Effect of darkness and CO2starvation on NH4+and NO3−assimilation in the unicellular algaCyanidium caldarium

Abstract

Cyanidium caldarium(Tilden) Geitler, a non‐vacuolate unicellular alga, resuspended in medium flushed with air enriched with 5% CO2, assimilated NH4+at high rates both in the light and in the dark. The assimilation of NO3−, by contrast, was inhibited by 63% in the dark. In cell suspensions flushed with CO2‐free air, NH4+assimilation decreased with time both in the light and in the dark and ceased almost completely after 90 min. The addition of CO2completely restored the capacity of the alga to assimilate NH4+. NO3−assimilation, by contrast, was 33% higher in the absence of CO2and was linear with time. It is suggested that NO3−and NH4+metabolism inC. caldariumare differently controlled in response to the light and carbon conditions of the cell.