Cloud bourne bacteria: community composition and potential impact on atmospheric nucleation

Microorganisms were discovered in clouds over 100 years ago but detailed information on community structure and function is severely limited. Clouds may be a niche within which bacteria could thrive and influence dynamic cloud processes using ice nucleating and cloud condensing abilities. Gaining an understanding of the bacterial communities and their possible role in these processes might introduce another discipline into meteorology and climate modelling.

Cloud and rain samples were collected in 2003 from Bowbeat Windfarm in the Scottish Moorfoot Hills and two mountains in the Outer Hebrides. Community composition was determined using a combination of amplified 16S ribosomal DNA restriction analysis and sequencing. 100 clones from the Bowbeat sample revealed ten OTUs of which three contained more than two clones. 256 clones from the Hebrides samples revealed 111 OTUs of which 33 contained two or more clones. In all the cloud samples the largest OTUs were identified as fluorescent Pseudomonas sp.

To investigate bacterial metabolic activity in clouds a further four cloud samples were collected from Bowbeat in 2006. Reverse transcriptase and quantitative PCR did not
definitively reveal metabolic activity in cloud bacteria, however the methodology requires further testing.

Heterogeneous nucleation is central to the Bergeron-Findeisen process of raindrop formation. Several bacterial species act as heterogeneous nuclei by producing an ice
nucleation (IN) protein. PCR targeting the IN gene of Pseudomonas fluorescens (InaW) in Pseudomonas isolates and cloud DNA did not amplify the IN gene.
Freezing cultures using differential scanning calorimetry also failed to reveal the IN phenotype.

A finding which evolved from the research was all the fluorescent Pseudomonas cloud isolates displayed biosurfactant activity. Surfactants are very important in the process of activating aerosols into cloud condensation nuclei (CCN). It is also known that surfactants influence cloud droplet size and increase cloud lifetime and albedo.
Some bacteria are known to act as CCN and so it is conceivable that these fluorescent pseudomonads could be using surfactants to facilitate their activation from aerosols into CCN. This might allow water scavenging, counter desiccation and aid their dispersal.

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