Use of chlorophyll a fluorescence to elucidate the toxicity target of N-phenyl-2-naphthylamine on photosynthetic system of Cylindrospermopsis raciborskii (Cyanobacteria)

In order to investigate the toxicity targets of the secondary metabolite from aquatic plants, N-phenyl-2-naphthylamine, on Cylindrospermopsis raciborskii photosynthetic system II (PSII), we determined chlorophyll a (Chl a), specific growth rate (μ−d), photosynthetic rapid light curves and polyphasic Chl a fluorescence transients following exposure to N-phenyl-2-naphthylamine. The results showed significant decreases in Chl a following 48 h at high concentration (0.50 mg l−1) exposure to N-phenyl-2-naphthylamine. The PSII electron transfer rate and Chl a fluorescence transients appeared to be affected after the cells were exposed to concentrations of 0.50, 1.00 and 2.00 mg l−1. Exposure to higher concentration of N-phenyl-2-naphthylamine (> 0.50 mg l−1) mainly depressed the density of reaction centers (RC/CS0) and quantum energy flux ratios (ϕP0 and ϕE0) but increased the absorption flux per RC (ABS/RC), dissipated energy flux per RC (DI0/RC) and the average redox state of QA−/QA in the time span from 0 to tFmax (Sm/tFmax). However, exposure to low concentration (≤ 0.5 mg l−1) mainly inhibited the trapping energy fluxes per reaction center (TR0/RC), electron transport energy fluxes per reaction center (ET0/RC) and energy flux ratio (ψ0). This suggested that the donor and the acceptor sides of PSII were targets of N-phenyl-2-naphthylamine in C. raciborskii. Additionally, our results indicated that the toxicity target at low concentrations of N-phenyl-2-naphthylamine was at each step of electron transport between QA− and the Cyt b6/f complex; whereas, the reduction of QA was the potential target at high concentrations. We suggest that N-phenyl-2-naphthylamine may be a potential allelochemical to control harmful cyanobacterial blooms.