Copper toxicity to Phaeodactylum tricornutum: a survey of the sensitivity of various toxicity endpoints at the physiological, biochemical, molecular and structural levels.

Copper (Cu) is one of the most toxic metals in phytoplankton but the toxicity mechanisms of this metal are still not fully understood. This study examines the toxicity targets of Cu in the modeled marine diatom, Phaeodactylum tricornutum, at the physiological (cell division, DNA cell cycle), biochemical [pigments synthesis, reactive oxygen species (ROS) and malondialdehyde (MDA)], structural (subcellular observation by flow cytometry) and molecular (transcription of several metalloprotein genes) level. Cu toxicity was detectable at all these levels after 48 h of exposure to ≥20 μM total initial added Cu. The order of sensitivity of all the studied Cu toxicity endpoints was: G2/M phase > MDA > metalloproteins RNA of the photosynthetic electron transport chain (ETC) > metalloproteins RNA of the respiratory chains > G0/G1 phase > pigments ≈ S phase > propidium iodide > estimated cell yield > ROS. The relatively sensitive decrease of the transcription of metalloproteins RNA of the ETC in response to Cu exposure, if associated to an effective decrease in the expression of the proteins composing the ETC, may help to initially mitigate the ROS-mediated toxic effects of Cu in P. tricornutum. However, this cellular response to Cu was only transitory and the transcription of virtually all genes involved in redox electron transfer chains was up-regulated within an interval of 2 days. This study brings new insights into the cellular mechanisms of Cu toxicity by documenting the sensitivity and kinetics of multiple Cu-cellular targets in one marine alga.