The surface as a primary response zone in Microcystis colonies under turbulence: Insights from integrating...

Microcystis, defined by its distinctive colonial morphology, is a key species in harmful cyanobacterial blooms (cyanoHABs) in eutrophic lakes. Turbulence affects its growth in an intensity-dependent manner, characterized by a pattern of low-level promotion but high-level inhibition. However, the mechanisms by which turbulence affects both cell growth and morphological development in Microcystis colonies remain unclear. This study found biphasic relationships between the dissipation rate and both growth and colonial morphology (e.g., colony size, tightness, and colony crowding). Low-intensity turbulence, mimicking the median hydrodynamic conditions of shallow lakes, triggered a dramatic growth surge of 241.1%. This increase was observed relative to a baseline growth rate of 0.016 day^-1, a value comparable to typical growth rates observed in natural field environments. A key contrast emerged: low-intensity turbulence sharply increased division in surface cells but elicited only a weak response interiorly; in contrast, extreme turbulence inhibited surface cells but allowed protected interior cells to maintain their division frequency. This indicates that the impact of turbulence on cyanobacterial growth is intricately linked to colonial morphological traits, underscoring the necessity for integrated research encompassing physical hormesis, hydrodynamics, and morphology. We propose that hydrodynamic conditions driven by relatively low wind speeds may serve as a critical factor promoting cyanoHABs, warranting further validation through large-scale lake studies in the future.