<p>Metabolically active microorganisms are increasingly acknowledged as actors of cloud chemical reactivity able to use organic compounds present in clouds (e.g. organic acids, aldhedydes) for their metabolism (Vaïtilingom et al., 2013). Uncharacterized biological activity may play a major role especially during the night, while during daytime the abiotic degradation of organic compounds would be driven and dominated by hydroxyl radical (•OH) chemistry (Vaïtilingom et al., 2011). To better understand and predict the impact of biological activity on atmospheric chemical reactivity, the metabolic pathways of the whole cloud microbiome and their modulations by environmental conditions (temperature, light, oxidants) must now be assessed.</p> <p>The METACLOUD project addresses metabolic acclimatation of cloud microorganisms under two contrasted situations simulating a summer day (17°C, with solar light and presence of hydrogen peroxide) and a winter night (at 5°C, in the dark and without hydrogen peroxide). A focus is made on formaldehyde assimilations as this compound is a key intermediate both in cloud radical chemistry and in many C1 biological pathway, using fluxomics (LC-HRMS and IC-HRMS) on <sup>13</sup>C-formaldehyde supplemented samples. Experiments were conducted in specially designed photobioreactors, either on (1) freshly sampled cloud water from the research station at the top of the puy de Dôme station (1465m asl, PUY, France) including naturally present microorganisms, or (2) an artificial consortium assembled from microbial strains isolated from cloud water sampled at PUY and resuspended in an artificial medium mimicking the composition of marine cloud water (major inorganic and organic compounds).</p> <p>Metatranscriptomes and metabolomes indicate metabolic acclimations of the cloud microbiome to model summer/winter conditions, especially linked with fatty acid regulation and central metabolism (e.g. citrate cycle). First results with <sup>13</sup>C-formaldehyde showed carbon incorporation from this molecule into several classes of metabolites (e.g. nucleotides, amino acids, central metabolites), illustrating the complex biological fate of this compound in the environment. The data will be used to implement biological activity on cloud chemistry models.</p> <p> </p> <p>Vaïtilingom M. <em>et al. </em>(2011) Atmospheric chemistry of carboxylic acids: microbial implication versus photochemistry. <em>Atmos. Chem. Phys. </em>11, 8721-8733. doi: 10.5194/acp-11-8721-2011.</p> <p>Vaïtilingom M. <em>et al. </em>(2013) Potential impact of microbial activity on the oxidant capacity and organic carbon budget in clouds. <em>Proc. Nat. </em><em>Acad. Sci. USA </em>110, 559-564. doi: 10.1073/pnas.1205743110.</p>