Mild osmotic stress offers photoprotection in Chlamydomonas reinhardtii under high light

• Published in: Plant physiology and biochemistry 2024-11, Vol.216, p.109050, Article 109050
• Main Authors: Gunasekaran, Jerome Xavier, Yadav, Ranay Mohan, Ramachandran, Pavithra, Sharma, Shriya, Subramanyam, Rajagopal
• Format: Article
• Language: English
• Subjects: Chlamydomonas reinhardtii; Chlamydomonas reinhardtii - metabolism; Chlamydomonas reinhardtii - physiology; Chlamydomonas reinhardtii - radiation effects; Chlorophyll - metabolism; Light; Non-photochemical quenching; Osmotic Pressure; Osmotic stress; Photosynthesis; Polyethylene glycol; Polyethylene Glycols - pharmacology; Reactive oxygen species and supercomplexes
• ISSN: 0981-9428; 1873-2690; 1873-2690
• DOI: 10.1016/j.plaphy.2024.109050

The exposure of autotrophs to high light intensities significantly impacts their photosynthetic performance. When combined with unpredictable climate changes, the lethality of these effects is exacerbated and, often surpassing the organisms' threshold for tolerance. In this regard, our study centres on examining the mitigating effects of mild osmotic stress induced by 2% Polyethylene Glycol (PEG) in conjunction with high-light conditions, using Chlamydomonas reinhardtii as a model system. Cells were cultivated under low PEG-induced osmotic stress at various light intensities, and their responses were analyzed through biochemical and biophysical approaches. Remarkably, cells grown under lower PEG concentrations exhibited superior growth, increased biomass, and enhanced photosynthetic efficiency under high light compared to non-PEG-treated cells. Surprisingly, their non-photochemical quenching (NPQ) levels were lower, indicating the operation of a distinct photoprotective mechanism in PEG-grown samples. The PEG-grown cells demonstrated higher chlorophyll content but lower carotenoid content, supporting the NPQ data. Circular dichroism analysis suggested that the macro-organization of super-complexes was minimally disrupted in PEG-grown samples, even under high light. This was further supported by Blue native PAGE, which showed greater stability of the super-complexes in PEG-grown cells, implying heightened stability in pigment-protein interactions. Immunoblot analysis revealed minimal differences in core reaction center proteins between PEG-grown and non-PEG cells. Notably, this protective mechanism was absent in the cell wall-deficient mutant CC503. We propose that the partial photoprotection observed is attributed to the PEG shielding the cell wall. This result holds promise for enhancing algal biomass production under natural environmental conditions influenced by fluctuating light intensity. •PEG shielding protects cells from highlight-induced cellular damage, thereby increasing biomass.•Under highlight, the chlorophyll, the photoprotective pigments like violaxanthin and lutein have been increased.•PEG triggers ROS accumulations, stimulating signalling cascade and overcoming highlight-induced compromise in biomass.•PEG shielding stabilizes the thylakoid super-complex organization in its native state. [Display omitted]