Interactions microorganismes-nuage : activité glaçogène et survie

Abstract : Airborne microorganisms have long been considered as inert, passive particles dealing with hostile conditions. Recent studies highlighting metabolic activity in cloud water raised questions about the role these organisms may play on physical and chemical processes in clouds. Indeed, cloud droplets and ice crystals formation at temperature warmer than -36°C need the presence of particles called “cloud condensation nuclei” or “ice nuclei”. Bacteria could be one of them. In addition, several works revealed a potential importance of microorganisms in organic matter transformation in clouds. The objective of this thesis was to study the reciprocal interactions between microorganisms and physico-chemical conditions in clouds. First, cloud physico-chemical and microbiological compositions were described by cloud sampling at the puy de Dôme station (1465 m, France) and statistical analyses were performed to highlight correlations between physico-chemical and/or biological parameters. Secondly, five microbial strains belonging to genera frequently isolated from cloud water were subjected to four atmospheric stresses: sunlight, hydrogen peroxide, osmotic shocks occurring when water droplets condensate or evaporate and freeze-thaw cycles. Thus, it was pointed that sunlight and hydrogen peroxide at cloud concentration have no or little impact on cell viability. On the opposite, osmotic shocks and freeze-thaw can be highly deleterious depending on the considered strain. The third part of this thesis focused on the detection of ice nucleating bacterial strains in cloud water. Seven strains were thus identified and described, and one of them was selected as a model to study its behavior (survival and ice nucleation activity, INA) in a cloud simulation chamber (AIDA, Germany). In parallel, biological ice nucleation activity was measured directly on cloud samples and bacterial INA was estimated. All these experiments highlighted underestimations of ice nucleation active bacteria in models simulating microphysical processes in clouds. This new dataset may be used as new parameterization in this kind of models. Finally, in order to estimate the bacterial contribution in cloud chemistry, numerical means are needed. Therefore, the last study of this thesis focused on the determination of biological kinetic constants that may be implemented in atmospheric chemistry models. The biodegradation of three major organic compounds encountered in cloud water by three bacterial strains isolated from clouds was measured. A first approach confirmed precedent team results highlighting a considerable contribution of microorganisms on the transformation of these compounds.
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Muriel Joly. Interactions microorganismes-nuage : activité glaçogène et survie. Autre. Université Blaise Pascal - Clermont-Ferrand II, 2013. Français. ⟨NNT : 2013CLF22424⟩. ⟨tel-01136212⟩

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