Predicting Copper Toxicity with Its Intracellular or Subcellular Concentration and the Thiol Synthesis in a Marine Diatom

The accumulation and subcellular distribution of copper (Cu) in a marine diatom Thalassiosira weissflogii were examined under different nutrient conditions [nitrogen-(-N) and phosphorus-starved (-P) conditions as well as nutrient-enriched conditions with nitrate (+NO₃^-) and ammonium (+NH₄⁺) as the nitrogen source]. Cu accumulation was induced in the NO₃^--exposed (+NO₃ ^- and -P) cells, suggesting that Cu may be directly or indirectly required for intracellular NO₃^- reduction. The relationships between the response of the cell-specific growth rate or photosynthetic system II maximum quantum yield (Φ_M) to Cu exposure in different nutrient-conditioned cells and the free copper ion concentration, the intracellular Cu concentration (intra-Cu), and the distribution of Cu in different subcellular compartments were also examined. We found that the intra-Cu was the best Cu toxicity predictor, as it accounted for most of the Cu-induced Φ_M response between different nutrient-conditioned cells. The synthesis of intracellular cysteine and five low molecular weight (LMW) thiols was not notably induced at high Cu levels possibly because of the existence of some other Cu detoxification mechanisms. This was further demonstrated by the much lower cysteine and LMW thiol contents in the -N cells with a similar Cu tolerance to the nutrient-enriched cells. Overall, our results suggest that Cu toxicity could be better predicted with the intra-Cu and its subcellular distribution as compared with the currently used free ion activity model and biotic ligand model. However, the LMW thiols had unexpectedly little contribution to Cu detoxification in T. weissflogii. © 2007 American Chemical Society.