New Insights into Toxic Effects of Arsenate on Four Microcystis Species under Different Phosphorus Regimes

Very little information is available on the stressed growth of Microcystis imposed by arsenate (As(V)) under different phosphorus (P) regimes. In this study, we examined the growth characteristics and arsenic transformation of four Microcystis species exposed under As(V) with two P sources involving dissolved inorganic phosphorus (IP) and organophosphate (d-glucose-6-phosphate disodium salt, GP). Results showed that all the four chosen Microcystis species could grow and reproduce with GP as the only P source, and the difference was insignificant when compared with IP. From optical density (OD), chlorophyll a (Chla), and actual quantum yield (Yield), the tolerance to As(V) of the chosen species was following as FACHB 905 > FACHB 1028 > FACHB 1334 > FACHB 912. Specifically, the 96 h EC50 of As(V) for FACHB 905 in IP was approx. 4 orders of magnitude higher than that in GP, but for other three algal species, the 96 h EC50 values were similar under the two given different P conditions. Furthermore, all antioxidant enzyme activities of superoxide dismutase (SOD), peroxide dismutase (POD), glutathione S-transferases (GSTs), and metalloproteinase (MTs) in algal cells were significantly increased in GP conditions. Moreover, the enzyme activities of AKP, GSTs, and MTs were inhibited with increasing As(V) levels under both IP and GP conditions. In addition, arsenite (As(III)) and methylated As of monomethylarsonic acid (MMA) and dimethylthioarsinic acid (DMA) were found in FACHB 912 and FACHB 1334 media, indicating that these Microcystis could detoxify As(V) by As biotransformation under IP and GP conditions. Specifically, As(V) reduction was elevated in media of FACHB 1334 and FACHB 905, but was decreased in media of FACHB 912 under GP conditions. Our results highlight the different P sources that impact the toxic effects of arsenate exposure on Microcystis and subsequent As biotransformation.