Regulatory Aspects of Nh4+ Utilization in the Acidophilic Thermophilic Unicellular Red Alga Cyanidium Caldarium

Summary Cyanidium caldarium is an acidophilic thermophilic red alga. At pH 1.9 with cells grown in batch culture with excess ammonium, removal of CO2 or darkness caused immediate inhibition of ammonium assimilation. By contrast, with cells grown in chemostat culture with nitrogen limitation, assimilation of ammonium was not inhibited by darkness, but it was inhibited by CO2 deprivation. In the case of these cells, inhibition was not immediate and behaved as a time‐dependent phenomenon. Ammonium assimilation was restored when CO2was supplied in either light or darkness. In contrast, at pH 7.4, ammonium assimilation decreased with time both in the presence and absence of CO2, and it ceased completely after about 90 min incubation at this pH. The inhibition was not due to the high pH per se. When resuspended at pH 2, these inhibited cells were unable to assimilate ammonium during a 25 min period in the presence of CO2. Thereafter, they become able to assimilate ammonium but at a rate which was only 30% of that of the control. If they were pre‐incubated at pH 2 in nitrogen‐free medium for 3 h, however, they then assimilated ammonium immediately after its addition. It is suggested that a reversible inactivation at pH 1.9 and a denaturation at pH 7.4 of the system for ammonium uptake occurs, and that this uptake system is stabilized by carbon compounds against inactivation at pH 1.9 but not against denaturation at pH 7.4.