Colonies of the bloom-forming cyanobacterium Microcystis. Image credit: Yuri Sinzato (CC BY 4.0)
Anyone who has seen a lake turn green in summer has witnessed the visible sign of a cyanobacterial bloom. Cyanobacteria are photosynthetic microorganisms, and some species are toxic and cause major environmental problems. A common nuisance species is Microcystis, which lives in colonies – clusters of cells held together by a slimy material called extracellular polymeric substance, which acts like a biological glue.
These colonies can reach large dimensions and thus float faster than single cells and receive more light, making blooms harder to control. They can grow through cell division, in which newly formed cells remain attached, or through aggregation, in which separate cells or colonies collide and stick together. Water movement also plays a crucial role. Fluid flow can promote aggregation, but it can also generate shear forces that break colonies apart. So far, it was unclear which of these competing processes dominates under realistic lake conditions or during mitigation efforts such as artificial mixing.
To find out how fluid flow influences the size of Microcystis colonies, Sinzato et al. studied colonies from both laboratory cultures and field samples under controlled shear flow. They quantified changes in colony size and compared the results with a mathematical model that incorporates both fragmentation (breaking apart) and aggregation (sticking together) processes.
The results showed that Microcystis colonies are surprisingly mechanically robust and are not readily fragmented by wind-driven or artificial mixing. Moreover, colonies formed through cell division were significantly stronger than those formed through aggregation, most likely because aggregated cells do not have enough sticky material to form a strong bond.
Colony size of cyanobacteria affects buoyancy, light exposure and bloom development, which, in turn, influences the effectiveness of bloom forecasting and control strategies. The findings of Sinzato et al. shed light on the mechanisms underlying colony formation and could help improve predictive models and mitigation strategies for harmful cyanobacterial blooms. They may also assist engineers in understanding how dense cell aggregates interact with fluid flow in algal bioreactors.
More broadly, the combined approach of flow studies, imaging, and modeling used in this study could be applied to other biological aggregates exposed to flow, such as marine snow. However, further work is needed to test this framework across a wider range of species and environmental conditions.
