White Paper Title: 
Bioenergetics in deep-sea hydrothermal systems

The hydrothermal vents of the deep-sea are in some ways simpler than the shallow marine vents and terrestrial hot springs that are their shallower counterparts.  While shallow marine and terrestrial systems are infinitely easier to access, their ecology is complicated by the abundance of photosynthetic organisms and the significant fluxes from ecosystems outside the hydrothermal system itself.  Therefore, it might seem that the bioenergetics of chemosynthetic metabolisms would be more tractable in deep-sea environments.  However, our ability to calculate in situ bioenergetics is hampered by the difficulties in assessing fluid geochemistry at temperatures and in environments that are relevant to microbial metabolisms.  Recent studies of shallow marine hydrothermal vents and terrestrial hot springs shows that in situ measurements of redox sensitive species are essential to properly evaluate the energetics of metabolic reactions at in situ conditions (Shock et al. 2010, Rogers & Amend, 2006; Amend et al., 2003).  Much of the early work on deep-sea hydrothermal geochemistry focused on end-member fluids.  However, recent advances in electrochemical sensors and voltammetric methods have extended these measurements to include lower temperature and diffuse flow systems. Furthermore, in situ microbial incubators that are situated in chimney walls are helping to elucidate the geochemistry of mixed fluids where microbial communities are thriving.  Finally, coordinated geochemical and microbiological studies are beginning to illustrate the relationship between these two datasets.

Despite these advances, it is still not possible to develop a complete picture of in situ bioenergetics in deep-sea vent systems.  Geochemical models can be used to estimate fluid compositions for mixing environments (e.g. McCollom & Shock, 1997) and also to predict the potential abundance of otherwise unknown compounds (Schulte & Rogers, 2004; Schulte, 2010), which in turn can guide culturing efforts in hydrothermal systems.  Our efforts to link hydrothermal geochemistry and microbial diversity and function can also be aided by laboratory experiments that quantify energy consumption by thermophiles under geochemically distinct and relevant conditions.  The geochemical and microbiological datasets acquired through decades of RIDGE research can be combined and hopefully extended to develop a more complete picture or the bioenergetics of microbial metabolisms in deep-sea hydrothermal systems.   Furthermore, methods can be designed to use current datasets to guide microbiological experiments in order to further our understanding of bioenergetic requirements of thermophiles at in situ conditions.

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