Effects of atmospheric conditions on ice nucleation activity of Pseudomonas
Although ice nuclei from bacterial origin are known to be efficient at the highest temperatures known for ice catalysts, quantitative data are still needed to assess their role in cloud processes. Here we studied the effects of three typical cloud conditions (i) acidic pH (ii) NO2 and O3 ex- posure and (iii) UV-A exposure on the ice nucleation activ- ity (INA) of four Pseudomonas strains. Three of the Pseu- domonas syringae strains were isolated from cloudwater and the phyllosphere and Pseudomonas fluorescens strain CGina- 01 was isolated from Antarctic glacier ice melt. Among the three conditions tested, acidic pH caused the most significant effects on INA likely due to denaturation of the ice nucle- ation protein complex. Exposure to NO2 and O3 gases had no significant or only weak effects on the INA of two P. sy- ringae strains whereas the INA of P. fluorescens CGina-01 was significantly affected. The INA of the third P. syringae strain showed variable responses to NO2 and O3 exposure. These differences in the INA of different Pseudomonas sug- gest that the response to atmospheric conditions could be strain-specific. After UV-A exposure, a substantial loss of viability of all four strains was observed whereas their INA decreased only slightly. This corroborates the notion that un- der certain conditions dead bacterial cells can maintain their INA. Overall, the negative effects of the three environmental factors on INA were more significant at the warmer temper- atures. Our results suggest that in clouds where temperatures are near 0 ◦C, the importance of bacterial ice nucleation in precipitation processes could be reduced by some environ- mental factors.