Phytoplankton are major players in the world’s biogeochemical cycles, and are the foundation of most aquatic food webs. Phytoplankton and other marine organisms are facing unprecedented environmental change that threatens to dramatically alter their current geographical ranges and cause massive changes in diversity and abundance across the world’s oceans. However, given high rates of reproduction and massive population sizes, some phytoplankton may be capable of rapid evolution in response to (e.g.) ocean acidification and warming, thus mitigating some ecological impacts of global climate change. A number of recent high-profile evolution experiments have sought to understand how phytoplankton evolve to cope with environmental change, though to date most have focused on ocean acidification due to increased atmospheric PPCO2. In an attempt to explore the evolution of phytoplankton traits in response to temperature selection, we have maintained the marine diatom Thalassiosira pseudonana in replicate populations evolving at two different temperatures (16 and 31 °C) for 18 months (~400 generations).
Results/Conclusions
We predicted and have now observed evolutionary change in several traits associated with the thermal reaction norm, including divergence in the optimal temperature (Topt) and the upper critical temperature (TUcrit) for population growth between low- and high-temperature selected populations, as well as an increase in maximum growth rate at Topt (µopt). Concurrent with these changes, many replicate populations have experienced significant reductions in nitrate affinity at their respective selection temperatures, indicating that evolution of high maximum growth rate in response to temperature selection carries with it a cost to competitive ability for NO3 (a trade-off). These results are an important step not only to understanding how phytoplankton adapt to temperature change, but also the implications of thermal adaptation in phytoplankton for global marine biogeochemistry.