Stefan Sievert, Jeff Seewald, Craig Taylor (Woods Hole Oceanographic Institution), Costantino Vetriani (Rutgers University), Dionysis Foustoukos (Geophysical Laboratory, Carnegie Institution of Washington), Ramunas Stepanauskas (Bigelow Laboratory)

Chemolithoautotrophic microorganisms are at the nexus of hydrothermal systems by effectively transferring the energy from the geothermal source to the higher trophic levels. While the validity of this conceptual framework is well established at this point, there are still significant gaps in our understanding of the microbiology and biogeochemistry of deep-sea hydrothermal systems. This includes information on the diversity of chemolithoautotrophic microorganisms mediating critical reactions in different geothermal systems, the metabolic pathways used by the microbes, the rates of the catalyzed reactions, and the amount of organic carbon being produced. In particular, there is currently a notable lack of process-oriented studies that would allow us to assess the larger role of these ecosystems in global biogeochemical cycles. Important questions in this regard are:

A) How much and at what rates is carbon being produced? B) What are the dominant autotrophs? C) Which metabolic pathways are they using to conserve energy and to fix carbon? D) How efficiently is the energy being utilized and transformed into biomass? E) How do microbes attach to surfaces? F) How does community wide gene expression in fluid and biofilm communities compare? What is the role of biofilms for the functioning of the ecosystem?

While we can start integrating existing data to fill some of these gaps, ultimately a focused, interdisciplinary research program will be required to characterize the complexity of microbially-catalyzed processes at deep-sea vents at a qualitatively new level. Here we advocate to pursue an integrated approach that couples an assessment of taxonomic diversity using cultivation-dependent and -independent approaches with methodologies addressing genetic diversity, including metagenomics (genetic potential and diversity of community), single cell genomics (genetic potential and diversity of uncultured single cells), metatranscriptomics (identification and function of active community members), and metaproteomics (realized potential of community). To assess the functional component, these approaches would be combined with 1) measurement of in situ rates of chemoautotrophic production, 2) geochemical characterization of microbial habitats, and 3) shipboard incubations at in situ conditions (hypothesis testing under controlled conditions). This would provide unique insights into the functioning of deep-sea vent microbial communities and the constraints regulating the interactions between the microbes and their abiotic and biotic environment, ultimately enabling us to put these systems in a quantitative framework and thus a larger global context.