An important oceanic source of micro-organisms for cloud water at the Puy de Dome (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 Wine summit (1465 m a.s.l.). Total bacterial and fungal cells were about, respectively, 8.1 x 10(4) and 5.9 x 10(3) mL(-1), with more than 10% of the fungi but < 1% of bacteria recovered by cultivation at 15 or 27 degrees 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 degrees 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 microorganisms 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. (c) 2007 Elsevier Ltd. All rights reserved.