Microorganisms isolated from the water phase of tropospheric clouds at the Puy de Dôme: major groups and growth abilities at low temperatures.

This work constitutes the first large report on aerobic cultivable microorganisms present in cloud water. Seven cloud-event samples were collected at the Puy de Dôme summit, and cultivation was performed leading to the isolation of 71 bacterial, 42 fungal and 15 yeast strains. Most of the fungi isolated were of Cladosporium or Trametes affiliation, and yeasts were of Cryptococcus affiliation. Bacteria, identified on the basis of their 16S rRNA gene sequence, were found to belong to Actinobacteria, Firmicutes, Proteobacteria (Alpha, Beta and Gamma subclasses) and Bacteroidetes phyla, and mainly to the genera Pseudomonas, Sphingomonas, Staphylococcus, Streptomyces, and Arthrobacter. These strains appear to be closely related to some bacteria described from cold environments, water (sea and freshwater), soil or vegetation. Comparison of the distribution of Gram-negative vs. Gram-positive bacteria shows that the number of Gram-negative bacteria is greater in summer than in winter. Finally, a very important result of this study concerns the ability of half of the tested strains to grow at low temperatures (5 degrees C): most of these are Gram-negative bacteria, and a few are even shown to be psychrophiles. On the whole, these results give a good picture of the microbial content of cloud water in terms of classification, and suggest that a large proportion of bacteria present in clouds have the capacity to be metabolically active there. This is of special interest with respect to the potential role of these microorganisms in atmospheric chemistry.