The initial average heterocyst density in all nitrate treatments was 8.3 ± 1.97 × 103 cells mL−1 andit increased by 1.42–6.83 × 104 cells mL−1 after 2 d of incubation, following which heterocyst accountedfor 3–16 % of the total cells counted (Figure 3a,c).During the 14-d incubation, N-starved A. variabilisproduced heterocysts in all nitrate treatments, and the highest heterocyst density was observedin 1.5 × 10−5 g L−1 nitrate treatment, followed by N-free treatment (Figure 4).Heterocyst numberrapidly increased within 2 d after incubation both in 1.5 × 10−5 g L−1 nitrate treatment and N-freetreatment.The heterocyst densities of these treatments were significantly higher than those of othernitrate treatments (F(3,8) = 7.645, p = 0.010).On day 4, they peaked at the highest densities of1.54 ± 0.52 × 105 cells mL−1 and 1.29 ± 0.34 × 105 cells mL−1, respectively, before a steep decreaseon day 6, and then reached an equilibrium upon reaching 0.5 × 105 cells mL−1 in most treatments(Figure 3a).Heterocyst density in 1.5 × 10−2 g L−1 nitrate treatment showed no significant differencecompared to that in 1.5 g L−1 nitrate treatment, in which heterocysts produced were the lowestamong all treatments (Figure 3a).On average, incubation under 1.5 × 10−5 g L−1 nitrate treatment
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