Studies of biouptake and transformation of mercury by a typical unicellular diatom Phaeodactylum tricornutum

Mercury (Hg) is a toxic heavy metal with its biogeochemical cycling in the ocean depending on the type and behavior of the oceanic microalgae. The present work aimed to evaluate bioaccumulation and transformation of Hg by Phaeodactylum tricornutum, a typical unicellular diatom, when exposed to the extremely high level of Hg in order to understand the possible mechanisms of acute stress response. P. tricornutum can accumulate Hg (its bioaccumulation factor is at 10(4) level), and the 96 h EC50 was estimated to be 145 mu g L-1. The amounts of surface-bound Hg being about 1.2 to 4.8 times higher than those of intracellular Hg under exposure to HgCl2 (from 20 to 120 mu g L-1 concentrations) suggested that the cell wall of P. tricornutum is an important "fence" towards Hg. After entering the P. tricornutum cell, Hg underwent transformation in its chemical form via interactions with high molecular weight sulfur-containing proteins (accounting for 68% of the intracellular Hg), and glutathione as well as the induced phytochelatins (PCs) (24% Hg) which alleviated the toxicity of HgCl2. In addition, the existence of organic ligands greatly influenced the uptake and transformation behavior of P. tricornutum towards HgCl2, especially in the case of cysteine (Cys), which increased the uptake of Hg, but alleviated the toxicity of Hg towards P. tricornutum due to the fact that Cys is an important precursor for the synthesis of PCs inside the cell. The uptake process of Hg by P. tricornutum was in agreement with the Freundlich isotherm, suggesting a typical heterogeneous sorption process. More importantly, we observed the conversion of HgCl2 into methylmercury inside the P. tricornutum cells and its release into the culture solution using HPLC/CVG-AFS and GC-MS, although the mechanism needs to be further investigated.