Nitrate uptake in the halotolerant cyanobacterium Aphanothece halophytica is energy-dependent driven by ΔpH

Abstract

The energetics of nitrate uptake by intact cells of the halotolerant cyanobacterium Aphanothece halophytica were investigated. Nitrate uptake was inhibited by various protonophores suggesting the coupling of nitrate uptake to the proton motive force. An artificially-generated pH gradient across the membrane (ΔpH) caused an increase of nitrate uptake. In contrast, the suppression of ΔpH resulted in a decrease of nitrate uptake. The increase of external pH also resulted in an enhancement of nitrate uptake. The generation of the electrical potential across the membrane (Δψ) resulted in no elevation of the rate of nitrate uptake. On the other hand, the valinomycin-mediated dissipation of Δψ caused no depression of the rate of nitrate uptake. Thus, it is unlikely that Δψ participated in the energization of the uptake of nitrate. However, Na+-gradient across the membrane was suggested to play a role in nitrate uptake since monensin which collapses Na+-gradient strongly inhibited nitrate uptake. Exogenously added glucose and lactate stimulated nitrate uptake in the starved cells. N, N′-dicyclohexylcarbodiimide, an inhibitor of ATPase, could also inhibit nitrate uptake suggesting that ATP hydrolysis was required for nitrate uptake. All these results indicate that nitrate uptake in A. halophytica is ATP-dependent, driven by ΔpH and Na+-gradient.