Energetics of Acidianus ambivalens growth in response to oxygen availability.

Abstract

All life requires energy to drive metabolic reactions such as growth and cell maintenance; therefore, fluctuations in energy availability can alter microbial activity. There is a gap in our knowledge concerning how energy availability affects the growth of extreme chemolithoautotrophs. Toward this end, we investigated the growth of thermoacidophile Acidianus ambivalens during sulfur oxidation under aerobic to microaerophilic conditions. Calorimetry was used to measure enthalpy (ΔHinc ) of microbial activity, and chemical changes in growth media were measured to calculate Gibbs energy change (ΔGinc ) during incubation. In all experiments, Gibbs energy was primarily dissipated through the release of heat, which suggests enthalpy-driven growth. In microaerophilic conditions, growth was significantly more efficient in terms of biomass yield (defined as C-mol biomass per mole sulfur consumed) and resulted in lower ΔGinc and ΔHinc . ΔGinc in oxygen-limited (OL) and oxygen- and CO2 -limited (OCL) microaerophilic growth conditions resulted in averages of -1.44×103 kJ/C-mol and -7.56×102 kJ/C-mol, respectively, and average ΔHinc values of -1.11×105 kJ/C-mol and -4.43×104 kJ/C-mol, respectively. High-oxygen experiments resulted in lower biomass yield values, an increase in ΔGinc to -1.71×104 kJ/C-mol, and more exothermic ΔHinc values of -4.71×105 kJ/C-mol. The observed inefficiency in high-oxygen conditions may suggest larger maintenance energy demands due to oxidative stresses and a preference for growth in microaerophilic environments.