Heterotrophic growth on formate is dependent on the maintenance of intracellular pH for the thermoacidophilic methanotroph Methylacidiphilum sp RTK17.1.

In this work, we investigated intracellular pH homeostasis within the thermoacidophilc methanotroph Methylacidiphilum sp. RTK17.1. Our findings show the proton motive force for this species is primarily generated by a pH gradient across the cellular membrane. In batch experiments, the addition of formate resulted in no observable cell growth and, correspondingly, acidification of the cytosol, decreased formate dehydrogenase activity and (presumably) cell-death. Nevertheless, we were able to demonstrable growth on formate as the sole source of metabolizable energy was possible in steady-state (continuous) cultures following the transition from methanol to formate. Under these conditions, biomass productivity yields on formate were 63% less than for growth on methanol. Transcriptome analysis revealed key genes associated with pH homeostasis, methane, methanol and formate metabolism were significantly regulated in response to growth on formate. Collectively, these results suggest environmental formate represents a utilisable source of energy/carbon to the acidophilic methanotrophs during periods of methane starvations and highlights potential short-comings of traditional batch-culture physiological characterisation studies in acidophilic species. Chemostat cell samples (3 total) of Methylacidiphilum sp. RTK17,1 were grown methylotrophically (pH 2.5, 45C) under oxygen limiting conditions in the presence of NH4+. Following the establishment of a steady state condition, methanol was replaced with an equimolar concentration of formate