Deciphering the Significant Role of Biological Ice Nucleators in Precipitation at the Organic Molecular Level

Biological particles, as a fraction of organic particles, potentially play a crucial role in ice nucleation processes. However, the contributions and relationships of biological components and organic matter (OM) to atmospheric ice nucleation remain largely unexplored. Here, total ice nucleating particles (INPs), heat-resistant INPs, lysozyme-resistant INPs, nanoscale INPs (<0.22 μm), and heat-resistant nanoscale INPs in precipitation collected at the summit of Mt. Lu, China, were determined using droplet freezing assays coupled with corresponding pretreatments. Heat-sensitive INPs and lysozyme-sensitive INPs were considered as biological INPs and bacterial INPs, respectively. Microorganisms and OM molecules in precipitation were identified by high-throughput sequencing technology and ultrahigh-resolution mass spectrometry, respectively. Results revealed a predominant biological (heat-sensitive) composition (78.8% and 93.2%) of total and nanoscale INPs at temperatures above −15°C. Specifically, bacterial (lysozyme-sensitive) INPs accounted for 36.1% of the biological INPs at temperatures above −15°C. A notable correlation between sulfur-containing organic compounds, mainly proteinaceous and lignin-like substances, and INPs was uncovered, with a co-occurrence network linking these compounds to Gram-positive bacteria and Agaricomycetes. This study underscored the possible significance of sulfur-containing organic compounds in the ice nucleation capacity of biological INPs, further shedding light on the ice nucleation mechanisms and potential sources of biological INPs.