Is there a difference of temperature sensitivity between marine phytoplankton and heterotrophs?

The temperature sensitivity of phytoplankton growth rates, parameterized as the activation energy (Ea) in the Boltzmann-Arrhenius equation, is critical to determining how global warming will affect marine ecosystems and the efficiency of the biological pump in the ocean. We applied both linear and nonlinear regression models to two laboratory temperature-growth experimental datasets to estimate the Ea of each taxon of phytoplankton and heterotrophic protists. We found that phytoplankton Ea and normalized growth rates depended strongly on community composition. Diatoms grew more rapidly and had lower Ea values, whereas cyanobacteria grew more slowly and had higher Ea values. The phytoplankton Ea was underestimated by a single OLS regression on the pooled dataset because slowly growing cyanobacteria dominated in warm, oligotrophic ocean gyres, and rapidly growing diatoms dominated in cold, nutrient-rich waters. By contrast, the median Ea values estimated from individual experiments did not differ between phytoplankton and heterotrophic protists. Our results suggest that phytoplankton community composition needs to be considered when trying to predict the effects of ocean warming on ecosystem productivity and metabolism.