The response and detoxification strategies of three freshwater phytoplankton species, Aphanizomenon flos-aquae, Pediastrum simplex, and Synedra acus, to cadmium

The response and detoxification mechanisms of three freshwater phytoplankton species (the cyanobacterium Aphanizomenon flos-aquae, the green alga Pediastrum simplex, and the diatom Synedra acus) to cadmium (Cd) were investigated. The cell growth of each species was measured over 10 days, and chlorophyll a fluorescence, Cd bioaccumulation (including surface-adsorbed and intracellular Cd), and phytochelatin (PC) synthesis were determined after 96-h exposures. The growth of the three phytoplankton species was significantly inhibited when Cd concentrations were >= 5 mg L-1. Compared with P. simplex, greater growth inhibition in S. acus and A. flos-aquae occurred. The changes in chlorophyll fluorescence parameters including the maximal quantum yield of PSII (Fv/Fm) and relative variable fluorescence of the J point (Vj) demonstrated that the increase in Cd concentration damaged PSII in all three species. After 96-h exposures, the accumulation of surface-adsorbed Cd and intracellular Cd increased significantly in all three species, with the increase of Cd concentrations in the media; total cadmium accumulation was 245, 658, and 1670 times greater than that of the control in A. flos-aquae, P. simplex, and S. acus, respectively, after exposure to 10 mg L-1. Total thiols exhibited a similar trend to that of Cd accumulation. PC3 was found in A. flos-aquae and P. simplex in all Cd treatments. Glutathione (GSH) and PC2 were also produced in response to exposure to high concentrations of Cd. PC4 was only discovered at exposure concentrations of 10 mg L-1 Cd and only in S. acus. The intracellular Cd/PCs ratio increased in all three phytoplankton with an increase in Cd concentrations, and a linear relationship between the ratio and the growth inhibition rates was observed with P. simplex and S. acus. Our results have demonstrated that metal detoxification mechanisms were dependent on the species. This study suggested that the variance of metal detoxification strategies, such as cadmium accumulation and PCs, might be an explanation why algal species have different sensitivity to Cd at various levels.