White Paper Title: 
Experimental Constraints on the Metabolic Rates and Growth Efficiency of Seafloor Microbial Communities across Redox and Thermal Gradients

Dionysis Foustoukos1 and Stefan Sievert2

1Geophysical Lab, Carnegie Institution of Washington, Washington DC 20015
2Department of Biology, Woods Hole Oceanographic Institution, Woods Hole, MA 02543

Understanding the basic microbiology and biogeochemistry of deep-sea hydrothermal ecosystems is essential to constrain the extent of deep-sea and subsurface biosphere and to evaluate the role of microbial metabolism on the fluxes of inorganic/organic nutrients and the bio-signatures imprinted in the chemical composition of low-temperature hydrothermal emissions. A number of field and laboratory culturing studies have been conducted to better describe these biogeochemical processes; however, there is presently a complete lack of experimental studies to evaluate the effect of redox gradients on the metabolic rates and growth efficiencies of anaerobic and microaerobic organisms at seafloor pressure conditions.

While thermodynamic models have emerged as very important tools to inform about potentially important biogeochemical processes occurring at vents, and to make estimates on overall biomass production, a number of assumptions go into these models, and there is a strong need for actual data derived from cultivated microbes. For example, existing thermodynamic models predict an abrupt transition between oxic (oxidation) and anoxic (reduction) conditions that reflects the relative distribution of redox species in the hydrothermal fluid/seawater mixture and the ΔGr energy for each individual redox reaction. Experimental data demonstrating inhibition of the Knallgas reaction at low temperatures (<100oC), however, suggest that active bacterial population in near-seafloor habitats could potentially utilize both H2(aq) and O2(aq), while anaerobic chemolithoautotrophic metabolism can be feasible at temperatures lower than 40oC due to H2(aq) persistence in the seawater/vent fluid mixtures. In effect, under H2-O2 disequilibria conditions the anoxic/oxic boundaries along mixing interface may not be as sharp and microbial-mediated H2(aq) oxidation could provide one of the largest energy sources available at the low-T diffuse flow vent sites.

Thus, novel experimental approaches need to be developed to facilitate controlled laboratory studies on microbial metabolism and biogeochemical processes under extreme conditions. For example, experimental designs can include continuous culture flow-through approaches that would allow for the assessment of specific relations between different metabolic pathways and microbial adaptability across a spectrum of anaerobic/aerobic conditions and by utilizing a range of substrate compositions (e.g. H2, CO2, SO4--,NO3-) while exposing microbial communities to physical conditions that resemble deep-sea environments. Such experimental studies are essential for defining and for thoroughly describing the mass transport and mechanisms of biomass production associated with microbial metabolism at sites of deep-sea hydrothermal activity, which is fundamental for:

- Identifying the effect of redox gradients (relative availability of e--donors/acceptors) present in seawater/hydrothermal fluid mixing zones on microbial growth and the metabolic pathways of (anaerobic) chemolithotrophs,

- Estimating bioavailable energy along anoxic/oxic boundaries in seafloor mixing zones, and identifying major metabolic pathways at the extreme conditions of active submarine hydrothermal systems,

- Constraining the extent and spatial variability of subsurface biosphere to define the C, N, S, and Fe biochemical cycling in mid-ocean ridges, based on experimental data on the rate of microbial metabolism and the flux of metabolic products in near-seafloor hydrothermal environments.