Temperature-dependent akinete formation strategies of the harmful cyanobacterium Dolichospermum circinale

Cyanobacteria from the orders Nostocales and certain Stigonematales form akinetes, spore-like dormant cells that allow them to survive adverse environmental conditions. Temperature is known to be one of the key factors affecting akinete formation, but there is currently little known about akinete formation during cell growth over a wide range of temperature conditions and its relation to the overall survival strategy of cyanobacteria. Therefore, in the current study, we conducted a temperature-controlled experiment to analyze the akinete formation of a harmful cyanobacterium Dolichospurmum circinale using a growth chamber. We measured the concentration and size of both vegetative cells and different types of akinetes (free, attached, and empty type) under varying temperatures (5–25 °C). We also analyzed the buoyant ability and vertical migration velocity of trichomes along with changes in the volume of vegetative cells and akinetes. The total akinete concentration and ratio (number of akinetes to total number of cells) were both found to be higher at high temperatures (20–25°C) than they were at low temperatures (5–10°C) (p<0.05). Meanwhile, the rate of formation of akinetes (both free and attached akinetes) was highest at low temperature (10 °C) and decreased with increasing temperature. The rate of empty akinete formation increased with increasing temperature and was highest at 25°C, indicating that most of the akinetes produced under high temperature conditions germinated. The change in vegetative cell size was proportional to the increase in the growth rate in response to increasing temperature (p<0.05). At high temperature, vegetative cells exhibited positive buoyancy and higher vertical migration velocity, while at low temperature, they exhibited negative buoyancy and relatively low migration velocity. Akinete size was larger at low temperature than it was at high temperature. These findings suggest that akinetes play an important role in maintaining populations in the water column, with a link between akinete formation and germination during summer cyanobacteria blooms. This information is expected to contribute to a deeper understanding of the D. circinale life cycle.