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L. R. Maki and K. J. Willoughby

Abstract

Bacteria which produce freezing nuclei active at temperatures warmer than −10°C have been isolated from leaves, water and snow. Those which produce freezing nuclei active at the warmest temperatures include strains of P. fluorescens and P. syringae. Only cultures of P. fluorescens were shown to be active in a cloud simulator. Activities as high as one freezing nucleus per bacterial cell were observed, while activities of one nucleus per 20–100 cells were more common. The nucleating activity appears to be associated with the cell wall or cell wall fragments of the bacteria.

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G. Vali, M. Christensen, R. W. Fresh, E. L. Galyan, L. R. Maki, and R. C. Schnell

Abstract

Transient appearance of ice nuclei active at temperatures of −2 to −5°C has been noted to accompany the natural decay of plant leaf materials. It was shown that the development of these nuclei results from the presence of a bacterium which was identified as Pseudomonas syringae. These bacteria produce highly active nuclei in a variety of growth media. Evidence points to the fact that the bacterial cells themselves are the nuclei, but that nucleating capacity is a rare and changeable property of the cells. The findings raise the possibility that bacteria may play a role in atmospheric precipitation processes.

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