Rapid flotation of mediated by high light exposure: implications for surface scum formation and cyanobacterial species succession.

Increasing occurrences of surface scum have been observed in the context of global climate change and the increase in anthropogenic pollution, causing deteriorating water quality in aquatic ecosystems. Previous studies on scum formation mainly focus on the buoyancy-driven floating process of larger colonies, neglecting other potential mechanisms. To study the non-buoyancy-driven rapid flotation of , we here investigate the floating processes of two strains of single-cell species ( and ), which are typically buoyant, under light conditions (150 μmol photons s m). Our results showed that exhibited fast upward migration and formed surface scum within 4 hours, while did not form visible scum throughout the experiments. To further explore the underlying mechanism of these processes, we compared the dissolved oxygen (DO), extracellular polymeric substance (EPS) content, and colony size of in different treatments. We found supersaturated DO and the formation of micro-bubbles (50-200 µm in diameter) in treatments. produces bubbles in small quantities and small sizes. Additionally, produced more EPS and tended to aggregate into larger colonies. had much more derived-soluble extracellular proteins and polysaccharides compared to . At the same time, contains abundant functional groups, which was beneficial to the formation of agglomerates. The surface scum observed in is likely due to micro-bubbles attaching to the surface of cell aggregates or becoming trapped within the colony. Our study reveals a species-specific mechanism for the rapid floatation of , providing novel insights into surface scum formation as well as succession of cyanobacterial species.