Effects of nitrate on intracellular nitrite and growth of Microcystis aeruginosa

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 CO2 and high CO2 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 CO2 conditions, M. aeruginosa showed lower intracellular nitrite concentrations and higher growth rates and \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\text{P}}_{\text{m}}^{{\text{chl}}a} $$\end{document}, \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\text{R}}_{\text{d}}^{{\text{chl}}a} $$\end{document}, αchla than under low CO2 conditions. These results suggest that the accumulation of intracellular nitrite could be the cause of inhibition of algal growth under high nitrate concentrations.