Measurements of NH4+ uptake by Cyanidium caldarium (Tilden) Geitler, an acidophilic thermophilic non-vacuolate unicellular red alga, were made with cells grown either in batch culture with excess ammonium or in continuous culture with nitrogen limitation.
Batch-grown cells absorbed NH4+ at a lower rate than chemostat-grown cells, and uptake was greatly inhibited in darkness (83 %) and by CO2 deprivation (74%). In chemostat-grown cells, in contrast, darkness inhibited uptake by only 20 to 30%. In chemostat-grown cells subjected, after the addition of ammonium, to alternate light/dark cycles over 7 h of incubation, the rate of NH4+ uptake in the successive dark periods progressively decreased and fell to only 15% of the initial rate after 5 h incubation. In the successive light periods, in contrast, there was a 15% decrease in rate from one light period to the next, and after 7 h the rate of uptake was still 50% of the initial value. At this time, transfer of the cells from light to darkness resulted in 85% inhibition of uptake. It is suggested that in C. caldarium the overall process of ammonium assimilation is controlled, at the level of ammonium entry into the cell, either through the involvement of two independently controlled uptake systems or through multiple interconvertible forms of a single system, sharing different regulatory properties.