Comparison of Microbial Communities Inhabiting Ultramafic- and Basalt-Hosted Hydrothermal Systems: Genetic Characterization of 16S rRNA Genes, Hydrogen Oxidation and CO2 Fixation Pathways
The host rock of a hydrothermal system defines that systems specific fluid chemical composition. Therefore the type of host rock (ultramafic rocks versus basalts) plays a major role in determining the most abundant energy sources which are available for microorganisms. This study was conducted to investigate the influence the host rock and resulting fluid chemistry have on the distribution of the indigenous microorganisms. For this purpose hydrothermal fluids were collected from the ultramafic-hosted Logatchev hydrothermal field (LHF) and the basalt-hosted Lilliput hydrothermal field. The microbial community analysis was based on 16S rRNA genes and fluorescence in situ hybridization. The functionality of chemoautotrophic communities was assessed in terms of putative CO2 fixation pathways and energy sources used. Ultramafic host rocks at the LHF result in fluids with extremely high hydrogen contents. Possibly as a consequence, phylogenetically diverse hydrogen-oxidizing chemolithotrophs were detected. High abundance in hydrogen concentrations seem to coincide with the diversification of putative hydrogen-oxidizing prokaryotes at the three sites investigated. By contrast, the fluids from the Lilliput field are very low in hydrogen but high in sulfide. Significantly less diverse hydrogen-oxidizing microorganisms were detected at the basalt-hosted Lilliput hydrothermal field. Nevertheless, diverse bacteria were related to organisms involved in sulfur cycling. This suggests that the host rock indirectly determines the presence of microorganisms with specific physiologies. However, in addition to the importance of the host rock it can be shown that mixing processes of hydrothermal fluids with ambient seawater have a significant impact on the distribution of microorganisms. Contrary, for CO2 fixation pathways the type of host rock is not relevant. Key enzymes of the two CO2 fixation pathways investigated tolerate different levels of oxygen. The presence of multiple environmental niches with different oxygen contents is reflected in the phylogenetic diversity of Calvin Benson-Bassham and reverse tricarboxylic acid cycle genes detected at the LHF. At the Lilliput hydrothermal field the diversity of these different genes suggests that the chemoautotrophic community is better adapted to low oxygen contents. The generally high diversity of CO2 fixation pathway genes demonstrates the ability of metabolically versatile prokaryotes to exhaust the potential of every available habitat.
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