Are temperature sensitivities of Prochlorococcus and Synechococcus impacted by nutrient availability in the subtropical northwest Pacific?

Temperature sensitivity of phytoplankton growth rate is crucial for predicting the effect of global warming on oceanic primary productivity and the efficiency of the biological carbon pump. To investigate how nutrient availability affects the temperature sensitivity of phytoplankton growth, we estimated the activation energy (Ea) of two dominant picocyanobacteria (Prochlorococcus and Synechococcus) in the subtropical northwest Pacific using short-term temperature modulated dilution experiments. We also conducted a meta-analysis on a compiled dataset of picocyanobacteria growth rate estimated by the dilution technique. Our results revealed that the Ea of Synechococcus growth rate under in situ nutrient conditions was lower than under nutrient-replete conditions. The growth response of Synechococcus to warming could, therefore, be weaker under nutrient-limiting conditions than in nutrient-replete waters. In contrast, Ea values of Prochlorococcus growth rate showed no difference between the two nutrient supply scenarios. We also found that the reduced Ea of Synechococcus growth was most likely related to the increasing trend of the half-saturation constants for growth with increasing temperature. The temperature sensitivity of half-saturation constants and the level of nutrient limitation can counteract the response of Synechococcus growth rate to increasing temperature. Our results highlight the importance of considering nutrient availability when evaluating the responses of phytoplankton growth and primary production to climate warming, especially in the oligotrophic ocean.