Ice nucleators, bacterial cells and Pseudomonas syringae in precipitation at Jungfraujoch

Ice nucleation is a means by which the deposition of an airborne microorganism can be accelerated under favourable meteorological conditions. Analysis of 56 snow samples collected at the high-altitude observatory Jungfraujoch (3580ma.s.l.) revealed an order-of-magnitude-larger dynamic range of ice-nucleating particles active at 8 degrees C (INPs(-8)) compared to the total number of bacterial cells (of which on average 60% was alive). This indicates a shorter atmospheric residence time for INPs(-8). Furthermore, concentrations of INPs(-8) decreased much faster, with an increasing fraction of water precipitated from the air mass prior to sampling, than the number of total bacterial cells. Nevertheless, at high wind speeds (> 50 km h(-1))the ratio of INPs(-8) to total bacterial cells largely remained in a range between 10(-2) and 10(-3), independent of prior precipitation, likely because of recent injections of particles in regions upwind. Based on our field observations, we conclude that ice nucleators travel shorter legs of distance with the atmospheric water cycle than the majority of bacterial cells. A prominentice- nucleating bacterium, Pseudomonas syringae, has been previously supposed to benefit from this behaviour as a means to spread via the atmosphere and to colonise new host plants. Therefore, we targeted this bacterium with a selective cultivation approach. P. syringae was successfully isolated for the first time at such an altitude in 3 of 13 samples analysed. Colonyforming units of this species constituted a minor fraction (10(-4)) of the numbers of INPs(-8) in these samples. Overall, our findings expand the geographic range of habitats where this bacterium has been found and corroborate theories on its robustness in the atmosphere and its propensity to spread to colonise new habitats.