ABSTRACTIce nucleation-active (INA) bacteria may function as high-temperature ice-nucleating particles (INP) in clouds, but their effective contribution to atmospheric processes, i.e., their potential to trigger glaciation and precipitation, remains uncertain. We know little about their abundance on natural vegetation, factors that trigger their release, or persistence of their ice nucleation activity once airborne. To facilitate these investigations, we developed two quantitative PCR (qPCR) tests of theinagene to directly count INA bacteria in environmental samples. Each of two primer pairs amplified most alleles of theinagene and, taken together, they should amplify all known alleles. To aid primer design, we collected many new INA isolates. Alignment of their partialinasequences revealed new and deeply branching clades, including sequences fromPseudomonas syringaepv.atropurpurea,Ps. viridiflava,Pantoea agglomerans,Xanthomonas campestris, and possiblyPs. putida,Ps. auricularis, andPs. poae. qPCR of leaf washings recorded ∼108inagenes g−1fresh weight of foliage on cereals and 105to 107g−1on broadleaf crops. Much lower populations were found on most naturally occurring vegetation. In fresh snow,inagenes from various INA bacteria were detected in about half the samples but at abundances that could have accounted for only a minor proportion of INP at −10°C (assuming oneinagene per INA bacterium). Despite this, an apparent biological source contributed an average of ∼85% of INP active at −10°C in snow samples. In contrast, a thunderstorm hail sample contained 0.3 INA bacteria per INP active at −10°C, suggesting a significant contribution to this sample.
Isolation, Characterization, and Genetic Diversity of Ice Nucleation Active Bacteria on Various Plants