Na⁺-coupled Energetics of a Psychrotrophic Bacterium Isolated from Lake Vanda, Antarctica

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Respiration-dependent H⁺ and Na⁺ pumps and the driving force for ATP synthesis of a psychrotrophic bacterium, strain 13A, isolated from the bottom layer of Lake Vanda, Antarctica, were examined with intact cells. This bacterium was able to grow well over a pH range of 5 to 10 at 25°C in a synthetic medium containing sodium succinate as a carbon source. The growth at alkaline pH was resistant to a protonophore, carbonylcyanide m-chlorophenylhydrazone (CCCP). Endogenous respiration induced by the O₂-pulsing of an anaerobic cell suspension resulted in transient acidification and CCCP-dependent alkalization of the suspension at both pH 6.5 and 8.5. Strong inhibition of the acidification by CCCP indicated that the acidification is attributable to a respiration-dependent primary H⁺ pump. The inhibition by 2-n-heptyl-4-hydroxyquinoline N-oxide (HQNO) and Na⁺-dependence of the alkalization suggested that a respiration-dependent primary Na⁺ pump is responsible for the CCCP-dependent alkalization. In vivo, the synthesis of ATP through endogenous respiration at pH 6.5 and 8.5 was strongly inhibited by CCCP and HQNO, respectively. Therefore, it is suggested that ATP synthesis in this bacterium at pH 6.5 is driven by an electrochemical potential of H⁺ across membranes (Δμ_H⁺), and at pH 8.5 by Δμ_Na⁺ but not Δμ_H⁺. Even at a Na⁺ concentration as low as 2.1mM, this bacterium grew well and exhibited CCCP-resistant growth at alkaline pH. These results strongly suggest that Na⁺ is indispensable as a coupling ion of energy metabolism to sustain the growth of this bacterium not only in Na⁺-ubiquitous environments but also under alkaline conditions.