Ice nucleation activity of bacteria isolated from cloud water

Some Gamma–Proteobacteria can catalyze ice formation thereby potentially contributing to the induction of precipitation in supercooled clouds and subsequently to bacterial deposition. Forty-four bacterial strains from cloud water were screened for their capacity to induce freezing. Seven strains (16%) were active at −8 °C or warmer and were identified as Pseudomonas syringae, Xanthomonas spp. and Pseudoxanthomonas sp. Phylogenetic analysis revealed that the P. syringae strains in clouds at the Puy de Dôme belonged to clades that are among the most infrequently detected in the environment, while widespread clades were absent suggesting some extent of selection or unusual biogeography of the bacteria at the sampling site. Three strains induced freezing at −3 °C while the others nucleated ice at −4 °C to −6 °C. The freezing profiles revealed that the peaks of activity were centered around −3.5 °C, −5 °C and/or −8.5 °C depending on the strain. The frequency of ice-nuclei (IN) per cell at −6 °C was generally below 0.5% and reached up to 4.2% in one strain. We estimated that clouds influenced by vegetated areas would carry between less than 1 and ∼500 bacterial IN mL−1 of water active between −3 °C and −10 °C depending on the season. These data will contribute to modeling the impact of bacterial IN on precipitation at regional scales.