Alkaliphilic species that grow at pH 10.5 must cope with a low protonmotive force (- 50 mV) due to a reversed transmembrane pH gradient at least 2 pH units more acid inside. Here we demonstrate that strictly alkaliphilic RAB and two strains of (ATCC 27467 and DSM 485) grow exponentially in batch cultures with a doubling time of less than 1 h in 100 mm buffered medium, while the actual medium pH remains above 10.2. The ATCC strain continued to grow rapidly for at least 7 h, but the growth rate of the DSM strain declined dramatically after 3 h. However, both the strains, RAB and facultatively alkaliphilic OF4 were readily maintained for at least 24 h between pH 10.4 and 10.6 in a chemostat where nutrients were constantly replenished. A critical nutrient may be limiting in batch cultures of the DSM strain of . The facultative alkaliphile grew equally well in batch cultures at an initial pH of 7.5 or 10.5. Its molar growth yield (23 mg dry wt mmol) on malate ( ) was the same at the two pH values and was comparable to for grown at neutral pH. RAB and ATCC 27467 grown at pH 10.5 also showed values at least as high as the neutralphile, indicating efficient use of the energy source even at low protonmotive force. Moreover, the phosphorylation potential of OF4 grown at pH 7.5 (45.2 kJ mol)or pH 10.5 (46 kJ mol) was in a conventional range for bacteria.


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