Phytoplankton With Flexible Pigment Content Disadvantaged by Projected Future Decrease in Variability of the Ocean Light Spectrum.

Phytoplankton are key components of ocean ecosystems that play a critical role in regulating Earth's climate. However, how climate-driven changes in light availability in the ocean will affect marine phytoplankton remains poorly understood. Here, we assess the impact of climate-induced shifts in the spectral quality of the underwater light field on the relative fitness of phytoplankton with distinct pigment traits using a global ecosystem model. We focus on Synechococcus pigment types, comparing light color specialists with a chromatic acclimator capable of adjusting its pigment composition. Under a high-emission scenario, the model simulation projected an increase in the average blue-to-green ratio across 76% of the ocean area by the end of the 21st century, while 24% of the simulated ocean showed a shift toward greener wavelengths. Regions characterized by larger seasonal variability in blue-to-green ratio values appeared to be reduced due to climate-driven spectral changes. We find that reduced variability in the ocean light field makes the chromatic acclimators' plasticity less advantageous, and this pigment type was most negatively affected. These findings highlight the potential of Synechococcus pigment types as functional bioindicators of ecosystem change and underscore the importance of incorporating functional diversity in global models to better predict phytoplankton responses to changing ocean conditions.