Temperature acclimation alters phytoplankton growth and production rates

Temperature is a major driver of phytoplankton growth and physiology, but despite decades of study on temperature effects, the influence of temperature fluctuations on the growth acclimation of marine phytoplankton is largely unknown. To address this knowledge gap, we subjected a coastal phytoplankton species, Heterosigma akashiwo, to ecologically relevant temperature shifts of 2-3 degrees C, cumulatively totaling 3-16 degrees C across a range from 6 degrees C to 31 degrees C over a 3-week period. Using a symmetric design, we show time dependent differences between growth rates and that these changes were related to the magnitude of the temperature shift, but not the direction. Cell size scaled inversely with temperature at a rate of -1.9 to -3.3%degrees C-1 at all except the highest temperature treatments > 25 degrees C. Intraspecific variability in growth rates increased exponentially with cumulative thermal shifts, suggesting thermal variability may be a driver of intraspecific variation. The observed acclimation effects on phytoplankton growth rates suggest that ignoring acclimation effects could systematically under or overestimate temperature-dependent primary production. Empirical results, contextualized with in situ coastal ocean temperature record, demonstrated that daily primary production could differ from current model assumptions utilizing acclimated rates by -33% to +36%. If broadly applicable to diverse phytoplankton species, these results have ramifications for predicting the ecology and production of phytoplankton in present day dynamic ecosystems and in future climate scenarios where thermal variability is expected to increase.

Automated summary

This study investigated the impact of temperature fluctuations on the growth acclimation of marine phytoplankton, revealing that the magnitude of temperature shifts affected growth rates, while the direction did not. Thermal variability may drive intraspecific variation, and ignoring acclimation effects could lead to systematic underestimation or overestimation of temperature-dependent primary production.