Effects of phosphate on the toxicity and bioaccumulation of arsenate in marine diatom Skeletonema costatum

The effects of nutrient phosphate (P) at environmentally relevant levels on the toxicity of arsenic (As) in marine microalgae have been rarely known. In the present study, we explored the toxicity and bioaccumulation of As in a globally distributed diatom species Skeletonema costatum at different ambient P concentrations. The results showed that As toxicity was suppressed at elevated P concentrations. Intracellular As content ([As]intra) and the molar ratio of in-tracellular As to P ([As:P]) were negatively correlated with the ambient P concentrations. The trends of As bioaccumula-tion were substantially different between the relatively low (0, 0.5 and 1.5 mu M) and high P concentrations (7.5 and 37.5 mu M). Both [As]intra and [As:P] suggested that As bioaccumulation was a better factor to explain the As toxicity com-paring to the ambient As concentration. The 4 h As uptake kinetics at different P concentrations followed Michaelis- Menten kinetic pattern. The maximum uptake rates (Vmax) decreased with the increase in P concentrations, and the half-saturation constants (Kd) remained constant except for that at extremely high P concentration (37.5 mu M-P), suggest-ing the depression of P on As uptake was mainly due to the non-competitive effect. Overall, these results demonstrate that the P concentration in seawater is an important factor affecting As toxicity and bioaccumulation in the marine diatom. This study therefore helps us better understand the effects of eutrophication on the toxicity and biogeochemistry of As in the marine environment.