Light-driven intracellular and extracellular polymer dynamics regulate colony morphology and buoyancy in Microcystis

Microcystis blooms are a major ecological and water-quality concern in eutrophic freshwaters because persistent surface accumulation exacerbates water-quality deterioration and complicates management. Light intensity shapes growth and the dynamics of intracellular polymers (IPS) and bound extracellular polymers (bEPS), but their respective roles in colony development and buoyancy regulation remain unclear. Here, natural Microcystis-dominated communities were exposed to five light treatments (0-216 μmol photons m^-2 s^-1) for 54 days, and colony growth, colony size, IPS, bEPS, and sinking dynamics were quantified. Growth generally increased with irradiance, except under the highest light treatment, where early proliferation was suppressed. IPS and bEPS were negatively correlated with growth, indicating a trade-off between cell proliferation and polymer accumulation. Colony enlargement was more closely associated with bEPS, whereas high irradiance favored IPS accumulation and increased sinking propensity. Floating colonies consistently maintained lower IPS than sinking colonies, indicating a closer association of IPS with buoyancy loss. Under prolonged high light, substantial sinking persisted even at relatively low polymer levels, indicating the involvement of additional processes in buoyancy loss. Overall, these results show that light regulates Microcystis buoyancy through distinct effects on IPS, bEPS, and colony structure, thereby influencing surface scum persistence and downward redistribution of bloom biomass and providing a mechanistic basis for bloom-risk assessment and for identifying critical stages for monitoring and management in eutrophic freshwaters.