Variations in the buoyancy response of Microcystis aeruginosa to nitrogen, phosphorus and light

Microcystis aeruginosa displays a range of variability in buoyancy in response to light which is dependent upon the previous nutrient or light history of the cell. The short-term buoyancy response to light is nested into a longer term response, over a period of days, as cyanobacteria respond to their nutrient and light climate, which may be manifested in their gas vesicle volume, photosynthetic rate and carbohydrate metabolism. Microcystis aeruginosa cultures were subjected to a range of nitrogen (N), phosphorus (P) or light pre-treatments before exposure to saturating irradiance, and these results are used to illustrate that populations can display a range of buoyancy responses dependent upon subtle changes in nutrient and light conditions. N-limited (0 μM) cells suffered a dilution in gas vesicle volume and increased carbohydrate content, which resulted in a loss of buoyancy. Cells in 10 μM N increased their gas vesicle content during light exposure; however, this was insufficient to maintain buoyancy in the majority of cells as carbohydrate increased. Cells in 100 μM N increased their gas vesicle volume, metabolized carbohydrate more efficiently than the N-limited treatments and retained positive buoyancy. During light exposure, there was minimal change in buoyancy in 0 μM P pre-treated cells, although there was a dilution in gas vesicle volume and an increase in carbohydrate. In 0.5 μM P pre-treated cells, the proportion of individuals floating did not change, although gas vesicle volume and carbohydrate increased. There was a significant increase in gas vesicle volume in 10 μM P pre-treated cells during light exposure, and some buoyancy loss due to carbohydrate accumulation. Cells grown in the range of light intensities tested all increased their gas vesicle volume during the 24 h light treatment. The actual rate of gas vesicle production was equivalent for all three light pretreatments; however, the growth-dependent dilution rate was determined by the previous light history, which resulted in different relative gas vesicle volume per cell. There was minimal increase in carbohydrate per cell during the light period, yet all treatments displayed the classical buoyancy loss response.