Low polyethylene glycol (PEG) concentration contributes to photoprotection in Chlamydomonas reinhardtii under highlight stress

The highlight is one of the major problems encountered by all the autotrophs in the natural environment, as it affects the photosynthetic performance of these organisms, they were equipped themself with the diverse photoprotective mechanism. However, the drastic climate change in recent years had a more lethal effect on these organisms as it is beyond their threshold to withstand. In the past decade, scientists have unravelled many photoprotective mechanisms like qE, qZ, qT and qI (NPQ) used by autotrophs to combat highlight stress and the thirst for discovering such a new mechanism remains constant. In this regard, we studied the effect of mild osmotic stress (2%PEG) in alleviating high-light stress using Chlamydomonas (C) reinhardtii as a model system. The cells were grown in low Polyethylene glycol (PEG)-induced osmotic stress at varying light intensities; their response to these treatments had been examined via biochemical and biophysical approaches. The PEG-treated cells showed better growth and photosynthetic efficiency even at highlight than control, but their NPQ level is lesser than control, suggesting a unique or novel photoprotective mechanism is operating in PEG-treated samples. The Circular dichroism and ATG8 localization assay suggest that supercomplexes organization is not much disturbed in PEG-treated samples irrespective of light intensity. Also, the latter indicates that PEG-treated cells were healthier than the control at the highlight. This result is promising as we can improve the algal biomass under natural environmental conditions with fluctuating light intensity. As algal biomass has immense commercial importance in biofuel production, cosmetics and pharmaceutical application. This mechanism can be exploited to promote the socio-economic status of our nation. Highlights PEG application triggers ROS accumulations, stimulating signalling cascade and overcoming highlight-induced compromise in biomass. Under highlight, the photosynthetic pigment (chlorophyll and carotenoid) constantly increased with PEG except for C-250. Specifically, the photoprotective pigment like violaxanthin and lutein has been increased. PEG shielding stabilizes pigment-protein interaction, confirming that the supercomplex organization is in its native state. PEG keeps low ATG8 accumulation, preventing cells and enabling it to combat highlights more effectively. Graphical abstract