Are temperature sensitivities of Synechococcus and Prochlorococcus growth suppressed by resource supply in the oligotrophic Western Pacific Ocean?

Temperature sensitivity of phytoplankton growth rates is critical for predicting the effect of global warming on marine primary production and the efficiency of biological carbon pump. To investigate how resource availability affects the temperature sensitivity of phytoplankton growth rate, we estimated the temperature sensitivity (activation energy, E_a) of two dominant phytoplankton population (i.e., Synechococcus and Prochlorococcus) in the oligotrophic Western Pacific Ocean under two nutrient supply scenarios through short-term temperature modulation dilution experiments and meta-analysis on a compiled dataset of phytoplankton growth rate estimated by dilution experiments. Our results showed that E_a values of Synechococcus growth rate under in situ nutrient condition were significantly lower than those estimated under nutrient-replete condition, indicating that the response of Synechococcus growth in the oligotrophic ocean to ocean warming could be weaker. In contrast, E_a values of Prochlorococcus growth rate showed no difference between the two nutrient supply scenarios because the in situ nutrient concentration was sufficient for their growth. In addition, we found the reduced E_a of Synechococcus growth was most likely related to the enzyme kinetics, because the half-saturation constants for nutrient uptake also increased with increasing temperature. The temperature sensitivity of half-saturation constants and the level of nutrient limitation can counteract the response of phytoplankton growth rate to increasing temperature. Therefore, we highlighted the importance of considering nutrient availability when evaluating the responses of phytoplankton growth and production to climate warming especially in oligotrophic ocean.