Response of antioxidant prenyllipids to exposure to Cd, Cr, and Cu ions in the green microalga Chlamydomonas reinhardtii

Copper, cadmium, and chromium are known to disrupt metabolic processes and induce oxidative stress. Chloroplasts are the main site of reactive oxygen species (ROS) production in photosynthetic cells under light. Exposure to various abiotic stress factors, including heavy metal (HM) ions, leads to increased ROS formation. Prenyllipid antioxidants, including carotenoids, tocopherols (Tocs), and plastoquinol, play an important protective role in chloroplast membranes. To examine the response of prenyllipid antioxidants to HM-induced stress in more detail, culture growth, the content of photosynthetic pigments, Tocs, plastochromanol, as well as plastoquinone (PQ) pool and the extent of its reduction were measured day by day in Chlamydomonas reinhardtii, exposed to Cu, Cr, and Cd ions in two experimental setups, i.e. stress evoked at the beginning of culture growth and stress evoked at the beginning of stationary phase. Algae exposed to stress from the very beginning were more susceptible to HM ions than grown cultures. The Cu ions displayed the highest toxicity of all the HMs tested. Exposure to HMs was expected to lead to the increase in prenyllipid content. However, grown cultures exposed to HMs showed either no changes or a decrease in Tocs and PQ content. In algae exposed to HM ions from the very beginning, the content of prenyllipids was increased during the first four days of growth compared to control, but the main reason for it was a slowdown in cell division and chlorophyll synthesis rather than an enhanced production. An increase in prenyllipid content appears to be more important during long-term acclimation to HMs than during short-term responses.