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| Titel |
Immersion freezing of ice nucleation active protein complexes |
| VerfasserIn |
S. Hartmann, S. Augustin, T. Clauss, H. Wex, T. Šantl-Temkiv, J. Voigtländer, D. Niedermeier, F. Stratmann |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1680-7316
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| Digitales Dokument |
URL |
| Erschienen |
In: Atmospheric Chemistry and Physics ; 13, no. 11 ; Nr. 13, no. 11 (2013-06-14), S.5751-5766 |
| Datensatznummer |
250018702
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| Publikation (Nr.) |
 copernicus.org/acp-13-5751-2013.pdf |
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| Zusammenfassung |
Utilising the Leipzig Aerosol Cloud Interaction Simulator (LACIS), the
immersion freezing behaviour of droplet ensembles containing monodisperse
particles, generated from a Snomax™ solution/suspension, was
investigated. Thereto ice fractions were measured in the temperature range
between −5 °C to −38 °C. Snomax™ is an
industrial product applied for artificial snow production and contains
Pseudomonas syringae} bacteria which have long been used as model
organism for atmospheric relevant ice nucleation active (INA) bacteria. The
ice nucleation activity of such bacteria is controlled by INA protein
complexes in their outer membrane.
In our experiments, ice fractions increased steeply in the temperature range
from about −6 °C to about −10 °C and then levelled
off at ice fractions smaller than one. The plateau implies that not all
examined droplets contained an INA protein complex. Assuming the INA protein
complexes to be Poisson distributed over the investigated droplet
populations, we developed the CHESS model (stoCHastic modEl of similar and
poiSSon distributed ice nuclei) which allows for the calculation of ice
fractions as function of temperature and time for a given nucleation rate.
Matching calculated and measured ice fractions, we determined and
parameterised the nucleation rate of INA protein complexes exhibiting class
III ice nucleation behaviour. Utilising the CHESS model, together with the
determined nucleation rate, we compared predictions from the model to
experimental data from the literature and found good agreement.
We found that (a) the heterogeneous ice nucleation rate expression
quantifying the ice nucleation behaviour of the INA protein complex is
capable of describing the ice nucleation behaviour observed in various
experiments for both, Snomax™ and P. syringae bacteria,
(b) the ice nucleation rate, and its temperature dependence, seem to be very
similar regardless of whether the INA protein complexes inducing ice
nucleation are attached to the outer membrane of intact bacteria or membrane
fragments, (c) the temperature range in which heterogeneous droplet freezing
occurs, and the fraction of droplets being able to freeze, both depend on the
actual number of INA protein complexes present in the droplet ensemble, and
(d) possible artifacts suspected to occur in connection with the drop
freezing method, i.e., the method frequently used by biologist for
quantifying ice nucleation behaviour, are of minor importance, at least for
substances such as P. syringae, which induce freezing at comparably
high temperatures. The last statement implies that for single ice nucleation
entities such as INA protein complexes, it is the number of entities present
in the droplet population, and the entities' nucleation rate, which control
the freezing behaviour of the droplet population. Quantities such as ice
active surface site density are not suitable in this context.
The results obtained in this study allow a different perspective on the
quantification of the immersion freezing behaviour of bacterial ice
nucleation. |
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