Microbial communities near the oxic/anoxic interface in the hydrothermal system of Vulcano Island, Italy

Hydrothermal fluids and sediments from subaerial and shallow submarine sites at Vulcano Island, Italy were investigated for relations between the thermophilic microbial communities, as analysed by fluorescence in situ hybridization, and their geochemical environment, as assessed by photometry, chromatography, and in situ microsensor measurements. Mixing between hydrothermal fluids and seawater in the sediment pore space was reflected in the chemical composition of the emitted fluids, in depth profiles of pore water oxygen and sulfide concentrations, and in the structure of the benthic microbial community. Organic compounds did not accumulate in the vent fluids (< 10 μM fatty acids) or in the sediments (< 0.1% Corg), suggesting that efficient utilization supported microbial populations on the order of 104 cells per ml fluid and 108 cells per cm3 sediment. Groups of thermophiles that typically gain metabolic energy from the fermentation of organic matter (Thermococcales, Thermotoga/Thermosipho spp., and Bacillus sp.) were detected in significant abundances at all study sites. Also abundant were thermophiles capable of oxidizing organic acids with oxygen, nitrate, or sulfate. Aerobic thermophiles (Aquificales and Thermus sp.) were more abundant at oxic sites than at anoxic sites. Increasingly oxygenated habitats were associated with decreasing abundance of anaerobic (hyper)thermophiles belonging to the order Archaeoglobales.