Environment and evolutionary history determine the global biogeography of phytoplankton temperature traits

Background/Question/Methods

Functional traits vary across broad spatial gradients, reflecting a combination of adaptation to local conditions, species interactions, and both ecological and evolutionary constraints. Characterizing worldwide patterns in these traits can therefore reveal signals of historical selection, as well as the factors constraining community composition and species distributions. Understanding these contributors to trait variation can also improve our ability to predict the ecological and evolutionary consequences of environmental change.

We examined global variation in important temperature-response traits in 442 isolates (252 species) of phytoplankton isolated across >150 degrees of latitude. The traits are: 1) optimal temperature for growth, 2) maximum persistence temperature, 3) minimum persistence temperature, 4) temperature niche width, and 5) maximum growth rate. We tested for variation in these traits attributable to latitudinal differences, environment type (marine vs. freshwater), and evolutionary history (or functional group). 

Results/Conclusions

We found that latitude, environment type, and functional group together explained 60-90% of the variance in all of these traits. Trait variation across latitude and environment type is consistent with selection caused by environmental gradients in mean temperature and temperature variability. Most notably, major functional groups exhibited strong differences in trait-environment relationships. Temperature traits of all groups were very similar in the tropics, where environmental temperature variation is low, but they diverged at temperate latitudes. Moreover, trait differences at temperate latitudes are consistent with seasonal succession patterns at these latitudes (e.g. diatoms have the lowest optimum temperatures and bloom in spring and fall; cyanobacteria have the highest optima and bloom in summer). Our results suggest that selection has driven niche partitioning of the temperature spectrum in variable temperate environments, while the low variability of tropical environments has led to trait convergence.

Our study illustrates the importance of using trait-environment relationships to elucidate how environmental variation, evolutionary history, and selection interact to determine physiological patterns, thereby informing predictions of future ecological and evolutionary change.