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Abstract

The increased occurrence of heatwaves and microplastic pollution may be decreasing the productivity of aquatic ecosystems. In response to changing environmental conditions, phytoplankton adjust their lipid profile and consequently alter their nutritional quality, which has implications for the productivity of their consumers. Elevated temperatures reduce the nutritional quality of phytoplankton, largely by decreasing their polyunsaturated fatty acid (PUFA) content, but it is unclear whether heatwaves elicit a similar or stronger response. The effect of microplastics, and the combination of heatwaves and microplastics, on the nutritional quality of phytoplankton is similarly unknown. To address these knowledge gaps, I conducted a laboratory experiment to examine the effects of constant warming, heatwave, and microplastics on the population growth, the lipidome, and the proteome of a freshwater green microalga, Tetradesmus obliquus. I also investigated their ability to recover once returned to ambient conditions. Although I applied the same total amount of heat in the constant warming and heatwave treatments, I saw clear differences in the lipid profile. The lipidomic response under heatwave involved a transient induction of more numerous and acute changes, whereas changes accumulated over time with constant warming. Extraplastidial membrane lipids and PUFA significantly decreased under both heat treatments, but neutral lipids decreased only under heatwave. Interestingly, plastidial lipids increased in all treatments, suggesting a protective response within photosynthetic organelles. The proteomic response followed a similar pattern, with a stronger but transient response under peak heatwave temperature, and an increase in differentially expressed proteins over time under constant warming. I also found that the co-occurrence of microplastics did not induce a unique lipidomic response under heatwave, but did so in the constant warming group. Upon return to ambient conditions, T. obliquus showed signs of recovery from both treatments, particularly with MP present, but maintained a slightly different lipid profile one-week post-heating. My findings suggest that future global change may have significant consequences for freshwater algal nutritional quality and thus the productivity of aquatic food webs. Further investigation into these effects at the community level and in multi-stressor scenarios is needed to better predict broader ecological responses to our rapidly changing environment.