Coordinating photosynthesis and respiration: Evolution and functional integration of mitochondria and chloroplasts in algae

The coordination of photosynthesis and respiration is central to cellular energy balance, yet in algae this relationship exhibits exceptional diversity. Shaped by successive endosymbioses, algal lineages represent natural experiments in merging two energy systems of distinct bacterial ancestry: the chloroplast and the mitochondrion. Their structural proximity, shared redox pathways, and dual-targeted proteins enable dynamic communication between photosynthetic and respiratory metabolism. Recent imaging and multi-omics studies reveal that this interaction is highly responsive to environmental variables such as light intensity, nutrient availability, and oxidative stress. In diatoms, mitochondria envelop the plastid to exchange ATP and reducing power, whereas in green algae and euglenoids, malate/oxaloacetate shuttles, alternative oxidases, and cyclic electron flow collectively stabilize chloroplast redox states. This functional coupling optimizes CO2 fixation and photoprotection under stress and underlies the metabolic flexibility of mixotrophic species such as Euglena gracilis. This review synthesizes the current understanding of mitochondria–chloroplast integration in algae from evolutionary, structural, and mechanistic perspectives, highlighting photosynthesis–respiration coordination as a unifying physiological principle. By elucidating how inter-organelle networks sustain carbon assimilation and redox homeostasis, these insights advance our understanding of algal productivity and resilience and inform strategies for improving energy efficiency in photosynthetic systems.