Abstract
Chlamydomonas (C.) reinhardtii is a potential microalga for lipid production. Autophagy-triggered lipid metabolism in microalgae has not being studied so far from a mutant of proton gradient regulation 1 like (PGRL1) and proton gradient regulation 5 (PGR5). In this study, C. reinhardtii cells (wild-type CC124 and cyclic electron transport dependant mutants pgrl1 and pgr5) were grown photoheterotrophically in high light 500 μmol photons m–2 s–1, where pgr5 growth was retarded due to an increase in reactive oxygen species (ROS). The lipid contents were increased; however, carbohydrate content was decreased in pgr5. Further, the Nile Red (NR) fluorescence shows many lipid bodies in pgr5 cells under high light. Similarly, the electron micrographs show that large vacuoles were formed in high light stress despite the grana stacks structure. We also observed increased production of reactive oxygen species, which could be one reason the cells underwent autophagy. Further, a significant increase of autophagy ATG8 and detections of ATG8-PE protein was noticed in pgr5, a hallmark characteristic for autophagy formation. Consequently, the triacylglycerol (TAG) content was increased due to diacylglycerol acyltransferases (DGAT) and phospholipid diacylglycerol acyl-transference (PDAT) enzymes’ expression, especially in pgr5. Here the TAG synthesis would have been obtained from degraded membrane lipids in pgr5. Additionally, mono, polyunsaturated, and saturated fatty acids were identified more in the high light condition. Our study shows that the increased light induces the reactive oxygen species, which leads to autophagy and TAG accumulation. Therefore, the enhanced accumulation of TAGs can be used as feedstock for biodiesel production and aqua feed.
Highlights
Light is vital for microalgae for efficient photosynthesis
Our results showed that the cell growth pattern of the WT was increased 30%, whereas pgrl1 and pgr5 growth were decreased 21 and 30% compared to the WT under high light (500 μmol photon m−2 s−1) after 3rd day and normalized optical density (OD) data was shown in Supplementary Table 1
Cyclic electron transport around PSI requires the functions of proton gradient regulation 1 like (PGRL1) and proton gradient regulation 5 (PGR5) to generate a proton gradient over the thylakoid membrane
Summary
Light is vital for microalgae for efficient photosynthesis. CO2 fixation by Calvin Benson cycle occurs through photosynthesis that primarily synthesizes carbohydrates, leading to the synthesis of lipid stored as triacylglycerols (TAG) (Mondal et al, 2017). Autophagy Induced Lipid Accumulation conditions, the rate of light absorbed is equal to the pace of photosynthesis, but when light intensity increases, the system cannot tolerate over-excitation. In this condition, the primary by-product of photosynthesis formed as reactive oxygen species (ROS), which regulate the autophagy (ATG) mechanism. In a group of ATG (autophagy-related) proteins, the accumulation of ATG8 and ATG3 increased in a conditional repression line of a chloroplast protease (ClpP1), suggesting the chloroplast proteolysis systems and autophagy partially complement each other in C. reinhardtii (Ramundo et al, 2014) These proteins were shown a similar function to that of other organisms (PérezPérez et al, 2010, 2016). The mechanism of autophagy caused by high light stress in C. reinhardtii is poorly understood
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