As the polar ice caps continue to thin and vanish under the relentless advance of global warming, the underwater world beneath them undergoes transformations that are as profound as they are unexpected. Recent groundbreaking research led by marine biologists Monika Soja-Woźniak and Jef Huisman at the University of Amsterdam reveals a fundamental shift in the spectrum of light penetrating the ocean’s surface in polar regions—a shift that could cascade through marine ecosystems, altering the very foundation of Arctic and Antarctic food webs.
When sea ice covers polar oceans, it acts as a selective filter for sunlight, scattering and reflecting much of the incoming radiation. Despite this, the thin veil of ice allows a broad spectrum of visible light, encompassing a wide range of colors, to reach the underlying waters. This spectrum supports the unique photosynthetic communities that thrive beneath the ice, particularly ice algae—microscopic plants that anchor the polar marine food chain. However, as the ice melts, the ocean is increasingly exposed directly to sunlight filtered solely through the clear blue waters, dramatically changing the underwater light environment.
This alteration in underwater illumination arises from a fundamental difference in how ice and liquid water interact with light at a molecular level. In liquid seawater, the mobility of H₂O molecules gives rise to dynamic molecular vibrations—phenomena that generate specific absorption bands at certain wavelengths. These absorption bands carve “spectral niches” within the underwater light spectrum, essentially creating distinct windows of light in which various photosynthetic organisms have evolved specialized pigments that exploit these unique spectral signatures.
Conversely, in the solid, crystalline lattice of sea ice, water molecules are locked rigidly in place, suppressing these molecular vibrations and their associated absorption bands. As a result, light transmitted through ice maintains a broader and more continuous spectrum, enabling ice-associated algae to harness a fuller palette of colors for photosynthesis. The melting of sea ice thus eliminates these broad spectral advantages, confining the underwater light environment predominantly to the blue wavelengths that penetrate the ocean depths.
This spectral compression has profound ecological implications. Algae thriving under ice have pigments finely tuned by evolution to capture the widest range of available light wavelengths, optimizing photosynthesis under low-light conditions. As the ice retreats, they find themselves in a monochromatic world dominated by blue light—a scenario for which their pigments are poorly adapted. This results in diminished photosynthetic efficiency and may weaken their competitive edge compared to free-living phytoplankton species that have evolved pigments better matched to blue-dominated light spectra.
The research team employed sophisticated optical modeling and in situ spectral measurements to quantify these changes. These analyses confirm that the shift in spectral quality of underwater light does not simply reduce the amount of light available for photosynthesis—it fundamentally alters the composition and dynamics of photosynthetic communities. Blue-adapted algal species are poised to outcompete ice algae, potentially restructuring community assemblages that have persisted for millennia beneath the ice.
Professor Huisman emphasizes that these seemingly microscopic changes in photosynthetic pigment efficiency and species composition can ripple upwards through the marine food web. Ice algae form the base of the Arctic and Antarctic food chains, sustaining a diverse array of organisms ranging from tiny zooplankton to large fish, seabirds, and marine mammals. Shifts at this foundation may therefore influence the abundance, distribution, and survival of these higher trophic levels, with consequences for biodiversity and ecosystem stability.
Beyond biological effects, photosynthetic activity in polar oceans plays a critical role in the global carbon cycle. The ocean is one of the planet’s largest sinks for atmospheric CO₂, and photosynthesis by marine algae facilitates this uptake. Alterations to photosynthetic efficiency and species dominance may, therefore, modulate the ocean’s capacity to sequester carbon, feeding back into climate dynamics.
Central to this phenomenon are the interactions of molecular vibrations with electromagnetic radiation. In liquid water, these vibrations induce absorption bands that selectively filter specific light wavelengths, sculpting the underwater light spectrum. In ice, the immobilization of water molecules disrupts these absorption processes, preserving wavelengths otherwise diminished. This physics-chemistry interplay delineates how climate-induced phase transitions of water—from solid ice to liquid sea water—manifest at ecological scales.
This research underscores the urgency of integrating nuanced optical and biological parameters into climate models and oceanographic forecasts, particularly for polar regions where environmental change is fastest and most dramatic. Existing models often account simplistically for sea ice extent and thickness but overlook critical spectral changes in underwater light quality that influence marine productivity and ecosystem structure.
As polar environments warm and ice retreats, the very fabric of light that shapes photosynthesis and marine biodiversity is being rewoven. The transformation from ice-filtered broad-spectrum light to blue-dominated liquid water spectra is a silent yet powerful driver of ecological change, one that scientists are only beginning to unravel. Understanding these processes offers a window into the complex mechanisms through which climate change reverberates through the biosphere.
The pioneering study, published in Nature Communications, brings together expertise from marine biology, physical chemistry, and environmental sciences, employing cutting-edge spectral measurements and modeling to illuminate these subtle yet consequential shifts. The team’s findings highlight the intricate connections between physical climate processes and biological systems that sustain life in Earth’s coldest regions.
As our planet progresses toward a future with less ice and more open water, the fate of polar photosynthetic communities and their myriad dependent species hangs in the balance. This research opens new avenues for investigating how microscopic changes at the molecular level can drive broad-scale transformations in ecosystem function and resilience against the backdrop of global change.
Subject of Research: Impact of sea ice loss on underwater light spectra and photosynthetic ecosystems in polar regions.
Article Title: Loss of sea ice alters light spectra for aquatic photosynthesis.
News Publication Date: 2025.
Web References: http://dx.doi.org/10.1038/s41467-025-59386-x
References: 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.
Image Credits: Photo by Lars Chresten Lund-Hansen.
Keywords: Ecology, Environmental sciences, Applied sciences and engineering.
