Oxygen underpins the temperature-size relationship of protist plankton.

UNLABELLED: Temperature is known to affect organismal size, yet the underlying mechanism has been debated. Here, we develop a simple growth model of heterotrophic plankton and explain the temperature-size effect based on the demand and supply of oxygen. The model applies the growth rate-temperature relationship based on protein stability, capturing six data sets of a planktonic ciliate Urotricha. The model then computes the oxygen demand, based on the growth rate, which must be balanced by the diffusive uptake of oxygen. Because the smaller cell size favors oxygen uptake per cell volume, the predicted volume-temperature relationship is roughly the inversion of the growth rate-temperature relationship. With this inversive pattern, models are able to capture the data. This model-data consistency indicates that Urotricha may adjust their body size to meet the oxygen demands. Given that similar inversive relationships have been observed in other organisms, we hypothesize that oxygen demand and uptake are the key factors in constraining the volume of heterotrophic planktonic organisms.

IMPORTANCE: The temperature-size relationship is a famous rule in physiological ecology based on common observations, but the mechanism underlying this rule has remained unclear. In this study, we develop a simple growth model of heterotrophic plankton, which explains the temperature-size relationship based on the demand and supply of oxygen. The model predicts temperature change will cause growth rate change; with higher growth rates, the increase of O2 demand causes a smaller cellular size. The model results also capture six datasets of Urotricha ciliates, suggesting that these organisms may adjust their body size to meet oxygen requirements. Given that similar patterns have been observed in other organisms, we hypothesize that oxygen demand and uptake are key factors constraining cell volume. Our study highlights oxygen as an intermediate factor underpinning temperature and cell volume, providing an explanation for the temperature-size relationship.