Energetic investigations of Acidianus ambivalens metabolism during anaerobic sulfur reduction and comparisons to aerobic sulfur oxidation.

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Acidianus ambivalens is a metabolically flexible facultative anaerobe that can oxidize and reduce elemental sulfur with O2 and H2, respectively. In this work, the growth energetics of Acidianus ambivalens were determined under anaerobic conditions at 76°C with H2 oxidation by elemental sulfur serving as the energy-yielding catabolic reaction. The biomass yields (C-mol of biomass per mol of H2 consumed) ranged from approximately 0.004 to 0.01. Growth rates ranged from 0.003 to 0.012h-1. Gibbs energies of incubation based on macrochemical equations of cell growth ranged from - 881 to - 3349kJ/C-mol. Enthalpies of incubation determined from calorimetric measurements ranged from - 610 to - 4090kJ/C-mol. The Gibbs energy consumed by anaerobic cultures was compared to sulfur-oxidizing cultures under aerobic and microaerobic conditions to determine the effects of environmental and substrate redox state on energetics. This comparison revealed that aerobic cultures were inefficient relative to microaerobic and anaerobic conditions. These results suggest that aerobic conditions induce a measurable oxidative stress on cultures. The similarities in growth efficiency and energy budgets under microaerobic and anaerobic conditions may allow Acidianus ambivalens to be competitive in natural environments either by oxidizing or reducing elemental sulfur.