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.

The biological characteristics and competitive predominance of cyanobacteria,as well as the formation and maintenance mechanism of cyanobacteria blooms,have aroused continuous interests among researchers.As an important nutrient for cyanobacterial growth,the assimilation of nitrogen is intimately coupled with the photosynthesis.However,nitrogen exists in various chemical forms in water,which affect the cyanohacterial growth and the formation of blooms in distinct ways.To delve into the formation mechanism of cyanobacterial blooms,it is important to understand the cyanobacterial photosynthetic feature as well as the mechanism and strategy by which the nitrogen of different forms are assimilated into cyanobacteria.This paper reviewed the processes and characteristics of photosynthesis in cyanobacteria,such as light energy capture and excitation energy transfer,primary photosynthetic reaction,CO 2 concentrating mechanism and assimilation.Then,it illustrated the transport and assimilation of different nitrogen forms（NO 3 - ,NO 2 - ,NH 4 + ,urea and N 2 ）,the regulation of these processes,as well as the mechanisms by which cells coordinate and regulate the carbon/nitrogen metabolism.The paper also summarized the latest research advances in cyanobacterial blooms and suggested on the content and direction of further research.

The critical role of nitrogen in the global proliferation of cyanobacterial blooms is arousing increasing attention. However, the mechanism underlying the algal responses to differential nitrogen forms remains unclarified. The physiological and transcriptomic changes of Microcystis aeruginosa supplied with different nitrogen forms (nitrate and ammonium) were highlighted in this study. The results indicated that ammonium behaves better in stimulating the initial growth in N-limited cells than nitrate. However, a concomitant side effect is that cellular growth and photosynthesis decreased due to photosystem II damage induced by excess absorbed light energy under 10 mg L-1 ammonium. By contrast, adequate nitrate supply favored more efficient photosynthesis, higher biomass yield and microcystin quotas than ammonium. Depending on the supplied nitrogen form, different transcriptomic patterns were observed in M. aeruginosa. Under nitrate, the upregulation of genes involved in Arg biosynthesis, ornithine-urea cycle and photosynthesis increased nitrogen storage and cellular growth, while genes involved in cyclic electron flow around photosystem I and CO2-concentrating mechanism were heightened to dissipate excess energy under high ammonium. These insights provided important clues for understanding the physiological and molecular effects of available nitrogen forms on the frequent outbreaks of cyanobacteria.