AEM Accepts, published online ahead of print on 23 October 2009

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Appl. Environ. Microbiol. doi:10.1128/AEM.01639-09
Copyright (c) 2009, American Society for Microbiology and/or the Listed Authors/Institutions. All Rights Reserved.

Non-Fermentative Thermoalkaliphilic Growth is Restricted to Alkaline Environments

Duncan G. G. McMillan, Stefanie Keis, Michael Berney, and Gregory M. Cook^*

Department of Microbiology and Immunology, Otago School of Medical Sciences, University of Otago, P.O. Box 56, Dunedin, New Zealand

^* To whom correspondence should be addressed. Email: gregory.cook{at}otago.ac.nz.

Caldalkalibacillus thermarum strain TA2.A1 grew in pH-controlled batch culture containing a fermentable growth substrate (i.e. sucrose) from pH 7.5 to 10.0 with no significant change in the specific growth rate, suggesting that this bacterium was a facultative alkaliphile. However, when strain TA2.A1 was grown on non-fermentable carbon sources like succinate or malate, no growth was observed until the external pH was > 9.0, suggesting that this bacterium was an obligate alkaliphile. Succinate and sucrose transport by strain TA2.A1 showed a similar pH profile to that of growth on these carbon sources and the molar growth yield on sucrose was higher at pH 9.5 compared to pH 7.5, despite the increased energy demands on the cell for intracellular pH regulation. Succinate transport, succinate-dependent oxygen consumption, succinate dehydrogenase and F₁F_o-ATPase specific activities were all significantly lower in cultures of strain TA2.A1 grown at pH 7.5 compared to those cultured at 9.5. No significant ATP synthesis via the F₁F_o-ATP synthase was detected until the external pH was > pH 8.5. On the basis of these results, we propose that non-fermentative thermoalkaliphilic growth is specialized to function at high pH values, but not at pH values near neutral.