (delta C-13 similar to -50 parts per thousand) characteristic for certain CO2-reducing methanogens synthesizing lipids via acetyl CoA.