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Although nitrate is a macronutrient and can serve as good nitrogen source for many species of phytoplankton, high nitrate concentrations do not benefit the growth of phytoplankton. We hypothesise that algae cultured under high nitrate concentrations can accumulate intracellular nitrite, which is produced by nitrate reductase (NR) and can inhibit the growth of algae. To assess the validity of this hypothesis, Microcystis aeruginosa was grown under different nitrate concentrations from 3.57 to 21.43 mM in low CO 2 and high CO 2 conditions for 15 days. We observed that, with increasing nitrate concentrations, the intracellular nitrite concentrations of the alga increased and the growth rates and photosynthesis declined. When grown under high CO 2 conditions, M. aeruginosa showed lower intracellular nitrite concentrations and higher growth rates and P chla m , R chla d , α chla than under low CO 2 conditions. These results suggest that the accumulation of intracellular nitrite could be the cause of inhibition of algal growth under high nitrate concentrations.

The occurrence of microcystins (MC) during aquatic blooms of cyanobacteria in eutrophicated freshwater body has increasingly become an environmental and health concern worldwide. An experimental study was conducted to investigate the adsorption of two common variants of microcystins, MCLR and MCLW, onto SPM at different pH values. The results showed that most microcystins spiked into an SPM solution with the concentration of 0.25g/l was adsorbed onto solids at pH3 (>95%) and 7 (>85%).At pH13, the proportion of adsorbed microcystin decreased to 8-29% for MCLR and 38-47% for MCLW. The adsorption of MCLR and MCLW onto SPM fitted well with S type of Freundlich isotherm except for pH3 at which L type of Freundlich isotherm was suitable for MCLW. At pH3 and 7, linear isotherm was also able to describe the adsorption of MCLR and MCLW. The adsorption of MCLR and MCLW decreased significantly with the elevated pH, which is consistent with the pH-dependent hydrophobicity.