Harmful freshwater cyanobacterial blooms of Microcystis form buoyant colonies held together by extracellular polymeric substances (EPS). Research combining controlled shear flows with microscopic imaging reveals that colonies formed via cell division are remarkably mechanically robust and resist fragmentation from natural wind mixing or artificial aeration systems. In contrast, colonies formed by flow-induced aggregation rely on weak intercellular bonds, demonstrating that natural bloom expansion is primarily driven by cell division rather than fluid-driven sticking, except within ultra-dense surface scums.
Physical Mechanisms of Fragmentation and Aggregation
When exposed to extreme shear stress, division-formed colonies undergo fragmentation through an erosion mechanism, continuously shedding single cells or tiny clusters from their outer layer rather than splitting in half. Cell division allows sufficient time for secreted EPS to fill intercellular spaces, endowing the colony matrix with substantial structural integrity and plastic yield resistance. Conversely, when single cells collide and aggregate under flow, incomplete EPS coverage creates fragile bonds that quickly rupture under mild hydrodynamic stress.
Dynamic Regimes and Bloom Mitigation
A mathematical population model using a dimensionless ratio (Ag) maps out distinct operational regimes based on fluid dissipation rates and cell concentrations. Under normal lake conditions, cell division strictly dominates colony growth, whereas flow-induced aggregation becomes significant only during dense surface scum events. Crucially, standard artificial mixing methods like bubble plumes cannot generate enough shear to tear apart division-formed colonies; instead, they successfully mitigate toxic blooms by dispersing buoyant colonies throughout the water column to suppress surface scum formation.