The Na+ cycle of extreme alkalophiles: A secondary Na+/H+ antiporter and Na+/solute symporters

Extremely alkalophilic bacteria that grow optimally at pH 10.5 and above are generally aerobic bacilli that grow at mesophilic temperatures and moderate salt levels. The adaptations to alkalophily in these organisms may be distinguished from responses to combined challenges of high pH together with other stresses such as salinity or anaerobiosis. These alkalophiles all possess a simple and physiologically crucial Na⁺ cycle that accomplishes the key task of pH homeostasis. An electrogenic, secondary Na⁺/H⁺ antiporter is energized by the electrochemical proton gradient formed by the proton-pumping respiratory chain. The antiporter facilitates maintenance of a pH_in that is two or more pH units lower than pH_out at optimal pH values for growth. It also largely converts the initial electrochemical proton gradient formed by respiration into an electrochemical sodium gradient that energizes motility as well as a plethora of Na⁺/solute symporters. These symporters catalyze solute accumulation and, importantly, reentry of Na⁺. The extreme nonmarine alkalophiles exhibit no primary sodium pumping dependent upon either respiration or ATP. ATP synthesis is not part of their Na⁺ cycle. Rather, the specific details of oxidative phosphorylation in these organisms are an interesting analogue of the same process in mitochondria, and may utilize some common features to optimize energy transduction.