Growth and formaldehyde degradation of photoheterotrophic Methylobacterium within radiation fogs.

UNLABELLED: The atmosphere contains thousands to millions of bacterial cells per cubic meter. However, it remains unclear if microbes are at all active or growing in situ or whether they are merely being transported in an inactive state. Based on the analyses of 32 overland radiation fog events over a 2-year period, we show that fog waters, with bacterial concentrations similar to those in continental or marine bodies of water, contain microbiomes well differentiated in composition from those in the dry aerosol microbiomes that occur locally before, during, or after fog events. They are consistently and strongly enriched in photoheterotrophic Methylobacterium species, suggesting that fog populations may be metabolizing volatile C1 compounds in situ, although phototrophy seems much less important. Indeed, metabolically active bacteria in the fog, and representative isolates of the main field populations, can degrade formaldehyde at unprecedently high rates; most of this activity seems to play a detoxification role. The increase in bacterial aerobiome counts upon intervening fog events, the dependence of microbial concentration on ambient temperature, the increases in cell size and frequency of dividing cells in fog water with respect to cells in interstitial aerosols of fogs, in addition to their metabolic capacity, all suggest that the fog water microbiome is actually growing. Consequently, droplets of atmospheric water should be considered a potential aquatic microhabitat. Our results highlight the fog microbiome's role in atmospheric chemistry and have implications for fog harvesting as a source of fresh water for human use.

IMPORTANCE: While bacteria are common in the atmosphere, their activity in situ has remained unclear. Using stagnant radiation fogs as new study systems where sampling is optimal, the dynamics, composition, cellular characteristics, and metabolic rates of fog water microbiomes, dominated by Methylobacterium sp., show that they are a hub of active detoxification of atmospheric formaldehyde and likely growing in situ on the basis of heterotrophic or photoheterotrophic metabolism of volatile C1 compounds, with implications for atmospheric chemistry and fog harvesting as sources of freshwater.