Within cloud water, microorganisms are metabolically active and, thus, are expected to contribute to the atmospheric chemistry. This paper investigates the interactions between microorganisms and the reactive oxygenated species that are present in cloud water because these chemical compounds drive the oxidant capacity of the cloud system. Real cloud water samples with contrasting features (marine, continental and urban) were taken from the puy de Dôme mountain (France). The samples exhibited a high microbial biodiversity and complex chemical composition. The media were incubated in the dark and subjected to UV radiation in specifically designed photo-bioreactors. The concentrations of H2O2, organic compounds and the ATP/ADP ratio were monitored during the incubation period. The microorganisms remained metabolically active in the presence of *OH radicals that were photo-produced from H2O2. This oxidant and major carbon compounds (formaldehyde and carboxylic acids) were biodegraded by the endogenous microflora. This work suggests that microorganisms could play a double role in atmospheric chemistry; first, they could directly metabolize organic carbon species, and second, they could reduce the available source of radicals through their oxidative metabolism. Consequently, molecules such as H2O2 would no longer be available for photochemical or other chemical reactions, which would decrease the cloud oxidant capacity.