Advancing our understanding of photosynthesis: discoveries and insights from aquatic plants.

Aquatic plants inhabit highly dynamic environments characterized by rapid and extreme fluctuations in CO2 availability, light intensity, hydrodynamics, temperature, and pH. These conditions have driven the evolution of exceptional photosynthetic plasticity. This review synthesizes recent advances in understanding how higher aquatic plants deploy a diverse array of carbon-concentrating mechanisms (CCMs), including canonical C3 and C4 pathways, C4-like and CAM-like metabolic flexibility, and efficient biophysical CCMs based on HCO3 - uptake, proton extrusion, and the coordinated action of internal and external carbonic anhydrases. We highlight key evolutionary innovations unique to aquatic lineages, such as single-cell C4 photosynthesis, dimorphic chloroplasts, and heterophylly, which together facilitate rapid optimization of carbon assimilation under shifting ecological conditions. A detailed comparison between terrestrial model species and aquatic macrophytes further illustrates how contrasting environmental pressures have shaped distinct morphological, anatomical, and physiological strategies. We also emphasize emerging model systems, including Ottelia ovalifolia, O. alismoides, Hygrophila difformis, and Rorippa aquatica, which provide powerful platforms for investigating the evolution, regulation, and integration of CCMs with phenotypic plasticity. Collectively, aquatic plants represent underexplored yet promising systems with significant potential for advancing carbon neutrality strategies, freshwater ecosystem management, crop improvement, and synthetic biology.