Adaptive phenotypic evolution of Skeletonema costatum to ocean acidification and warming with trade-offs from a multi-year outdoor experiment

Human-induced climate change is increasing variability in marine
environments, significantly affecting marine organisms and ecosystems.
While marine diatoms can adapt to ocean acidification and warming in
stable laboratory settings, their responses to long-term environmental
changes under natural variability remain unclear. To investigate this, we
cultivated Skeletonema costatum in outdoor semi-continuous cultures for
over three years, exposing them to fluctuating natural light and
temperature that tracked the in situ sea surface temperatures. We
simulated current and future ocean conditions through four treatments:
ambient CO2 and temperature (LTLC), elevated CO2 (LTHC), elevated
temperature (+ 4°C, HTLC), and combined increases (HTHC). After 1,396
days, we assessed populations in two assay environments (20°C, 400 ppm CO2
and 24°C, 1,000 ppm CO2) for adaptations in growth rate, pigment
composition, and photosynthesis. The HTLC-selected group showed highest
growth rates in the HTHC assay environment, while the LTLC-selected group
grow fastest in the LTLC assay environment, indicating adaptive evolution.
Furthermore, populations selected under elevated conditions exhibited
lower fitness in LTLC environments, highlighting a trade-off and
underscoring the complexity of evolutionary adaptation in marine diatoms.
Understanding these mechanisms is crucial for predicting phytoplankton
dynamics and their role in marine ecosystems, especially in response to
climate change.