Effects of gradual warming and marine heatwaves on the growth and metabolism of Phaeocystis globosa

The effects of extreme climate events on phytoplankton, particularly under different warming patterns, remain uncertain. This study simulated three temperature scenarios: constant temperature (control), gradual warming and a marine heatwave (MHW), and investigated their impacts on the growth and metabolism of Phaeocystis globosa (P. globosa). Both warming treatments significantly inhibited total cell growth of P. globosa but resulted in different effects: growth inhibition was manifested mainly as the inhibition of the growth of colonial cells, whereas the growth of solitary cells was inhibited only during phase III of gradual warming, indicating that colonial cells are more sensitive to warming. In terms of metabolism, gradual warming significantly increased the content of transparent exopolymer particles (TEPs) and hemolytic toxins per cell; the MHW increased only the TEP content per cell but did not significantly influence the dimethylsulfide (DMS) content per cell. These findings suggested that P. globosa may adopt growth inhibition and metabolism enhancement strategy in response to temperature stress. This strategy exhibited the following temporal dynamic characteristics: the response to gradual warming occurred in phases II (days 6–10) and III (days 11–15), whereas the response to the MHW was delayed and became significant only in phase III after the MHW had ended. Overall, this study highlights the differences in the effects of different warming modes (gradual and extreme) on phytoplankton, and the consideration of the lag effect is crucial for assessing the ecological influence of MHWs, providing critical insights for predicting the population dynamics of phytoplankton under climate change.