Cytosolic distribution of copper in the gills of field-collected oysters with different copper bioaccumulation.

Oysters can hyper-accumulate copper (Cu) without apparent toxicity, but the mechanism of sequestering excessive cytosolic Cu in oysters remains unclear. We here investigated the Cu distribution in the cytosolic proteins (CPs) in the gills of oysters (Crassostrea hongkongensis) through size-exclusion chromatography coupled to inductively coupled plasma mass spectrometry (SEC-ICP-MS). Oysters collected from the southern coast of China contained a gradient of gill Cu concentrations ranging from 132 to 3540 µg g-1 (dry weight), with 7-41 % of Cu distributed in the CPs fraction. The CPs-Cu concentrations were 8.6 times higher in oysters with high Cu concentrations compared to low concentrations. In the CPs, Cu was dispersed with a broad range of molecular weight, suggesting the involvement of various cytosolic proteins in Cu binding. Among the 10 major Cu peaks, peaks 2 (>600 kDa) and peak 8 (18 kDa) contained substantial Cu and showed obvious differences in response to the variation of CPs-Cu levels. Peak 8 contained metallothionein-like proteins that decreased their role in Cu binding as CPs-Cu concentrations increased. LC-MS/MS analysis revealed that peak 2 contained macromolecular protein complexes (MPCs), which played a critical role in binding excess Cu. The comparison with other bivalve species further suggested that sequestering excess CPs-Cu in MPCs was a special strategy employed by oysters in response to high Cu accumulation. This study provides valuable insights into the mechanism of hyper-accumulation and sequestration of Cu in oysters and helps to better understand Cu biomonitoring by oysters.

The influences of dissolved inorganic and organic phosphorus on arsenate toxicity in marine diatom Skeletonema costatum and dinoflagellate Amphidinium carterae.

In this study, arsenate (As(V)) uptake, bioaccumulation, subcellular distribution and biotransformation were assessed in the marine diatom Skeletonema costatum and dinoflagellate Amphidinium carterae cultured in dissolved inorganic phosphorus (DIP) and dissolved organic phosphorus (DOP). The results of 3-days As(V) exposure showed that As(V) was more toxic in DOP cultures than in DIP counterparts. The higher As accumulation contributed to more severe As(V) toxicity. The 4-h As(V) uptake kinetics followed Michaelis-Menten kinetics. The maximum uptake rates were higher in DOP cultures than those in DIP counterparts. After P addition, the half-saturation constants remained constant in S. costatum (2.42-3.07 µM) but decreased in A. carterae (from 10.9 to 3.8 µM) compared with that in the respective P-depleted counterparts. During long-term As(V) exposure, A. carterae accumulated more As than S. costatum. Simultaneously, As(V) was reduced and transformed into organic As species in DIP-cultured S. costatum, which was severely inhibited in their DOP counterparts. Only As(V) reduction occurred in A. carterae. Overall, this study demonstrated species-specific effects of DOP on As(V) toxicity, and thus provide a new insight into the relationship between As contamination and eutrophication on the basis of marine microalgae.

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 intracellular As to P ([As:P]) were negatively correlated with the ambient P concentrations. The trends of As bioaccumulation were substantially different between the relatively low (0, 0.5 and 1.5 µM) and high P concentrations (7.5 and 37.5 µM). Both [As]intra and [As:P] suggested that As bioaccumulation was a better factor to explain the As toxicity comparing 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 µM-P), suggesting 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.