Surviving the estuary: how colony morphology and nutrient history shape Microcystis salt tolerance

Harmful freshwater cyanobacteria, such as Microcystis, are increasingly reported in brackish estuaries, yet the extracellular mechanisms that allow freshwater colonies to withstand salt stress in nature remain unresolved. This study combined field surveys with in situ bottle and enclosure experiments to examine how colony size, extracellular polysaccharides (EPS), and upstream nitrogen availability shape the salinity tolerance of Microcystis transported from upstream lakes to the brackish water. Larger colonies were more abundant under higher salinity in the estuary, and redundancy analysis showed that salinity and water temperature were the primary correlates of colony size distributions. In situ suspended bottle experiments demonstrated that larger colonies maintained positive growth at higher salinity thresholds than small colonies, and that EPS accumulation increased with colony size. Small colonies showed rapid EPS synthesis under low-salt stress but were unable to sustain this response once salinity exceeded 10-12‰. Enclosure experiments further revealed that low nitrogen availability in upstream freshwater promoted the formation of larger colonies, enhanced EPS production, and strengthened physiological resistance to salt stress during downstream transport. These findings indicate that colony morphology and EPS act as key mediators of Microcystis survival along freshwater-estuarine gradients, and that nitrogen limitation upstream can facilitate bloom persistence in brackish water. Integrating colony size, EPS dynamics, and nutrient regimes into monitoring and management frameworks will improve early warning capacity for bloom propagation in estuaries under increasing anthropogenic and climate-driven pressures.