Quantifying evolutionary changes to temperature-CO2 growth response surfaces in <i>Skeletonema marinoi</i> 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-CO₂ growth response surfaces of two strains of <i>Skeletonema marinoi</i> that were each previously adapted to four different temperature × CO₂ combinations. These adapted strains were then grown under a factorial combination of five temperatures and five CO₂ concentrations to capture the temperature-CO₂ response surfaces for their unacclimated growth rates. The development of the first complete temperature-CO₂ response surfaces showed the optimal CO₂ concentration for growth to be substantially higher than expected future CO₂ levels (~6000 ppm). There was minimal variation in the optimal CO₂ concentration across the tested temperatures, suggesting that temperature will have a greater influence on growth rates compared to enhanced CO₂. Optimal temperature did not show a unimodal response to CO₂, either due to the lack of acclimation or the highly efficient CO₂ concentrating mechanisms, which diatoms (e.g. <i>Skeletonema</i>) can up-/downregulate depending on the CO₂ conditions. We also found that both strains showed evidence of evolutionary shifts as a result of adaptation to temperature and CO₂. 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.