Survival and ice nucleation activity of Pseudomonas syringae strains exposed to simulated high-altitude atmospheric conditions

Abstract

The epiphytic bacterium Pseudomonas syringae produces the most efficient and well-studied biological ice nuclei (IN) known. Bioaerosols containing these cells have been proposed to influence cloud glaciation, an important process in the initiation of precipitation. The presence of this species has been reported on rain, snow, and cloud water samples, but how these organisms can survive the harsh conditions present on the high atmosphere still remains to be better understood. In this study, the impact caused by this type of environment on P. syringae was assayed by measuring their viability and IN activity. Two strains, of the pathovars syringae and garcae, were compared to Escherichia coli. While UV-C radiation effectively inactivated these cells, the Pseudomonas were much more tolerant to UV-B. The P. syringae strains were also more resistant to \"environmental\" UV radiation from a solar simulator, composed of UV-A and UV-B. The response of their IN after long exposures to this radiation varied: only one strain suffered a relatively small 10-fold reduction in IN activity at -5 {degrees}C. Desiccation at different relative humidity values also affected the IN, but some activity at -5 {degrees}C was still maintained for all tests. The pathovar garcae tended to be more resistant to the stress treatments than the pathovar syringae, particularly to desiccation, though its IN were found to be more sensitive. Compared to E. coli, the P. syringae strains seemed relatively better adapted to survival under conditions present on the atmosphere at high altitudes.\n\nIMPORTANCEThe plant-associated bacterium Pseudomonas syringae produces on its outer membrane highly efficient ice nuclei which are able to induce the freezing of supercooled water. This ability has been linked to increased frost damaged on colonized leaves and also to the formation of ice in clouds, an important process leading to precipitation. P. syringae has been found on rain, snow, and cloud water samples, confirming its presence on the atmosphere. This study aimed to assess the survival of these cells and the maintenance of their ice nucleation activity under stressing conditions present in high altitudes: ultraviolet radiation and desiccation. P. syringae strains were shown to at least partially tolerate these factors, and their most efficient ice nuclei, while affected, could still be detected after all experiments.