Growth and bioenergetics of alkaliphilic Bacillus firmus OF4 in continuous culture at high pH

Abstract

The effect of external pH on growth of alkaliphilic Bacillus firmus OF4 was studied in steady-state, pH-controlled cultures at various pH values. Generation times of 54 and 38 min were observed at external pH values of 7.5 and 10.6, respectively. At more alkaline pH values, generation times increased, reaching 690 min at pH 11.4; this was approximately the upper limit of pH for growth with doubling times below 12 h. Decreasing growth rates above pH 11 correlated with an apparent decrease in the ability to tightly regulate cytoplasmic pH and with the appearance of chains of cells. Whereas the cytoplasmic pH was maintained at pH 8.3 or below up to external pH values of 10.8, there was an increase up to pH 8.9 and 9.6 as the growth pH was increased to 11.2 and 11.4, respectively. Both the transmembrane electrical potential and the phosphorylation potential (delta Gp) generally increased over the total pH range, except for a modest fall-off in the delta Gp at pH 11.4. The capacity for pH homeostasis rather than that for oxidative phosphorylation first appeared to become limiting for growth at the high edge of the pH range. No cytoplasmic or membrane-associated organelles were observed at any growth pH, confirming earlier conclusions that structural sequestration of oxidative phosphorylation was not used to resolve the discordance between the total electrochemical proton gradient (delta p) and the delta Gp as the external pH is raised. Were a strictly bulk chemiosmotic coupling mechanism to account for oxidative phosphorylation over the entire range, the deltaGp/deltap ration (which would equal the H+/ATP ratio) would rise from about 3 at pH 7.5 to 13 at pH 11.2, dropping to 7 at pH 11.4 only because of the rise in cytoplasmic pH relative to other parameters. Moreover, the molar growth yields on malate were higher at pH 10.5 than at pH 7.5, indicating greater rather than lesser efficiency in the use of substrate at the more alkaline pH.