Investigating low-dose UV-C radiation stress for improved cultivation of microalgae for biofuel and bio- products

Abstract

The use of food crops for biofuel production has led to concerns about agricultural productivity and global food supply, creating a "food vs fuel" dilemma. Microalgae have emerged as a potential alternative feedstock for biofuels, but commercial viability remains a challenge. To address this, alternative methods are needed to induce metabolic modifications in microalgae without compromising biomass yield. This study investigates the effects of non-lethal, low doses of ultraviolet C (UV-C) radiation as a potential stressor for metabolic modifications in microalgae. The objective was to quantify the physiological and metabolic effects of UV-C exposure, focusing on understanding its impact rather than generating mutants. Exposure of the biflagellate green microalgae Chlamydomonas reinhardtii and Dunaliella salina to low doses of UV- C (500, 1000 or 2000 μJ/cm2) was followed by 24 h incubation in either light/dark cycle conditions or in full darkness. In addition, a single round of UV-C exposure was compared with three rounds of exposure. The main findings from the study were that repetitive exposure to UV-C stimulated lipid accumulation in irradiated C. reinhardtii and this increased significantly up to 3-fold with the increase in UV-C dose. This was concluded to be a stress response since the highest UV-C doses decreased cell growth and viability, inhibited photosynthetic activity, and increased reactive oxygen species (ROS) production. UV-C exposure also increased cell sedimentation up to 90% that was likely due to flagella detachment and loss of motility. Furthermore, the 24 h period of dark after the UV-C exposure enhanced the stress response to the radiation but also negatively affected the non-irradiated control cells. Compared to C. reinhardtii, D. salina was found to be more tolerant to the UV-C radiation. This was proposed to be due in part to increased antioxidant activity in the salt tolerant and carotenoid-accumulating D. salina which showed a strong negative correlation between β-carotene concentration and ROS abundance. The increase in β-carotene was approximately 2-fold under the highest salt concentration NaCl of 1.2 M. Finally, a transcriptomic analysis found that UV-C exposure and/or 24 h dark incubation caused substantial changes in gene expression in C. reinhardtii. This included up-regulation of protective mechanisms such as heat shock proteins, and genes related to photosynthesis, down-regulation of genes associated with flagella function, and a complex response for genes related to starch and lipid metabolism, with many genes down-regulated but also many up-regulated. In conclusion, low dose UV-C stimulation of lipid, starch and antioxidants could potentially become an important tool for enhancing microalgae production in the future.