|(übersetzt):||The activity and diversity of prokaryotic communities in special deep-sea environments were investigated during several research projects. Water-bodies with different portions of hydrothermal fluids were examined in the central North Fiji Basin (South-West Pacific). Changes within the bacterial community structure could be observed with increasing distance from the vent sites and decreasing concentrations of reduced compounds. In the hydrothermal vent fields, sulfur-oxidizing bacteria take a major part in building up organic biomass. Aerobic methane-oxidizing bacteria were isolated from the surface of cold seep sediments in the Makran accretionary prism (off Pakistan). Among the derived isolates there are representatives of a, thus far, unknown genus belonging to the Methylococcaceae. Anoxic, hydrothermally-influenced sediments were investigated at the edge of a vulcanic structure in the Bransfield Strait (Antarctica). Groups of previously unknown archaea were detected in sediment horizons where anaerobic methane oxidation could be measured.