Marine sources of ice-nucleating proteins in the Arctic and their impact on atmospheric processes

The Arctic is particularly vulnerable to climate change due to a decrease in surface albedo caused by declining ice and snow cover. Aerosol-cloud feedbacks modulate Arctic warming, with clouds profoundly affecting the radiative balance of the region through both cooling and warming effects. The concentration and type of ice-nucleating particles (INP) are key factors controlling cloud ice formation which directly influences cloud radiative properties and lifetime. It has recently been proposed that microbially-produced INPs, which come from marine environments and can trigger freezing at low supercooling, are important for the formation of mixed-phase clouds in the Arctic. These clouds commonly form at low altitudes within the temperature range, where biogenic INPs are key drivers of ice formation. Despite their importance, it remains unclear which microorganisms are responsible for the production of marine INPs and under which conditions these are produced. This lack of knowledge limits our quantitative understanding of how high-temperature INPs from marine environments impact cloud formation in the Arctic.

To investigate marine-sourced INPs and their sources, we collected a series of marine- (i.e. seawater, sea-surface microlayer, and sea ice) and atmospheric aerosol samples from the west coast of Greenland between 2016 and 2023. We performed droplet-freezing measurements with the micro-PINGUIN setup to quantify INPs, along with chlorophyll a measurements, δO¹⁸ analysis, and amplicon sequencing of marker genes using Illumina MiSeq to determine the composition of bacteria (16S rRNA genes) and microalgae (18S rRNA genes) and identify potential producers of INPs. Using filtration analysis and heat treatments, we investigated the type of INPs identified in marine systems. We carried out field experiments and laboratory simulations using a modified cold-finger to study incorporation of INP from seawater into sea ice. Finally, we employed laboratory simulations using AEGOR the sea-spray tank to study emissions of bioaerosols and marine INP.

In the fjords, we observed a significant contribution of terrestrial sources to INPs in the marine waters during the early melting season with enhanced terrestrial runoff. These reflected in elevated INP concentrations, which were up to 10,000-fold higher that previously reported, with properties distinct from known marine INPs. In the open sea, we found that INP concentrations in seawater increased with latitude, independent of terrestrial freshwater input. While INP concentrations linked to marine microbial communities, they were surprisingly not tightly associated to phytoplankton blooms as previously suggested. We identified annual sea ice as a key reservoir of INPs, which exhibited INP concentrations up to 100-fold higher than the seawater below sea ice. INPs did not preferentially incorporate into the ice from seawater but were likely produced by the heterotrophic bacterial community in the early phase of sea ice growth. As the sea ice melts in the spring, these INPs are released into the surface seawater significantly contributing to the marine INP pool. Finally, through both field measurements and sea-spray experiments, we observed the transfer of marine INPs and microbial cells into the air. Ultimately, our research significantly enhances the understanding of marine microorganisms and their pivotal role in atmospheric processes within the Arctic region.