Bioenergetic Properties of the Thermoalkaliphilic Bacillus sp. Strain TA2.A1

ABSTRACT The thermoalkaliphilic Bacillus sp. strain TA2.A1 was able to grow in pH-controlled batch culture containing a nonfermentable growth substrate from pH 7.5 to 10.0 with no significant change in its specific growth rate, demonstrating that this bacterium is a facultative alkaliphile. Growth at pH 10.0 was sensitive to the protonophore carbonyl cyanide m-chlorophenylhydrazone, suggesting that a proton motive force (Δp) generated via aerobic respiration was an obligate requirement for growth of strain TA2.A1. Strain TA2.A1 exhibited intracellular pH homeostasis as the external pH increased from 7.5 to 10.0; however, the maximum ΔpH generated over this pH range was only 1.1 units at an external pH of 9.5. The membrane potential (Δψ) was maintained between −114 mV and −150 mV, and little significant change was observed over the pH range for growth. In contrast, the Δp declined from −164 mV at pH 7.5 to approximately −78 mV at pH 10.0. An inwardly directed sodium motive force (ΔpNa+) of −100 mV at pH 10.0 indicated that cellular processes (i.e., solute transport) dependent on a sodium gradient would not be affected by the adverse Δp. The phosphorylation potential of strain TA2.A1 was maintained between −300 mV and −418 mV, and the calculated H+/ATP stoichiometry of the ATP synthase increased from 2.0 at pH 7.5 to 5.7 at pH 10.0. Based on these data, vigorous growth of strain TA2.A1 correlated well with the ΔpNa+, phosphorylation potential, and the ATP/ADP ratio, but not with Δp. This communication represents the first report on the bioenergetics of an extremely thermoalkaliphilic aerobic bacterium.