An important oceanic source of micro-organisms for cloud water at the Puy de Dôme (France)

A description of the microbial content of cloud water based on samples collected along an almost 2-year-period is presented. Cloud water from 14 events was sampled at the Puy de Dôme summit (1465 m a.s.l.). Total bacterial and fungal cells were about, respectively, 8.1×104 and 5.9×103 mL−1, with more than 10% of the fungi but <1% of bacteria recovered by cultivation at 15 or 27 °C. However, ATP concentration of about 0.40 pmol mL−1 shows that a large majority of these cells are likely viable but not cultivable and remain alive in clouds. A high variability is noticed in the microbial content, and local meteorological variations are not involved. A seasonal effect is shown, with a general increase in the concentrations of cultivable micro-organism and of total fungal cells during summer and autumn. Moreover, psychrotolerant micro-organisms, with respect to those growing only at 27 °C, are more numerous during winter. The concentrations of micro-organisms (total and cultivable) were clearly linked to the chemical composition of cloud water: an increase with increasing oceanic contribution is pointed, and bacteria concentration decreases with increasing anthropic influence. A preferential integration of micro-organisms emitted by the ocean into cloud droplets, compared to micro-organisms from other sources, is likely to occur, making the ocean a major source of micro-organisms for cloud water. It also suggests that the toxicity of polluted cloud water could disturb an eventual multiplication of cells in atmospheric droplets.