Short-term regulation of nitrate uptake by a ‘pump and leak’ mechanism in the acidophilic nonvacuolated alga, Cyanidium caldarium

Abstract

In the nonvacuolated acidophilic thermophilic red alga, Cyanidium caldarium , nitrate uptake and reduction can be separated by measuring disappearance of nitrate from the suspension medium using, in in vivo experiments, cyanate, a competitive inhibitor of algal nitrate reductase. Cyanate selectively inhibited nitrate reduction at concentrations that did not significantly affect nitrate uptake, photosynthesis or respiration. Its use proved that short-term control of intracellular nitrate through the increase of a carrier-mediated nitrate efflux took place when nitrate reduction was inhibited. The occurrence of the high- and low-affinity nitrate uptake systems in cells grown in nitrogen-limited conditions, as previously reported, suggests a ‘pump and leak’ mechanism operating at the plasmalemma level to regulate nitrate uptake and intracellular nitrate: the high-affinity nitrate transport system mediated by proton cotransport (irreversible) operates the influx, while the low-affinity transport (reversible) operates influx or efflux according to cell requirements. Kinetic analysis of cyanate inhibition in cells taken from low-nitrate medium supports this hypothesis and reveals that, in Cyanidium , intracellular nitrate is probably compartmented in the cytosol.