Laboratory Study of Effects of Atmospheric Pollutants on Ice Nucleation Activity of Bacteria

Abstract

Ice nuclei of some bacterial origin as ice catalysts can initiate ice nucleation at temperatures as warm as -2°C in certain laboratory experiments. The ice nucleation activities of airborne bacteria in the real atmosphere may be different from those experiments. To estimate the impact of typical atmospheric pollutants including monocarboxylic acids (MCAs), dicarboxylic acids (DCAs) and ammonia sulfate on ice nucleation activity of P.syringae pv lachrymans and P.syringae pv.panici, we have conducted some experiments by means of the modified Vali's droplet freezing testing method in the immersion freezing mode with the mixture of the pure water and the polluted water. Our results show that the ice nucleation activity of bacterial origins can be regulated by such pollutant compounds even though the onset freezing temperatures of water droplets mainly depend on the concentrations of ice nucleation-active bacteria. Atmospheric acids can decrease ice nucleation activity of P.syringae pv lachrymans and P.syringae pv.panici. However, the onset freezing temperatures of water droplets immersed with ice nucleation-active P.syringae pv lachrymans will be enhanced by the low concentration of such atmospheric pollutants.

The Diversity and Role of Bacterial Ice Nuclei in Rainwater from Mountain Sites in China

“…Bacterial IN appear to be very efficient immersionfreezing nuclei owing to the outer membrane proteins or proteinaceous compounds (Christner et al, 2008b). Indeed, the initial freezing temperature of PS (10 8 cells mL -1 ) was around -2.5°C, which is similar to the previously reported value (Du et al, 2015). Since biological IN would be inactivated by heat treatment, whereas IN of non-biological origin would not.…”

“…For example, the droplet freezing assay deviates from the actual natural process because the droplet size is significantly larger than the actual size of a cloud droplet, and because the singlefreezing mode (immersion freezing) was employed, which can result in inaccurate calculation of the cumulative nucleus concentration (Mortazavi et al, 2008;Knopf and Alpert, 2013). Nonetheless, the drop-freezing assay is widely adopted in many studies because it can provide a relatively direct method for detection of a nucleation event (e.g., Feng et al, 2002;Christner et al, 2008a, b;Du and Ariya, 2008;Attard et al, 2012;Morris et al, 2013;Du et al, 2015). It is difficult to reproduce atmospheric conditions in the laboratory for evaluating the importance of bioaerosols in the atmospheric process resulting in precipitation.…”

“…Fortyseven droplets of 10 µL each were distributed on a sterile copper plate with a cooling rate of 2°C per min, with five repetitions, for a total number of 235 droplets tested in each sample (Wang et al, 2012;Du et al, 2015). This modified device could automatically detect the drop-frozen signals and calculate the experimental data (Supporting information [SI], Fig.…”

“…Lachrymans (PS) were provided by the State Key Laboratory for Biology of Plant Diseases and Insect Pests (SKLBPI) of the Chinese Academy of Agricultural Sciences. Suspensions of PS (10 8 cells·mL -1 ) were obtained with the cultivated bacteria in sterilized ultra-pure Milli-Q water (Du et al, 2015). As a control, the suspensions of PS were also subjected to heat and filtration treatment.…”

Evidence for a missing source of efficient ice nuclei

“…Lachrymans 24 ( PS ) suspension (10 8 cells mL −1 ) as controls. PS initiated freezing at −3.0 °C and produced 10 2.6 IN per milliliter at −5.6 °C, at which the freezing process of all droplets ended; the ultra-pure Milli–Q water initiated freezing at −15.4 °C.…”