Getting out of the water and into the air: Understanding aerosolization of the bacterium Pseudomonas syringae from aquatic environments

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Virginia Tech

Aquatic environments contain a great diversity of microorganisms, some of which may be aerosolized and transported long distances through the atmosphere. The bacterium Pseudomonas syringae can be found in aquatic environments and in the atmosphere and may express an ice nucleation protein (bacteria expressing the protein are Ice+ and bacteria not expressing the protein are Ice-). Ice+ bacteria may be involved in cloud formation and precipitation processes due to their ability to freeze water at warmer temperatures. Freshwater aerosolization processes are not well understood, particularly the role the Ice+ phenotype may play. Water samples were collected from Claytor Lake, Virginia, USA and screened for Ice+ P. syringae. Results indicated that between 6% and 15% of Pseudomonas colonies assayed were Ice+. Preliminary phylogenetic analysis of cts (citrate synthase) sequences from strains of P. syringae showed a surprising diversity of phylogenetic subgroups present in the lake. A Collison nebulizer was used to aerosolize an Ice+ and an Ice- strain of P. syringae under artificial laboratory conditions. The aerosolization of P. syringae was not influenced by water temperature between 5° and 30°C. In general, the culturability (viability) of P. syringae in aerosols increased with temperature between 5 and 30°C. The Ice+ strain was aerosolized in greater numbers than the Ice- strain at all temperatures studied, suggesting a possible connection between the Ice+ phenotype and aerosol production. A quantitative empirical assessment of aerosolized droplets was generated using a laboratory flume and high-speed video. Droplet diameter and initial velocity upon leaving the water surface were examined at four wind speeds (3.5, 4.0, 4.5, and 5.0 m/s), and the results showed that droplet diameter and velocity had a gamma distribution and droplet mass flux increased exponentially with wind speed. An estimate of the potential amount of bacteria capable of aerosolizing was made for each wind speed. An interdisciplinary unit for advanced high school students has been developed presenting biological aerosolization and ice nucleation. This interdisciplinary work combines modeling and experimental approaches across biology and engineering interfaces, with the goal of increasing our understanding of microbial aerosols from aquatic environments that may impact our planet's water cycle.

ice nucleation, Pseudomonas syringae, bioprecipitation, aerosolization, water cycle, interdisciplinary curricula