Metabolic Rates and Growth Efficiency across Redox and Thermal Gradients: An Experimental Study to Constrain Biomass Production at Vents

Collaborators:
Dionysis Foustoukos (PI, Geophysical Lab, Carnegie Inst., Washington DC)
George Cody (Co-PI, Geophysical Lab, Carnegie Inst., Washington DC)
Larry Nittler (Co-PI, DTM, Carnegie Inst., Washington DC)

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 kinds of microorganisms mediating these reactions in different geothermal systems, the metabolic pathways employed by the microbes, the rates of the catalyzed reactions, the amount of carbon being produced, and the larger role of these ecosystems in global biogeochemical cycles. 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, e.g. how efficiently organisms utilize their energy sources, to be able to improve and better constrain these models. Existing thermodynamic models create a sharp boundary 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 (<100^oC), however, suggest that active bacterial population in near-seafloor habitats could potentially utilize both H_2(aq) and O_2(aq), while anaerobic chemolithoautotrophic metabolism can be feasible at temperatures lower than 40^oC due to H_2(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 H_2(aq) oxidation could provide one of the largest energy sources available at the low-T diffuse flow vent sites. Here, we propose a 3-year interdisciplinary research program to obtain critical data on growth efficiencies, metabolic rates, and isotopic composition of N-bearing respiratory products of a number of anaerobic nitrate reducing and H₂-oxidizing Epsilonproteobacteria and Aquificales that are representative of groups that are of considerable ecological relevance at deep-sea vents. The proposed research is based on a series of controlled laboratory experiments at in-situ seafloor pressures using pure cultures or defined co-cultures combining geochemical, stable isotope, NanoSIMS, NMR and microbiological approaches.

The key objectives of this proposed research are:

• Investigate the metabolic rates and growth efficiency of mesophilic and thermophilic anaerobic Epsilonproteobacteria and Aquificales at a range of H_2(aq)/CO_2(aq) molar ratios and by utilizing NO₃^- as electron acceptor, while imposing pressure effects to simulate deep sea hydrothermal conditions.

• Determine the ¹⁵N/¹⁴N isotopic fractionation between seawater NO₃^- and the aqueous species (e.g. N_2(aq),NH_3(aq)) produced during respiratory denitrification and nitrate ammonification processes. Evaluate and constrain d¹⁵N kinetic isotope effects based on metabolism and phylogeny as well as the imposed thermal, redox, and fluid pH conditions.

• The role of redox gradients on the NO₃^- reduction through denitrification or ammonification. What is the effect of microaerobic conditions on the δ¹⁵N isotope fractionation between NO₃^- and N_2(aq)? Constrain the O_2(aq)-tolerance of microaerobic bacteria and the effects on anaerobic H_2(aq) oxidation, and finally, its impact on biomass formation.

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This project is funded by the NSF Ridge 2000 program.