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

## TL;DR

Climate change is altering ocean light patterns, which could disadvantage phytoplankton that can adjust their pigments to different light conditions.

## Contribution

The study introduces a global model showing how reduced spectral variability affects the competitive advantage of flexible phytoplankton pigment types.

## Key findings

- Reduced variability in ocean light fields makes flexible pigment phytoplankton less competitive.
- Synechococcus pigment types could serve as bioindicators of ecosystem changes due to climate.
- Transitional ocean zones with variable light spectra are shrinking under climate change.

## Abstract

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.

Climate change is altering the spectral quality of the underwater light field, yet its global impact on phytoplankton pigment strategies remains unclear. Our model projections show that while the ocean is shifting toward bluer or greener states, the transitional zones characterized by variable spectral light fields are contracting. Using Synechococcus as a study organism, we demonstrate that this reduction in spectral variability diminishes the competitive advantage of phytoplankton with flexible pigment content (Chromatic Acclimators) relative to specialists adapted to blue or green environments. These findings highlight how resolving functional diversity enhances our ability to diagnose ecosystem changes driven by climate change.