From Arctic soils to the atmosphere: microbial controls on biological ice-nucleating particles at high latitudes

The Arctic is experiencing rapid climate change, with warming rates exceeding three to four times the global average. This has a profound impact on cloud and precipitation formation. Bioaerosols are critical for cloud processes as they can act as high-temperature ice nucleating particles (INPs). Despite their importance, the understanding of bioaerosol-cloud interactions remains highly uncertain, primarily due to limited information on the types, concentrations, and sources of biogenic INPs. To reduce these uncertainties, we combined analyses of Arctic soils as potential reservoirs of biogenic INPs with multi-year atmospheric observations of bioaerosols and INP in the High Arctic.

We first investigated Arctic soils as reservoirs of biogenic INPs by analyzing fungal community composition and INP concentrations across 78 soil samples collected from seven sites spanning southern to northern Greenland. To determine whether INPs from soils are transferred into the atmosphere, we performed the first multi-year (2021–2023) study of bioaerosol abundance and composition, together with quantifying high-temperature INPs from the High Arctic, collected at Villum Research Station at a 3.5-day time resolution. Soils were sieved and INPs associated with particles <5 µm as well as INPs found in the soluble fraction (<0.22 µm) were obtained using the Micro-PINGUIN assay. Fungal and bacterial communities were characterized using ITS2 and 16S rRNA gene amplicon sequencing. Source tracking was used to determine the contribution of local sources to airborne microbial cells and INP.

In the soils, we found that higher INP concentrations were associated with higher latitudes. Based on their high-temperature activity, we suggest that these INPs are proteinaceous. Using multivariate analyses, we identified annual mean air temperature as the dominant explanatory variable, followed by soil pH. The composition of the fungal community varied significantly among sites, and several taxa, including Leptosphaeria, Pseudogymnoascus, Tetracladium, and Microdochium, showed significant positive correlations with high-temperature INP concentrations, suggesting that members of the fungal community are producing soil-derived INPs. We found that the INPs were present in the soluble fraction of the soils, which is also consistent with fungal origin. As suggested for temperate regions, these INPs can disassociate from fungal hyphae and bind to clay particles, getting emitted to the atmosphere on inorganic particles. Analyzing aerosol samples, we found that atmospheric INP concentrations ranged from 2.2 × 10^-2 to 7.2 × 10¹ m^-3, and airborne bacterial concentrations from 2.7 × 10⁰ to 4.2 × 10³ m⁻³. We observed seasonal shifts in microbial community composition, with spore-forming taxa dominating during in spring and more diverse, locally sourced communities in summer. Both bacterial abundance and diversity were positively correlated with warm-temperature INP concentrations, indicating that these were associated with emissions from environments with dense and diverse bacterial communities, such as soils.

Together, our results allow us to link high-latitude terrestrial microbial communities to atmospheric INP, and we demonstrated that Arctic soils, particularly at northern latitudes, represent key reservoirs of biogenic INPs, which disperse into the atmosphere. By integrating studies of the microbial soil communities and long-term atmospheric observations we can constraint biological aerosol–cloud interactions and their potential sensitivity to the ongoing Arctic warming.