Determining first principles behind metabolic rate/yield trade-offs in anaerobic hydrogenotrophs

Microbial energy and mass transfers have profound ecological and biogeochemical consequences, yet the controls directing microbial metabolic trade-offs remain poorly understood. The proposed project will study metabolic shifts in energy and mass transfers by microorganisms living through some of the earliest-evolving metabolisms known on Earth. By focusing efforts towards microbes driving primary production anaerobically with molecular hydrogen — through a metabolic process known as chemosynthesis — the project will test whether metabolic trade-offs can be predicted as a function of (i) growth temperature and/or (i) the amount of free energy associated with anaerobic respiration. State-of-the-art continuous cultivation techniques will be applied to compare changes in respiration rates, cell yields, and cell doubling times within and among chemosynthetic microbes under steady-state conditions. These experiments are key for integrating parameters describing microbial metabolic changes into mathematical and/or thermodynamic models of microbial growth and its biogeochemical consequence. Overall, this work will transform understanding of the underlying metabolic behavior of unicellular life at the base of the Universal Tree of Life and at the base of dark ecosystems. The objective of the proposed research is to determine whether microbial metabolic rate/yield trade-offs of hydrogenotrophic NO3--, Fe3+- and CO2-reducing microorganisms exhibit predictable adaptations to temperature and to the oxidation-reduction (redox) chemistry of catabolism during chemosynthesis. Specifically, we will test the hypotheses that (i) microbial metabolic rate-to-yield ratios will increase for all proposed microbial processes when at higher temperatures and that (ii) microbes relying on generally less exergonic catabolic redox reactions will systematically display higher anabolic efficiencies than those relying on more exergonic catabolic redox reactions. To test these hypotheses, the project will employ experimental microbiological and geochemical approaches to compare microbial respiration rates, cell (and total biomass) yields, and cell doubling times of different anaerobic chemosynthetic microorganisms during batch and continuous growth. The resulting metabolic and environmental data will demonstrate how microbial metabolic behaviors are shaped by their abiotic environments. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.