Biochemical characterization of vent fluids and the microbial community

In this study, microbial communities in hydrothermal fluids from four different venting areas (White Flames, Cage Site, Woody and Babylon) within the Menez Gwen hydrothermal field (Mid-Atlantic Ridge) were analyzed. Samples were taken along mixing gradients, including diffuse fluid discharge points, their immediate surroundings, and the buoyant parts of hydrothermal plumes, selected based on visible venting, temperature readings and gas concentrations as indicated by in situ mass spectrometry (ISMS) spectra. High throughput 16S rRNA gene amplicon sequences obtained by Illumina paired-end sequencing using the primer combination Bakt_341F and Bakt_805R were analyzed. Total cell counts were determined and fluorescence in situ hybridization was conducted using probes specific for Archaea and Bacteria as well as for different bacterial subgroups. Additionally, three metagenomes were sequenced (Illumina MiSeq paired-end shotgun), assembled, binned, compared and analyzed for key metabolic pathways. Molecular analyses were combined with geochemical analyses and thermodynamic calculations. The study revealed that close to diffuse venting orifices dominated by chemolithoautotrophic Epsilonproteobacteria, in areas where environmental conditions still supported chemolithoautotrophic processes, microbial communities enriched for versatile heterotrophic Alpha- and Gammaproteobacteria were present. The potential for alkane degradation could be shown for several genera and yet uncultured clades. It was proposed that hotspots of chemolithoautotrophic life support a "belt" of heterotrophic bacteria significantly different from the dominating oligotrophic microbiota of the deep sea.