Hydrothermal fluid samples were retrieved during dives made by the remotely operated vehicle (ROV) QUEST (MARUM, University of Bremen) during the HYDROMAR I (M60/3, 2004) and HYDROMAR II (M64/2, 2005) cruises to the Logatchev hyrothermal field. The samples were retrieved using a pumped flow-through system (Kiel Pumping System KIPS) specially designed for the ROV QUEST. Once on board the ship, the liquids intended for microbiological studies were concentrated on 0.2 µm pore size polycarbonate filters (Sartorius) and stored at -20°C. The pH and sulfide concentrations were determined immediately after sample recovery. The pH was measured (Mettler electrodes with Ag/AgCl reference electrode) at 25°C in unfiltered sample aliquots. Sulfide concentrations were determined photometrically following the methylene blue method or, for samples with low concentrations, by voltammetry (Metrohm Application Bulletin 199/3e). Methane was analyzed on board by applying a purge and trap technique. For on-board measurements of dissolved hydrogen the water sample was degassed into a high-grade vacuum. Aliquots of the released gas were analyzed by gas chromatography (Thermo Electron Corporation Trace GC Ultra with a pulsed discharge detector). The abundance of bacterial and archaeal taxa was investigated by sequencing of the 16S rRNA gene. The diversity of the cbbL, cbbM and aclb was investigated by sequencing of the genes.

Gene sequences obtained in this study were submitted to DDBJ/EMBL/GenBank database and assigned the accession numbers.