The disappearance of sea ice in polar regions due to global warming not only increases the amount of light entering the ocean, but also changes its color. These changes have far-reaching consequences for photosynthetic organisms such as ice algae and phytoplankton. That is the conclusion of new research published in Nature Communications, led by marine biologists Monika Soja-Woźniak and Jef Huisman from the Institute for Biodiversity and Ecosystem Dynamics (IBED) at the University of Amsterdam.
The international research team, which also included physical chemist Sander Woutersen (HIMS/UvA) and collaborators from the Netherlands and Denmark, investigated how the loss of sea ice alters the underwater light environment. Sea ice and seawater differ fundamentally in how they transmit light. Sea ice strongly scatters light and reflects much of it, while allowing only a small amount to penetrate. Yet, this limited amount of light still contains almost the full range of visible wavelengths. In contrast, seawater absorbs red and green light, while blue light penetrates deep into the water column. This is what gives the ocean its blue color.
Molecular vibrations of water
Another key difference between ice and liquid water lies in the role of molecular vibrations. In liquid water, H₂O molecules are free to move and vibrate, which leads to the formation of distinct absorption bands at specific wavelengths. These bands selectively remove portions of the light spectrum, creating gaps in the light available for photosynthesis.
Previous research by Maayke Stomp and Prof. Huisman demonstrated that these molecular absorption features create 'spectral niches'—distinct sets of wavelengths available for photosynthetic organisms. Phytoplankton and cyanobacteria have evolved a diversity of pigments tuned to the different spectral niches, shaping their global distribution across oceans, coastal waters, and lakes.
In ice, however, water molecules are locked into a rigid crystal lattice. This fixed structure suppresses their ability for molecular vibrations and thereby alters their absorption features. As a consequence, ice lacks the absorption bands of liquid water, and hence a broader spectrum of light is preserved under sea ice. This fundamental difference plays a key role in the spectral shift that occurs as sea ice melts.
Ecological implications
As sea ice disappears and gives way to open water, the underwater light environment shifts from a broad spectrum of colors to a narrower, blue-dominated spectrum. This spectral change is crucial for photosynthesis.
"The photosynthetic pigments of algae living under sea ice are adapted to make optimal use of the wide range of colors present in the little amount of light passing through ice and snow," says lead author Monika Soja-Woźniak. "When the ice melts, these organisms suddenly find themselves in a blue-dominated environment, which provides a lesser fit for their pigments."
Using optical models and spectral measurements, the researchers showed that this shift in light color not only alters photosynthetic performance, but may also lead to changes in species composition. Algal species specialized in blue light may gain a strong competitive advantage in comparison to ice algae.
According to Prof. Huisman, these changes can have cascading ecological effects. "Photosynthetic algae form the foundation of the Arctic food web. Changes in their productivity or species composition can ripple upward to affect fish, seabirds, and marine mammals. Moreover, photosynthesis plays an important role in natural CO2 uptake by the ocean."
The study highlights that climate change in the polar regions does more than melt ice—it causes fundamental shifts in key processes such as light transmission and energy flow in marine ecosystems.
The results underscore the importance of incorporating light spectra and photosynthesis more explicitly in climate models and ocean forecasts, especially in polar regions where environmental change is accelerating at an unprecedented rate.
Publication data:
Soja-Woźniak M, Holtrop T, Woutersen S, van der Woerd HJ, Lund-Hansen LC & Huisman J. 2025. Loss of sea ice alters light spectra for aquatic photosynthesis. Nature Communications 16: 4059.
URL: https://doi.org/10.1038/s41467-025-59386-x
