Rising temperature accelerates buoyancy regulation and vertical migration of the bloom-forming cyanobacterium Microcystis.

Eutrophication and global warming intensify the formation of harmful cyanobacterial blooms worldwide. Many bloom-forming cyanobacteria can regulate their buoyancy, which results in vertical migration through the water column. Density changes during vertical migration depend largely on carbohydrate production in the light and carbohydrate respiration in the dark, which are temperature-dependent processes. However, the effects of temperature on the vertical migration of Microcystis have not been fully investigated yet. Therefore, this study experimentally investigated density regulation at a range of temperatures (17.5, 20, 25 and 30 °C), and developed a mathematical model to predict the temperature-dependent impact of density regulation on vertical migration patterns for colonies of different sizes and in waters with different turbidities. Our experimental results show that elevated temperature enhances carbohydrate production rates in the light and carbohydrate respiration rates in the dark, and thereby accelerate changes in cellular density. Consequently, the model predicts that higher temperature leads to faster migration cycles and increases the number of migration cycles per day. Small colonies display chaotic migration at 17.5 °C, while they synchronize their vertical migration with the day-night cycle in warmer waters. An increasing colony size promotes faster vertical migration, whereas increasing turbidity of the water column reduces migration depth. The modelled migration patterns show that migration at 25 °C yields the highest net daily growth rate. To conclude, our results reveal that rising temperature will accelerate buoyancy changes of Microcystis, which modifies their vertical migration patterns and thereby affects the growth and bloom formation of these harmful cyanobacteria.