The toxicity of BDE-47 to the photosystem of Lemna minor fronds

Abstract

To elucidate the toxicity of 2,2ʹ,4,4ʹ-tetrabromodiphenyl ether (BDE-47) on photosynthetic primary processes, in vivo and in vitro treatments of BDE-47 were performed. The 20-d treatment in vivo (5 - 20 μg dm-3) suppressed the reproduction of duckweed (Lemna minor) and led to decline in chlorophyll (Chl) content of fronds. The most obvious features of BDE-47-treated fronds included a Chl a fluorescence rise at the J phase and a depression at the G phase, whereas significant fluorescence rises at the L, K, and J phases were found on the ΔVt curve of thylakoid membranes treated with 10 - 15 mg dm-3 BDE-47 for 4 h (in vitro). In both in vivo and in vitro experiments, the BDE-47 treatments significantly reduced the density of the active reaction centers (RC/CSo), affected the efficiency and speed of photosynthetic electron transfer [the maximum quantum yield of photosystem (PS) II photochemistry - Fv/Fm, quantum yield for electron transport (at t = 0) - φEo, electron transport flux per excited cross section - ETo/CSo, and net rate of reaction centers closure at 300 and 100 μs - dV/dto and dVG/dto, respectively], and increased energy dissipation [quantum yield for energy dissipation (at t = 0) - φDo, dissipated energy flux per reaction center - DIo/RC, and issipated energy flux per excited cross section - DIo/CSo]. The BDE-47 at 5 - 15 mg dm-3 had no impact on the minimum (initial) fluorescence (Fo) and total electron carriers per reaction center (Sm) of the thylakoid membranes, but PS II units were less tightly grouped (a positive L-band). On the contrary, there was no positive L-band on the difference between relative fluorescence intensities of the normalized induction curves from 50 μs to 300 μs (WK) of each BDE-47 treatment and control (ΔWK), and Fo and Sm increased after the treatment with BDE-47. The above results indicate that BDE-47 not only affected the permeability of thylakoid membranes, but also relaxed the structure of PS II, thereby affecting the function of PS II. In addition, BDE-47 could induce secondary damage to the PSs in duckweed fronds.