Studies in vivo on the control by ammonia of nitrate reduction to nitrite in the unicellular alga Cyanidium caldarium

Abstract

When intact cells of the acidophilic red alga Cyanidium caldarium, grown at pH 1.9 with nitrate as sole nitrogen source, are transferred to pH 7.5, they actively reduce nitrate to nitrite which accumulates in the external medium. At pH 7.5 nitrite is not further reduced to ammonia. Formation of nitrite from nitrate occurs in the light or in the dark, either in the presence or in the absence of oxygen. In cell suspensions supplied with ammonia in addition to nitrate, the formation of nitrite at pH 7.5 is strongly prevented. However ammonia is not able to prevent nitrite formation at pH 9.4. Nitrate reductase is 40% latent in cells resuspended at pH 7.5 in the presence of nitrate and up to 90% latent in cells resuspended in the presence of both nitrate and ammonia. It is proposed that the inhibition by ammonia of nitrate reduction to nitrite is based on the latent state that nitrate reductase exhibits in the presence of ammonia. In fact, at pH 9.4, when the production of nitrite is not inhibited by ammonia, nitrate reductase is only 14–25% latent even in cells resuspended in the presence of nitrate and ammonia.