Biotransformation of methanol and formaldehyde by bacteria isolated from clouds. Comparison with radical chemistry
Résumé
The kinetics of biodegradation of methanol and formaldehyde in phosphate buffer at pH 7 by 4 bacterial strains (Pseudomonas spp., Bacillus sp. and Frigoribacterium sp.) isolated from cloud water at the puy de Dôme mountain have been investigated using 1H and 13C NMR spectroscopy. We showed that biodegradation occurred at 5 °C and 17 °C, respectively average and summertime temperature considered within the cloud system at this site. They ranged from 10−19 to 10−21 mol cell−1 s−1 both at 5 and 17 °C for formaldehyde, and from 10−21 to 10−23 mol cell−1 s−1 at 5 and 17 °C for methanol. Metabolic intermediates were identified, with notably production of C3 compounds (glycerol, 1,2- and 1,3-propanediol) from formaldehyde by the strain Bacillus sp. In order to evaluate to which extent microbiological oxidation of organic compounds has to be considered as an alternative route to radical chemistry in cloud water, the biodegradation rates measured were compared with rates related to the reactivity of organic species with free radicals OH (daytime chemistry) and NO3 (nighttime chemistry) under two cloud situations (urban and remote cases). Clearly, measured biological and chemical reaction rates were in the same range of magnitude and their relative contribution varies according to the scenarios we tested, including the temperature of the clouds (5 or 17 °C), the category of the clouds (urban and remote) and the diurnal cycle (day and nighttime). Except for the degradation of methanol at 5 °C in remote clouds, our results show that biotransformation processes could be the main sink for C1 compounds in liquid clouds (T ≥ 5 °C ≡ "warm cloud") during the night and both in polluted and non polluted clouds.