Light, CO2, and carbon storage in microalgae.

Microalgae exhibit remarkable adaptability to environmental changes by integrating light and CO2 signals into regulatory networks that govern energy conversion, carbon fixation, and storage. Light serves not only as an energy source for photosynthesis but also as a regulatory signal mediated by photoreceptors. Specific light spectra distinctly influence carbon allocation, driving lipid or starch biosynthesis by altering transcriptional and metabolic pathways. The ratio of ATP to NADPH imbalances significantly impact carbon allocation toward lipid or starch production. To maintain this balance, alternative electron flow pathways play critical roles, while inter-organelle redox exchanges regulate cellular energy states to support efficient carbon storage. The CO2-concentrating mechanism (CCM) enhances photosynthetic efficiency by concentrating CO2 at Rubisco, energized by ATP from photosynthetic electron transport. This review examines how light receptors, energy-producing pathways, and the CCM interact to regulate carbon metabolism in microalgae, emphasizing their collective roles in balancing energy supply and carbon storage.