In this study, trace metal bioaccumulation and toxicity in marine phytoplankton were examined. For the metal bioaccumulation, the hypothesis that there is a direct relationship between the algal specific growth rate (μ) and metal uptake rate (ρ) was tested. For this purpose, μ was adjusted by changing the environmental factors (temperature and light) and ρ under each condition was then quantified. A hyperbolic relationship between μ and ρ of Cd and Zn was observed. And the potential changes in cell size, biochemical composition, and cell cycle with temperature and light would not affect ρ. Although a positive relationship was also observed between Fe uptake and cell growth, this relationship might not be true as Fe uptake is closely related to temperature and light....[ Read more ]

In this study, trace metal bioaccumulation and toxicity in marine phytoplankton were examined. For the metal bioaccumulation, the hypothesis that there is a direct relationship between the algal specific growth rate (μ) and metal uptake rate (ρ) was tested. For this purpose, μ was adjusted by changing the environmental factors (temperature and light) and ρ under each condition was then quantified. A hyperbolic relationship between μ and ρ of Cd and Zn was observed. And the potential changes in cell size, biochemical composition, and cell cycle with temperature and light would not affect ρ. Although a positive relationship was also observed between Fe uptake and cell growth, this relationship might not be true as Fe uptake is closely related to temperature and light.

Since the Fe requirements are different under different light and temperature conditions, then how the marine phytoplankton fulfilled the different Fe requirements was examined based on the three variables (cell growth, Fe uptake, and efflux). Although more Fe is required under the low light intensity to synthesize more pigments for light interception, ρ of Fe decreased with the decreasing light intensity. However, the cellular Fe concentration was kept constant or slightly increased at low light intensity as the cell growth was strongly inhibited. Contrastively, the high Fe requirement under the high temperature was fulfilled mainly through the Fe uptake induction. And μ was kept almost constant under different temperatures. Although there was an obvious Fe efflux out of the cell, it had no effects on the trend of intracellular Fe concentration under different light and temperature conditions.

As for the metal toxicity, the metal and algal species-dependent toxicity difference was examined. The cyanobacteria Synechococcus was found to be the most sensitive, as its cell growth and PAM parameters were the most dramatically inhibited at the same metal level and its NOEC (No-Observed Effect Concentration) was the lowest among the four species tested. The toxicity of the three metals was also different following the order: Cd > Cu > Zn from high to low toxicity according to their cellular concentration at NOEC. Meanwhile, the PAM technique had a similar sensitivity to μ as the metal toxicity endpoints for marine phytoplankton.

Metal toxicity to marine phytoplankton was thought to be dependent on the ambient free metal ion or the cell surface adsorbed metal concentration based on the Free Ion Activity Model and Biotic Ligand Model. However, these two models are based on several assumptions and a lot of exceptions have been reported. In this study, I quantified the metal concentrations in different subcellular compartments, which were then linked to their toxicity in algae. It was found that Cd concentration in the soluble fraction could account for most of its toxicity difference in different nutrient conditioned cells. However, it was not the case for Cu. Therefore, a more functionally based subcellular fractionation was required in the future.