Quantifying evolutionary changes to temperature-CO growth response surfaces in after adaptation to extreme conditions.

Global warming and ocean acidification are having an unprecedented impact on marine ecosystems, yet we do not yet know how phytoplankton will respond to simultaneous changes in multiple drivers. To better comprehend the combined impact of oceanic warming and acidification, we experimentally estimated how evolution shifted the temperature-CO growth response surfaces of two strains of that were each previously adapted to four different temperature × CO combinations. These adapted strains were then grown under a factorial combination of five temperatures and five CO concentrations to capture the temperature-CO response surfaces for their unacclimated growth rates. The development of the first complete temperature-CO response surfaces showed the optimal CO concentration for growth to be substantially higher than expected future CO levels (~6000 ppm). There was minimal variation in the optimal CO concentration across the tested temperatures, suggesting that temperature will have a greater influence on growth rates compared to enhanced CO. Optimal temperature did not show a unimodal response to CO, either due to the lack of acclimation or the highly efficient CO concentrating mechanisms, which diatoms (e.g. ) can up-/downregulate depending on the CO conditions. We also found that both strains showed evidence of evolutionary shifts as a result of adaptation to temperature and CO. The evolutionary response differed between strains, underscoring how genetic differences (perhaps related to historical regimes) can impact phytoplankton performance. Understanding how a dominant algal species responds to multiple drivers provides insight into real-world scenarios and helps construct theoretical predictions of environmental change.