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Current climate models have so far been unable to adequately reproduce the clouds over the Southern Ocean around Antarctica. An international team has now taken an important step toward filling this gap. The researchers were able to prove that the majority of ice nuclei in the atmosphere there are due to sugar compounds from marine microorganisms in the seawater.

They enter the clean air above the sea through sea spray and evaporation, causing water droplets to freeze and thus affecting cloud and precipitation formation. Ice formation in clouds has a major influence on the climate as ice particles in clouds reflect sunlight much more strongly than pure water clouds.

These results emphasize the importance of biological sources for precipitation formation in remote marine regions such as around the Antarctic. Researchers from the Leibniz Institute for Tropospheric Research (TROPOS) and the Arctic University of Norway in Tromsø recently their study in the journal Environmental Science & Technology.

Ice formation processes influence the radiation properties, precipitation formation and consequently the lifespan of clouds. Ice formation is made possible by so-called ice nucleating particles (INPs). In remote marine regions such as the Southern Ocean, where INP concentrations in the clean atmosphere are low, large differences in radiative effects between models and measurements have been observed. A better understanding of the sources and properties of ice nuclei, such as aerosol particles originating from sea spray, is therefore necessary to improve climate models.

It has been known for over a decade that ice-forming macromolecules produced by marine microorganisms such as fungi, protists or yeasts in seawater can enter the atmosphere via sea spray. For terrestrial sources, there is now some knowledge to be able to assign the macromolecules to specific proteins and polysaccharides.

In contrast, there was previously a lack of knowledge about the chemical identity of these ice-forming macromolecules from marine sources. "During the Polarstern expedition PS106 in 2017, we observed increased glucose concentrations in Arctic samples and concluded that this glucose could be an indicator of ice nuclei in seawater. The monosaccharide glucose is a degradation product of polysaccharides.

"It was therefore obvious to us that polysaccharides could be the missing piece of the puzzle," explains Dr. Sebastian Zeppenfeld from TROPOS.

A cosmos of microorganisms such as bacteria, algae, marine diatoms, haloarchaea, viruses, yeasts and fungi live in the surface film of the oceans that separates seawater from the atmosphere above. In addition to algae and bacteria, which primarily contribute to the production and decomposition of biomass, marine fungi are now also attracting scientific interest. As their potential role as ice nuclei was still largely unexplored, the researchers took a closer look at marine fungi.

"In this study, we investigated the ice nucleation of marine polysaccharides derived from marine fungi and protist, as well as commercially available standard polysaccharides," reports Dr. Susan Hartmann from TROPOS, who examined ice nucleation in the laboratory using the INDA (Ice Nucleation Droplet Array) droplet freezing test.

The result is a collection of data that indicates how many ice nuclei are formed at which temperatures by which components in the cloud droplets. The data now published are the first on protists and fungi from seawater that produce the aforementioned polysaccharides and catalyze ice formation.

It was already known that marine biology produces large numbers of ice nuclei in the atmosphere. The new study has now shown that the polysaccharides explain the total number of biological ice nuclei between around -15 and -20°C.

Together with the new data, various other studies provide a differentiated picture of which components in the unpolluted atmosphere of the southern high latitudes are responsible for ice in the cloud droplets: in warm clouds below -2°C these are mainly proteins, in medium-cold clouds below -10°C these are mainly the now proven polysaccharides and only in very cold clouds below -20°C the well-known mineral dust dominates.

However, as extensive sources of mineral dust (e.g., deserts) are scarce in the Southern Hemisphere, the importance of mineral dust for ice formation in the very clean air over the seas around Antarctica is much lower than in the Northern Hemisphere. The mixed-phase clouds with liquid water and ice are mostly located in the temperature range between -15 and -20°C, i.e., precisely in the range in which the polysaccharides are among the most important ice nuclei.

"In our simulations, we were able to show that at -15 to -16°C, the polysaccharides over the gigantic areas of the oceans in the clean Southern Hemisphere are probably the most important ice nuclei, i.e., they contribute more to ice formation than mineral dust emitted from the deserts, which is normally assumed to be the main type of ice nuclei in climate models.

"This is a new and important finding for climate models," summarizes Dr. Roland Schrödner from TROPOS, who analyzed the data using the TM5 global atmospheric chemistry transport model.

The study builds on years of preliminary work by three groups at TROPOS: The Aerosol Microphysics has been investigating ice formation in cloud droplets for a long time, Atmospheric Modeling is researching the influence of various types of particles on the climate, and Atmospheric Chemistry is analyzing the chemical composition.

The researchers had previously measured the concentrations of polysaccharides in the atmosphere during various expeditions, including the Spanish Antarctic expedition PI-ICE, the German Arctic expedition PASCAL/PS106, the MarParCloud campaign in the tropical Atlantic and measurements on Spitsbergen in the Arctic. These new findings were only made possible by combining this work.

From the researchers' point of view, this study emphasizes the importance of natural biological components in the atmosphere and that the biosphere and atmosphere are closely linked in the Earth system. If the ambitious climate protection goals of many countries are realized in the coming decades, man-made emissions are expected to decrease and natural aerosol particles will become even more important for cloud microphysics.

Clouds in a clean environment, i.e., with a low number of droplets, react more sensitively to fluctuations in aerosol number concentration. The clean Southern Hemisphere around Antarctica is therefore particularly exciting for cloud research: The "HALO-South" mission of the German research aircraft HALO will investigate the interaction of clouds, aerosols and radiation over the Southern Ocean around New Zealand in more detail from July to October 2025 under TROPOS leadership. The measurements in the air will be supplemented by measurements on the ground.

During the "goSouth-2" measurement campaign, researchers from TROPOS and Leipzig University will work with other partners to investigate the clouds of the Southern Ocean. To this end, the mobile aerosol and cloud remote sensing system LACROS will be deployed near Invercargill at the southern tip of New Zealand from September 2025 to March 2027.

The clouds of the less anthropogenically influenced Southern Hemisphere still harbor many secrets that the researchers from Leipzig hope to uncover over the next few years.