Light-dependent processes in algae across scales: from cellular physiology to ecological organization.

Sunlight provides both energy for photosynthesis and environmental information for algae. Beyond supporting a large share of global primary production, light encodes cues that regulate physiology, development, and ecological performance. This review examines how algae detect and interpret light and how these signals are integrated into cellular and organismal responses. Across photosynthetic algal lineages, many light responses arise from the integration of 2 input streams: photoreceptor-mediated sensory signaling and metabolic redox signals generated by photosynthetic electron transport. Across algal lineages, diverse photoreceptors-including rhodopsins, cryptochromes, phototropins, phytochromes, UV-B photoreceptor UVR8, and lineage-specific sensors such as aureochromes-detect spectral, directional, and temporal properties of light. Their outputs converge with chloroplast-derived signals-including redox state, reactive oxygen species, ion fluxes, and second messengers-to form recurring regulatory architectures that coordinate photosynthesis, photoprotection, pigment biosynthesis, metabolism, and gene expression. Through these pathways, light also regulates development and behavior, including motility, phototropism, circadian rhythms, cell-cycle progression, and life-cycle transitions. These processes operate across biological scales, from intracellular signaling to ecological organization, enabling algae to occupy diverse light environments. Emerging genomic, structural, and functional approaches will clarify how photoreceptor diversity and metabolic sensing are integrated across scales and may support predictive models linking cellular regulation to ecological performance under changing light climates.