Hydrothermal vents

Deep-sea hydrothermal vents are unique, highly productive oases amidst the predominantly food-limited deep-sea. The extreme habitats at vents are highly dynamic and characterized by steep temperature and chemical gradients partly featuring impressive chimney structures emitting hot hydrothermal fluids into the surrounding cold oxygenated sea-water. Life at hydrothermal vents is based on the oxidation of reduced compounds such as hydrogen sulphide, hydrogen, and methane dissolved in the venting fluids. At the Molecular Ecology Department research on free-living microbial communities at deep-sea hydrothermal vents is conducted within the framework of the DFG-funded Excellence Cluster MARUM. The aim of our interdisciplinary studies is a better understanding of geosphere and biosphere interactions at hydrothermal vents. Recent studies target structure and function of chemoautotrophic primary producers as well as heterotrophic communities at hydrothermal vents.

Vent
A schematic overview of the geological settings at hydrothermal vent sites. Sea water is entrained into the oceanic crust and heats up gradually while approaching the hot mantel rock. Magnesium and calcium are precipitated. Sulfate precipitates with calcium or is reduced to hydrogen sulfide. Various reduced compounds such as iron, zinc, copper and sulfide are dissolved from igneous rock and gasses like helium, hydrogen, methane and carbon-dioxide are released from the magma. The hot hydrothermal fluid is emitted back into the cold oxygenated sea water. At focused venting sites, metal sulfides precipitate creating massive chimney structures. After the venting stops, chimneys remain on the sea floor as massive sulfide deposits. Diffuse venting occurs, when hydrothermal fluids mixes with oxygenated sea water below the ocean floor, before it is emitted back into the bottom sea water. © Dimitri Meier
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© MARUM – Center for Marine Environmental Sciences, University of Bremen

Free-living chemoautotrophic microorganisms at hydrothermal vents

Primary production  at deep sea hydrothermal vents, is carried out by chemolithoautotrophic microorganisms, with the oxidation of reduced sulfur compounds being a major driver for microbial carbon fixation.

In collaboration with Kai Uwe Hinrich’s group at MARUM we integrated the analyses of IPL diversity and δ13C values (δ13Clipid) with 16S rRNA gene-based phylogeny to examine microbial carbon flow on active and inactive chimney structures (Reeves et al. 2014). Surficial crusts of active structures, dominated by Epsilonproteobacteria, all yielded bacterial δ13Clipid values higher than biomass δ13C (total organic carbon), implicating CO2 fixation via the reverse tricarboxylic acid cycle. The data also suggested that δ13Clipid values vary on individual active structures without accompanying changes in microbial diversity, which may be explained with temperature and/or dissolved substrate effects. In contrast, data suggested a shift to a more diverse community and an alternate carbon assimilation pathway after venting ceases. In follow up projects, we currently investigate the metabolic potential of these microbial communites in collaboration with the group of Thomas Schweder (University of Greifswald) in more depth.

 

Heterotrophic Bacteria at deep-sea hydrothermal vents

Until recently studies of microbial vent communities have focused primarily on autotrophic organisms and largely neglecting the heterotrophic part of the microbiel microbial communites building on this dark carbon fixation. In the study by Meier et al. (2016) we targeted this heterotrophic microbial community by analysed mixing gradients of hydrothermal fluids including diffuse fluid discharge points, their immediate surroundings, and the buoyant parts of hydrothermal plumes. Close to diffuse venting orifices dominated by chemolithoautotrophic Epsilonproteobacteria, in areas where environmental conditions still supported chemolithoautotrophic processes, we detected microbial communities enriched for versatile heterotrophic Alpha- and Gammaproteobacteria. The potential for alkane degradation could be shown for several genera and yet uncultured clades. We propose that hotspots of chemolithoautotrophic life support a “belt” of heterotrophic bacteria significantly different from the dominating oligotrophic microbiota of the deep sea. These may rely on dark carbon fixation at the vents, but also on organic matter present in the hydrothermal fluids. In a second study, we combined microbial community analysis with short-term inclubations and NanoSIMS analysis to investigated the acetate-assimilating microbial activity in sulfidic, diffuse hydrothermal fluids (Winkel et al. 2014). A novel epsilonproteobacterial group accounted for nearly all acetate-assimilating cells in the incubations of Menez Gwen (Mid-Atlantic Ridge) hydrothermal fluids, representing the first aerobic acetate-consuming member of the Nautiliales. In contrast, Gammaproteobacteria dominated the 13C-acetate-assimilating community in incubations from the back-arc hydrothermal system in the Manus Basin off Papua New Guinea. Here, 16S rRNA gene sequences were mostly related to mesophilic Marinobacter, likely reflecting the high content of sea water in these fluids. The rapid growth of microorganisms upon acetate addition suggests that acetate-consumers in diffuse fluids are copiotrophic opportunists, which quickly exploit their food sources whenever available under the spatially and temporally highly fluctuating conditions at hydrothermal vents. Both studies together underline the importance of the as yet poorly studied heterotrophic microbial populations at hydrothermal vents and sheds light on an under-investigated part of microbial carbon cycling at hydrothermal vents

Heterotrophie
Suggested schematic distribution of microbial lifestyles at hydrothermal vent fields. In yellow: chemolithoautotrophic microorganisms found at open venting fluid orifices, in subsurface chambers filled with venting fluid, and as symbionts of vent fauna. In green: heterotrophic organisms feeding on organic matter derived from primary production, vent fauna, or contained in the fluids. In purple: oligotrophic organisms present in the hydrothermally unaffected deep water column and in the plumes. (Figure modified from Meier et al. 2016)

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