Elevated carbon dioxide enhances polyunsaturated fatty acids in Isochrysis galbana and improves rotifer growth through algal feeding

Abstract Elevated atmospheric CO 2 levels directly affect marine primary producers by altering their photosynthetic efficiency and biochemical composition, thereby modifying nutrient transfer throughout marine food webs. This study examines the physiological, biochemical, and transcriptomic responses of the microalga Isochrysis galbana when cultured under elevated CO 2 level (1000 μ atm CO 2 ) and current ambient CO 2 level (420 μ atm CO 2 ) conditions, as well as the subsequent effects of feeding these algae to rotifer ( Brachionus plicatilis ). Under high CO 2 conditions, I. galbana exhibited enhanced photosynthetic efficiency, carbon fixation, and energy metabolism, driven by the upregulation of genes involved in photosynthesis, the Calvin‐Benson cycle, and glycolysis. These molecular adaptations increased the growth rate by 24% and increased the content of polyunsaturated fatty acids (PUFAs) by 8%, including docosahexaenoic acid, which improves the nutritional quality of the alga. Feeding experiments demonstrated that rotifers fed with I. galbana grown under high‐CO 2 conditions exhibited improved growth and enriched PUFA profiles, highlighting the potential for high‐CO 2 ‐induced changes in primary producers to enhance trophic transfer efficiency. This study provides new insights into the molecular mechanisms underlying I. galbana 's response to elevated CO 2 and its ecological implications. The findings emphasize the capacity of marine microalgae to adapt to environmental changes and highlight the cascading effects of elevated CO 2 on marine food web nutrition and dynamics.