The optimization of microbial metabolism has been described as the result of the balance between efficiency, or biomass yields, during anabolism and the kinetics of energy yielding reactions, or respiration rates, during catabolism. Through my C-DEBI postdoctoral research I have obtained experimental data that will help us understand how environmental factors relevant to deep-sea hydrothermal systems affect the trade-offs between efficiency and kinetics in deep-sea vent metabolisms. Particularly, I have been studying the metabolic trade-offs during chemosynthetic NO3- reduction, as well as its associated stable N isotope signatures, in endemic vent ε-Proteobacteria and Aquificae under different temperature regimes. We found, through culture-based approaches, that these bioenergetic traits were generally conserved between these two groups along with their physiologies, with no substantive overall differences in their N isotope effects. Further laboratory culturing of representative members of each group (Caminibacter mediatlanticus and Thermovibrio ammonificans) under seafloor pressures (20MPa) showed that N isotope effects were independent of pressure effects, while growth rates and NO3- reduction kinetic rate constants were significantly decreased under increased pressures. Currently, we are complementing our pure culture research with new studies using natural vent microbial communities to further constrain the role of NO3- reduction at in-situ seafloor temperatures and pressures. Additionally, I worked towards the isolation and characterization of a novel Fe(III) reducing microorganism, Deferrisoma paleochorii, in order to start research activities in the bioenergetics of Fe-based metabolisms. The goal is to increase our understanding of the relationship between bioenergetic landscapes, biogeography and biogeochemistry in these seafloor regions.