The Effect of Available Nitrate and Ammonium-Nitrogen on the Growth of Two Nitrogen-Fixing Blue-Green Algae

The effect of combined nitrogen supplied as nitrate or ammonium-nitrogen on the growth of two nitrogen-fixing blue-green algae, Nostoc entophytum and Calothrix scopulorum, has been studied. These species have been isolated from marine environments. Both algae grew as vigorously on elemental nitrogen as in the presence of combined nitrogen. Growth was equal at all levels of nitrate-nitrogen employed but high levels of ammonium-nitrogen proved inhibitory or even toxic to the algae. Nostoc was slightly more susceptible to high ammonium-nitrogen levels than was Calothrix. Increase in pH of the medium from 7.2 to 8.4 increased the toxic effect of ammonium-nitrogen although relative growth at the various levels of nitrate-nitrogen was not affected. The results suggest that the different effects of ammonium-nitrogen on the growth of freshwater and marine blue-green algae may be due in part at least to the different pH levels of freshwater and marine environments in which the algae grow, rather than to any inherent difference between the two groups.