Uncovering the Differential Growth of Microcystis aeruginosa Cultivated under Nitrate and Ammonium from a Photophysiological Perspective

Cyanobacteria have a relatively high affinity for NH4+, yet a NO3--rich environment is comparatively conducive to their proliferation. To date, little information is available on why NO3--N favors cyanobacterial biomass accumulation. This study investigated the dependence of biomass, nitrogen assimilation characteristics, and photophysiological performance of Microcystis aeruginosa on the forms of nitrogen supply, including NO3--N-only, NH4+-N-only, and NO3--N + NH4+-N. The results indicated that despite the retarded growth of M. aeruginosa, cells supplied with NO3--N-only maintained a simultaneous promotion in the growth rate, nitrogen assimilation efficiency, and photosynthetic capacity, resulting in high biomass production. Cells supplied with NH4+-N-only and NO3--N + NH4+-N were able to rapidly assimilate nitrogen during initial cell proliferation, showing a preferential use of NH4+-N and the inhibition of NO3- uptake by NH4+. However, growth repression occurred as cultivation time was prolonged when NH4+-N was excessively supplied, mainly due to PSII photodamage, intracellular redox imbalance, and increased electron energy accumulation. Thus, cells supplied with NH4+-N-only had to reduce light energy capture, increase photoprotection, and consume excess electrons to mitigate the damage. These findings are critical for improving our understanding of the role of different nitrogen forms in the regulation of cyanobacterial photophysiological performance and growth.