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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. 

Fusarium avenaceum is a filamentous fungus commonly associated with plants and soil. It is a causal agent of Fusarium head blight (FHB) on maize and small-grain cereals and blights on other plant species, and is one of the very few fungal species known to have ice nucleation activity (i.e., it catalyzes ice formation). Here, we report the draft genome of the ice-nucleation-active F. avenaceum strain F156N33 isolated from the atmosphere above Virginia. The genome assembly is 41,175,306 bp long, consists of 214 contigs, and is predicted to encode 11,233 proteins, which were annotated using RNA-sequencing data obtained from the same strain. 

ABSTRACT Mortierella alpina is a filamentous fungus commonly associated with soil and is one of very few fungal species known to include strains with ice nucleation activity. Here, we report the draft genome sequence of the ice nucleation-active M. alpina strain LL118, isolated from aspen leaf litter collected in Alberta, Canada. 

Plant microbiota play essential roles in plant health and crop productivity. Comparisons of community composition have suggested seed, soil, and the atmosphere as reservoirs of phyllosphere microbiota. After finding that leaves of tomato (Solanum lycopersicum) plants exposed to rain carried a higher microbial population size than leaves of tomato plants not exposed to rain, we experimentally tested the hypothesis that rain is a thus-far-neglected reservoir of phyllosphere microbiota. Therefore, rain microbiota were compared with phyllosphere microbiota of tomato plants either treated with concentrated rain microbiota, filter-sterilized rain, or sterile water. Based on 16S ribosomal RNA amplicon sequencing, 104 operational taxonomic units (OTUs) significantly increased in relative abundance after inoculation with concentrated rain microbiota but no OTU significantly increased after treatment with either sterile water or filter-sterilized rain. Some of the genera to which these 104 OTUs belonged were also found at higher relative abundance on tomato plants exposed to rain outdoors than on tomato plants grown protected from rain in a commercial greenhouse. Taken together, these results point to precipitation as a reservoir of phyllosphere microbiota and show the potential of controlled experiments to investigate the role of different reservoirs in the assembly of phyllosphere microbiota. 

Abstract. Decaying vegetation was determined to be a potentially important source ofatmospheric ice nucleation particles (INPs) in the early 1970s. The bacteriumPseudomonas syringae was the first microorganism with ice nucleationactivity (INA) isolated from decaying leaf litter in 1974. However, the icenucleation characteristics of P. syringae are not compatible withthe characteristics of leaf litter-derived INPs since the latter were foundto be sub-micron in size, while INA of P. syringae depends on muchlarger intact bacterial cells. Here we determined the cumulative icenucleation spectrum and microbial community composition of the historic leaflitter sample 70-S-14 collected in 1970 that conserved INA for 48 years. Themajority of the leaf litter-derived INPs were confirmed to be sub-micron insize and to be sensitive to boiling. Culture-independent microbial communityanalysis only identified Pseudomonas as potential INA.Culture-dependent analysis identified one P. syringae isolate, twoisolates of the bacterial species Pantoea ananatis, and one fungalisolate of Mortierella alpina as having INA among 1170 bacterialcolonies and 277 fungal isolates, respectively. Both Pa. ananatisand M. alpina are organisms that produce heat-sensitive sub-micronINPs. They are thus both likely sources of the INPs present in sample 70-S-14and may represent important terrestrial sources of atmospheric INPs, aconclusion that is in line with other recent results obtained in regard toINPs from soil, precipitation, and the atmosphere.