GROWTH RESPONSE OF A NITROGEN FIXER (ANABAENA FLOS‐AQUAE, CYANOPHYCEAE) TO LOW NITRATE1

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Abstract
Anabaena flos‐aquae (Lyngb.) Bréb. was grown in varying concentrations of nitrate. Specific growth rates, as estimated in batch culture, were constant and approached the maximum rate at all concentrations of NO3−‐N tested bewteen 0 and 400 μ/L. Steady‐state biomass, as determined in semicontinuous culture, did not vary with NO3− at slower dilution rates. However at a faster dilution rate, significantly less biomass occurred in intermediate concentrations of NO3− than in either higher or lower concentrations. The results indicate that both growth rate and standing crop are maximized by either N2 fixation or NO3− assimilation, but extracellular NO3− reduces the rate of N2 fixation. Consequently, at very low NO3− concentrations, growth is virtually maximized by N2 fixation alone, and at high concentrations of NO3−, N2 fixation is inhibited but growth is maximized by assimilation of NO3−. At intermediate concentrations of NO3−, growth becomes a function of NO3− assimilation augmented by N2 fixation. In this case, full growth potential is realized only if hydraulic residence time is sufficiently long to compensate for the reduced rate of N2 fixation. Growth rate and standing crop are not diminished in response to the large amount of energy allocated to N2 fixation. Instead, other cellular processes are probably affected negatively during N2 fixation.