Heterogeneous ice nucleation activity of bacteria: new laboratory experiments at simulated cloud conditions

Abstract

The ice nucleation activities of five different Pseudomonas syringae, Pseudomonas viridiflava and Erwinia herbicola bacterial species and of Snomax™ were investigated in the temperature range between −5 and −15 • C. Water suspensions of these bacteria were directly sprayed into the cloud chamber of the AIDA facility of Forschungszentrum Karlsruhe at a temperature of −5.7 • C. At this temperature, about 1% of the Snomax™ cells induced immersion freezing of the spray droplets before the droplets evaporated in the cloud chamber. The living cells didn't induce any detectable immersion freezing in the spray droplets at −5.7 • C. After evaporation of the spray droplets the bacterial cells remained as aerosol particles in the cloud chamber and were exposed to typical cloud formation conditions in experiments with expansion cooling to about −11 • C. During these experiments, the bacterial cells first acted as cloud condensation nuclei to form cloud droplets. Then, only a minor fraction of the cells acted as heterogeneous ice nuclei either in the condensation or the immersion mode. The results indicate that the bacteria investigated in the present study are mainly ice active in the temperature range between −7 and −11 • C with an ice nucleation (IN) active fraction of the order of 10 −4 . In agreement to previous literature results, the ice nucleation efficiency of Snomax™ cells was much larger with an IN active fraction of 0.2 at temperatures around −8 • C.

Immersion freezing of ice nucleation active protein complexes

“…Intact bacteria cells appear in the size range around 800 nm, visible as small bump in the size distribution. This is consistent with the size range in which bacteria cells for Snomax TM and P. syringae were found in Möhler et al (2008). The generated particles at sizes below the small bump are not made up of intact bacteria, but contain other Snomax TM ingredients such as e.g., cell constituents and nutrition remnants and also, as will later follow from our results, fragments of cell membrane with intact INA protein complexes.…”

“…The IN number size distribution shown in Fig. 6 is in contradiction to Möhler et al (2008) who speculated, based on polydisperse measurements, that only particles in the larger mode (D P ≥ 600 nm) of the bimodal size distribution are ice active. Results obtained in the study by Wood et al (2002) support our finding of INA protein complexes being present also in smaller size classes.…”

“…Since 2000, some studies (e.g., Wood et al, 2002;Möhler et al, 2008) investigated the ice nucleation behaviour of air suspended droplets containing Snomax TM as a convenient surrogate for living bacteria. Snomax TM consists of nonviable P. syringae cells.…”

“…Besides P. syringae, other ice nucleation active (INA) bacteria species, such as Pseudomonas fluorescens, Pseudomonas viridiflava, Pantoea agglomerans or Xanthomas campestris, have been described. These INA bacteria can induce heterogeneous freezing at temperatures as high as −1 • C (Morris et al, 2004) and are, therefore, among the most efficient ice nuclei (IN) known (e.g., Yankofsky et al, 1981;Levin and Yankofsky, 1983;Möhler et al, 2008). Most of the INA bacterial species typically grow on plant surfaces, where they cause massive frost damage at high subzero temperatures.…”

Ice nuclei properties within a Saharan dust event at the Jungfraujoch in the Swiss Alps

“…This statement is supported by field studies (DeMott et al, 2003b;Richardson et al, 2007;Klein et al, 2010) where high concentrations of mineral dust led to a significant increase in the ice crystal number concentration. Previous studies by Vali et al (1976); Schnell and Vali (1976); Möhler et al (2008) showed that bioaerosols and especially some specific bacteria like Pseudomonas Syringae initiate ice formation at warmer temperatures than mineral dust. Pratt et al (2009) found that one third of the ice particle residues in one cloud collected by a Counterflow Virtual Impactor (CVI) at high altitude over Wyoming contained biological markers.…”