Evidence for two transport systems for nitrate in the acidophilic thermophilic alga Cyanidium caldarium

Abstract

In the unicellular non-vacuolate red alga Cyanidium caldarium nitrate uptake occurs through two specific permease systems which, on the basis of kinetic constants can be defined as low affinity system and high affinity system. The high affinity system is saturated at very low nitrate concentrations (<1 μM), whereas the low affinity system is saturated only at high nitrate concentrations (Km=0.45±0.10 mM). The low affinity system is present in cells growing under conditions of nitrogen limitation as well as in cells growing in excess nitrate. In contrast, the high affinity system is present only in cells growing under conditions of nitrogen limitation. The high affinity system works only at acid pH and is inactive at neutral pH. The low affinity system is active both at acid and at neutral pH.