Ice Loss Is Transforming the Light-Absorption Properties of Seawater
As the Earth’s ice caps thaw, the absorption of light by polar seawater is undergoing a dramatic shift. That is the finding of Monika Soja-Woźniak from the University of Amsterdam and colleagues, as they considered how visible light is absorbed in different ways by frozen and liquid seawater [1]. Their results suggest that the ecology of photosynthetic organisms is likely undergoing a transformation in Earth’s polar regions, with far-reaching impacts on the marine ecosystems that depend on them.
When sunlight shines down on open seawater, the wavelengths it carries are absorbed at varying depths. While red light is absorbed just below the watery surface, longer wavelengths are only fully absorbed as the light penetrates deeper, where the spectrum is dominated by blue light.
This relation between wavelength and absorption depth isn’t completely smooth, however. At a handful of optical wavelengths, water molecules are driven into specific vibrational modes. At these wavelengths, any incoming photons are absorbed immediately, creating a series of sharp absorption peaks.
In 2007, researchers, including Soja-Woźniak’s colleague Jef Huisman, also of the University of Amsterdam, discovered a remarkable link between these peaks and the ecology of light-harvesting marine microorganisms. “These absorption features result in a series of ‘spectral niches’—distinct sets of wavelengths available for photosynthetic organisms,” Huisman explains. The team showed that the different pigments that had evolved in certain photosynthetic species are tuned to these different spectral niches. In the open ocean, for example, photosynthetic pigments are well adapted to harvest light in the blue spectral niche.
When seawater freezes, the situation changes entirely. For one, sea ice scatters light strongly but evenly across the visible spectrum, so that any sunlight that does manage to penetrate through to the liquid ocean beneath will carry all wavelengths in equal amounts.
However, this spectrum is also affected by the bonding structure of ice, where the water molecules are locked into a rigid crystal lattice. This fixed structure suppresses the ability of molecules to vibrate. As a result, the absorption peaks present in open seawater are almost entirely flattened out—creating a smooth, even spectrum.
Given the earlier findings of Huisman and his colleagues, Soja-Woźniak’s team predicted that the stark differences between the optical spectra of open and ice-covered seawater should have a profound impact on the photosynthetic organisms that inhabit each environment.
So far, these differences have proven extremely difficult to study with traditional methods. “Field campaigns are challenged by severe weather, and satellite observations suffer from persistent cloud cover or low winter light,” comments Bror Jönsson at the University of New Hampshire, who wasn’t involved in the study. That leaves biogeochemical ocean models as the main tool to understand and predict these systems. For researchers to glean useful results from these models, it is vital for them to reproduce the light spectra in both environments with pinpoint accuracy.
In their latest study, Soja-Woźniak, Huisman, and their colleagues approached this challenge from first principles. “We incorporated these optical properties into a state-of-the-art model in aquatic optics to compare spectra generated under sea ice and in open ocean water,” Soja-Woźniak describes. This model has been extensively applied to the open ocean but hasn’t yet been applied to sea ice.
Using this model, the researchers examined a wide range of marine ecosystems and considered many different types of ice. When focusing on the surface layers of the ocean, where most photosynthesis takes place, the team’s model confirmed that differences in the light-harvesting capabilities of organisms inhabiting open and ice-covered environments are directly linked to differences in the wavelengths of the light that reaches them.
Since sea ice allows a broader spectrum of light to penetrate, it encourages the evolution of photosynthetic organisms that combine multiple pigments to capture this full range. “Indeed, ice algae often look brown, as their pigments absorb many of the colors,” Soja-Woźniak says. In contrast, open ocean water selects for photosynthetic species with pigments that are specialized to a blue-light environment.
The team’s results are starkly relevant in a rapidly heating climate. As sea ice thaws and gives way to open water, these results suggest that the underwater light environment will shift from a broad spectrum of colors to a narrower, blue-dominated spectrum. In regions of ocean that once remained frozen for much of the year, the team predicts that this shift will transform the makeup of photosynthetic organisms that form the very basis of marine food webs: supporting animals ranging from small plankton to large fish and mammals.
“The paper provides very useful insights into how the light field differs between areas with and without ice cover that can be directly added to climate models,” Jönsson comments. As such, the results could help to improve researchers’ predictions about how Earth’s polar regions are affected by global warming.
–Samuel Jarman
Samuel Jarman is a science writer based in the UK.
References
- M. Soja-Woźniak et al., “Loss of sea ice alters light spectra for aquatic photosynthesis,” Nat. Commun. 16, 4059 (2025).