| Summary: | Nitrogen is globally limiting primary production in the ocean, but some species of cyanobacteria can carry out nitrogen (N) fixation using specialized cells known as heterocysts. However, the effect of N sources and their availability on heterocyst development is not yet fully understood. This study aimed to evaluate the effect of various inorganic N sources on the heterocyst development and cellular growth in an N-fixing cyanobacterium, <i>Anabaena variabilis</i>. Growth rate, heterocyst development, and cellular N content of the cyanobacteria were examined under varying nitrate and ammonium concentrations. <i>A. variabilis</i> exhibited high growth rate both in the presence and absence of N sources regardless of their concentration. Ammonium was the primary source of N in <i>A. variabilis.</i> Even the highest concentrations of both nitrate (1.5 g L<sup>−1</sup> as NaNO<sub>3</sub>) and ammonium (0.006 g L<sup>−1</sup> as Fe-NH<sub>4</sub>-citrate) did not exhibit an inhibitory effect on heterocyst development. Heterocyst production positively correlated with the cell N quota and negatively correlated with vegetative cell growth, indicating that both of the processes were interdependent. Taken together, N deprivation triggers heterocyst production for N fixation. This study outlines the difference in heterocyst development and growth in <i>A. variabilis</i> under different N sources.
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