Low temperature modulates the carbon allocation in different metabolic pathways to improve the tolerance of Arctic Chlorella to high light stress

Polar microalgae, renowned for their exceptional adaptability in extreme environments, are vital for carbon fixation and nutrient cycling. To understand their metabolic resilience under low temperature and high light stress, we conducted an extensive biochemical and full-length transcriptome analysis of Arctic Chlorella (Chlorella-Arc) and temperate Chlorella (Chlorella-Temp). This research aimed to comprehensively assess Chlorella's adaptability to the interactive effects of temperature and light stress. Under low-temperature stress, intense light inhibited Chlorella-Temp growth by 14.8 %, while it significantly promoted Chlorella-Arc growth by 88.8 %. The cold-adapted Chlorella-Arc demonstrates higher tolerance to high light stress. Elevated temperature (+0.80) and light (+0.47) directly increased cell density, revealing direct and indirect effects of temperature and light on Chlorella-Arc bioproducts via structural equation modeling. We then analyzed key molecules involved in lipid metabolism using orthogonal projections to latent structures discriminant analysis, confirmed that Chlorella-Arc maintained a relatively constant fatty acids composition under low temperature and high light stress. Compared to its temperate counterpart, Chlorella-Arc exhibits unique adaptability by modulating polysaccharide composition, downregulating carbon fixation, reducing nitrogen absorption, and maintaining stable unsaturated fatty acid levels with a higher unsaturated to saturated fatty acid ratio. Chlorella-Arc, a prime example of polar microalgae adaptability, exhibits unique metabolic responses. Compared to the low-temperature normal-light group, the high-light Chlorella demonstrates precise regulation of nitrogen uptake and carbon allocation, while activating multiple antioxidant pathways to mitigate the increase in ROS levels under light stress. Acclimated Chlorella-Arc tends to increase triacylglycerol accumulation, suggesting a shift in carbon flow towards lipid production. Our analyses reveal specialized adjustments, enabling survival in harsh conditions of low temperature and intense light. This research enhances our understanding of microalgal resilience in extreme environments, offers valuable insights into the utilization of polar microalgal bioproduct resources.