This discussion forum focuses on the nature of micro-organisms and other organisms that are ice nucleators (can catalyze freezing of supercooled water) and their role in cloud processes and precipitation.


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To make a droplet of cloud water

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1 To make a droplet of cloud water on Mon Jan 19, 2009 4:22 pm

Ice formation is one of several steps in the making of precipitation. Formation of water droplets is also critical. Like ice formation, the transition of water vapor to the liquid phase (condensation) can be mediated by catalysts. These catalysts are called "Cloud Condensation Nuclei" (CCN). Although the properties of CCN are rather different from those of ice nuclei (IN) (CCN are often soluble materials whereas IN are insoluble), micro-organisms can be CCN as well as IN.

The earliest reports of CCN activity of bacteria relative to atmospheric processes target the plant-pathogens Pseudomonas syringae and Erwinia carotovora:

  • Snider, J. R., Layton, R. G., Caple, G., Chapman, D. 1985. Bacteria as condensation nuclei. J. Rech. Atmos. 19: 139-145

  • Franc, G. D. 1988. Long distance transport of Erwinia carotovora in the atmosphere and surface water. PhD thesis, Plant Pathology. Fort Collins, Colorado State University: 131 pp.

  • Franc, G. D., DeMott, P. J. 1998. Cloud activation characteristics of airborne Erwinia carotovora cells. J. Appl. Meteorol. 37: 1293-1300


Bacteria produce divers molecules on their surfaces among which hygroscopic polysaccharides could confer the CCN properties. Another surface property related to the interaction of bacterial cells with water is confered by detergent-like molecules called biosurfactants. These molecules enhance surface wettability and many are also powerful toxins because they can lead to instability of cell membranes.

In 2007 the team of Bruce Moffett and Tom Hill at Univ. East London made some interesting suggestions about the role of biosurfactants in cloud and rain formation. They suggested that by facilitating the formation of cloud droplets, biosurfactnts could lead to more but smaller cloud droplets thereby fostering fog and limiting the aggregation of droplets into larger drops that would fall as rain. This is reported in:

  • Ahern, H. A., Walsh, K. A., Hill, T. C. J., Moffett, B. F. 2007. Fluorescent pseudomonads isolated from Hebridean cloud and rain water produce biosurfactants but do not cause ice nucleation. Biogeosciences 4: 115-124 http://www.biogeosciences.net/4/115/2007/bg-4-115-2007.pdf



The 12 Jan 2009 issue of New Scientist contains a new report about the possible role of microbial biosurfactants in cloud processes. This report, concerning the recent work of Barbara Nozière of Stockholm University, Sweden, and colleagues can be seen at

http://www.newscientist.com/article/mg20126903.800-groundbased-bacteria-may-be-making-it-rain.html

Cindy Morris
INRA-Avignon, France
19 Jan 2009

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