Changes in lipid and carotenoid metabolism in Chlamydomonas reinhardtii during induction of CO₂-concentrating mechanism: Cellular response to low CO₂ stress

Photosynthetic organisms strictly depend on CO₂ availability and the CO₂:O₂ ratio, as both CO₂/O₂ compete for catalytic site of Rubisco. Green alga Chlamydomonas reinhardtii, can overcome CO₂ shortage by inducing CO₂-concentrating mechanism (CCM). Cells transferred to low-CO₂ are subjected to light-driven oxidative stress due to decrease in the electron sink. Response to environmental perturbations is mediated to some extent by changes in the lipid and carotenoid metabolism. We thus hypothesize that when cells are challenged with changes in CO₂ availability, changes in the lipidome and carotenoids profile occur. These changes expected to be transient, when CCM is activated, CO₂ limitation will be substantially ameliorated. In our experiments, cells were transferred from high (5%) to low (air equilibrium) CO₂. qPCR analysis of genes related to CCM and lipid metabolism was carried out. Lipidome was analyzed both in whole cells and in isolated lipid droplets. We characterized the changes in polar lipids, fatty acids and ketocarotenoids. In general, polar lipids significantly and transiently increased in lipid droplets during CCM. Similar pattern was observed for xanthophylls, ketocarotenoids and their esters. The data supports our hypothesis about the roles of lipids and carotenoids in tackling the oxidative stress associated with acclimation to sub-saturating CO₂.