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

Aim Ecological and evolutionary forces shape the functional traits of species within and across environments, generating biogeographical patterns in traits. We aimed to: (1) determine the extent to which temperature traits of phytoplankton are adapted to their local environment, and (2) detect and explain differences in patterns of adaptation between functional groups (reflecting evolutionary history) and across ecosystems (freshwater versus marine). Location We used laboratory-measured data on phytoplankton strains isolated from marine (76 degrees N to 75 degrees S) and freshwater ecosystems (80 degrees N to 78 degrees S). Methods We studied variation in five temperature traits: optimum temperature for growth (T-opt), maximum and minimum persistence temperature (T-max, T-min), temperature niche width and maximum growth rate, estimated in 439 strains from over 200 species. We tested whether these traits change along environmental temperature gradients (across latitude and ecosystems) and also investigated differences in trait-environment relationships related to evolutionary history (functional group identity). We used mixed models to evaluate our hypotheses while accounting for intraspecific variation. Results We identified three patterns caused by adaptation and community assembly: (1) T-opt, T-max and T-min decline sharply with latitude; (2) T-opt, T-max and T-min are similar across all functional groups at the equator, where temperature variation is low; and (3) T-opt and T-max are higher in freshwater locations than marine locations at similar latitudes. Additionally, evolutionary history explained substantial variation in all traits: functional groups differ strongly in their niche widths and maximum growth rates, as well as their T-opt, T-max, and T-min relationships with latitude. Main conclusions Globally, phytoplankton temperature traits are well adapted to local conditions, changing across ecosystems and latitude. Functional groups differ strongly in their patterns of adaptation: traits are similar in hot tropical environments, but diverge at temperate latitudes. These differences reflect two possible evolutionary constraints: cyanobacterial inability to adapt to low temperatures and differences in nutrient requirements between groups.