Abstract
Earth’s radiation budget and frequency and intensity of precipitation are influenced by aerosols with ice nucleation activity (INA), i.e., particles that catalyze the formation of ice. Some bacteria, fungi, and pollen are among the most efficient ice nucleators but the molecular basis of INA is poorly understood in most of them. Lysinibacillus parviboronicapiens (Lp) was previously identified as the first Gram-positive bacterium with INA. INA of Lp is associated with a secreted, nanometer-sized, non-proteinaceous macromolecule or particle. Here a combination of comparative genomics, transcriptomics, and a mutant screen showed that INA in Lp depends on a type I iterative polyketide synthase and a non-ribosomal peptide synthetase (PKS-NRPS). Differential filtration in combination with gradient ultracentrifugation revealed that the product of the PKS-NRPS is associated with secreted particles of a density typical of extracellular vesicles and electron microscopy showed that these particles consist in “pearl chain”-like structures not resembling any other known bacterial structures. These findings expand our knowledge of biological INA, may be a model for INA in other organisms for which the molecular basis of INA is unknown, and present another step towards unraveling the role of microbes in atmospheric processes.
Highlights
Ice nucleation activity (INA), the ability of particles to induce the freezing of water above the freezing temperature of pure water (~−38 °C), affects many aspects of Earth’s environment
Comparative genomics and transcriptomics reveal putative LINA genes As a first step towards identification of putative genes responsible for INA in Lysinibacillus parviboronicapiens (Lp), we hypothesized that only Lysinibacillus strains with INA would contain genes responsible for INA while strains without INA would be missing these genes
By isolating the ice nucleation active, Gram-positive species Lp from precipitation, we previously expanded the list of INA bacteria that may contribute to atmospheric processes [9]
Summary
Ice nucleation activity (INA), the ability of particles to induce the freezing of water above the freezing temperature of pure water (~−38 °C), affects many aspects of Earth’s environment. The concentration and activity of ice nucleating particles (INPs), referred to as ice nuclei (IN), in mixed-phase clouds affects the ratio of frozen to liquid water in clouds because INPs trigger the freezing of super-cooled liquid water droplets and the growth of the resulting ice crystals by the Wegener–Bergeron–Findeisen (WBF) process [1]. This in turn affects how reflective clouds are, i.e., their albedo, and Earth’s radiation budget and, Earth’s temperature [2]. The ina gene encodes a cell wall protein that includes 50–80 repeats that are hypothesized to bind water molecules in a way to provide a scaffold for water to crystallize [10] and/or to orient water molecules to predispose them to freezing [11]
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