Differential sensitivity of five cyanobacterial strains to ammonium toxicity and its inhibitory mechanism on the photosynthesis of rice-field cyanobacterium Ge–Xian–Mi (Nostoc)

Abstract: Effects of two fertilizers, NH4Cl and KCl, on the growth of the edible cyanobacterium Ge–Xian–Mi (Nostoc) and four other cyanobacterial strains were compared at pH 8.3±0.2 and 25°C. Their growth was decreased by at least 65% at 10mmolL−1 NH4Cl but no inhibitory effect was observed at the same level of KCl. Meanwhile, the strains exhibited a great variation of sensitivity to NH4 + toxicity in the order: Ge–Xian–Mi> Anabaena azotica FACHB 118> Microcystis aeruginosa FACHB 905> M. aeruginosa FACHB 315> Synechococcus FACHB 805. The 96-h EC50 value for relative growth rate with regard to NH4 + for Ge–Xian–Mi was 1.105mmolL−1, which was much less than the NH4 + concentration in many agricultural soils (2–20mmolL−1). This indicated that the use of ammonium as nitrogen fertilizer was responsible for the reduced resource of Ge–Xian–Mi in the paddy field. After 96h exposure to 1mmolL−1 NH4Cl, the photosynthetic rate, F v/F m value, saturating irradiance for photosynthesis and PSII activity of Ge–Xian–Mi colonies were remarkably decreased. The chlorophyll synthesis of Ge–Xian–Mi was more sensitive to NH4 + toxicity than phycobiliproteins. Thus, the functional absorption cross section of Ge–Xian–Mi PSII was increased markedly at NH4Cl levels ≥1mmolL−1 and the electron transport on the acceptor side of PSII was significantly accelerated by NH4Cl addition ≥3mmolL−1. Dark respiration of Ge–Xian–Mi was significantly increased by 246% and 384% at 5 and 10mmolL−1 NH4Cl, respectively. The rapid fluorescence rise kinetics indicated that the oxygen-evolving complex of PSII was the inhibitory site of NH4 +. [Copyright &y& Elsevier]