Effect of nitrogen forms on growth, cell composition and N2 fixation of Cylindrospermopsis raciborskii in phosphorus-limited chemostat cultures
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The aim of this research was to test whether NH4+ and NO3− affect the growth, P demand, cell composition and N2 fixation of Cylindrospermopsis raciborskii under P limitation. Experiments were carried out in P-limited (200 μg l−1 PO4-P) chemostat cultures of C. raciborskii using an inflowing medium containing either 4,000 μg l−1 NH4-N, 4,000 μg l−1 NO3-N or no combined N. The results showed the cellular N:P and C:P ratios of C. raciborskii decreased towards the Redfield ratio with increasing dilution rate (D) due to the alleviation of P limitation. The cellular C:N and carotenoids:chlorophyll-a ratios also decreased with D, predominantly as a result of an increase in the chlorophyll-a and N content. The NH4+ and NO3− supply reduced the P maintenance cell quota of C. raciborskii. Consequently, the biomass yield of the N2-grown culture was significantly lower. The maximum specific growth rate of N2-grown culture was also the lowest observed. It is suggested that these differences in growth parameters were caused by the P and energy requirement for heterocyte formation, nitrogenase synthesis and N2 fixation. N2 fixation was partially inhibited by NO3− and completely inhibited by NH4+. It was probably repressed through the high N content of cells at high dissolved N concentrations. These results indicate that C. raciborskii is able to grow faster and maintain a higher biomass under P limitation where a sufficient supply of NH4+ or NO3− is maintained. Information gained about the species-specific nutrient and pigment stoichiometry of C. raciborskii could help to access the degree of nutrient limitation in water bodies.