Bubble biofilm: Bacterial colonization of air-air interface

Susanne Sjöberga, Courtney Stairsb, Bert Allardc, Rolf Hallberga, Felix Homab, d, Tom Martinb, Thijs J.G. Ettemab, d, and Christophe Dupraza

a Department of Geological Sciences, Stockholm University, Stockholm, Sweden
b Department of Cell and Molecular Biology, Uppsala University, Sweden
c Man-Technology-Environment Research Centre (MTM) Örebro University, Sweden
d Laboratory of Microbiology, Department of Agrotechnology and Food Sciences, Wageningen University, the Netherlands

Abstract
Microbial mats or biofilms are known to colonize a wide range of substrates in aquatic environments. These dense benthic communities efficiently recycle nutrients and often exhibit high tolerance to environmental stressors, characteristics that enable them to inhabit harsh ecological niches. In some special cases, floating biofilms form at the air-water interface residing on top of a hydrophobic microlayer. Here, we describe biofilms that reside at the air-air interface by forming gas bubbles (bubble biofilms) in the former Ytterby mine, Sweden. The bubbles are built by micrometer thick membrane-like biofilm that holds enough water to sustain microbial activity. Molecular identification shows that the biofilm communities are dominated by the neuston bacterium Nevskia. Gas bubbles contain mostly air with a slightly elevated concentration of carbon dioxide. Biofilm formation and development was monitored in situ using a time-lapse camera over one year, taking one image every second hour. The bubbles were stable over long periods of time (weeks, even months) and gas build-up occurred in pulses as if the bedrock suddenly exhaled. The result was however not a passive inflation of a dying biofilm becoming more fragile with time (as a result of overstretching of the organic material). To the contrary, microbial growth lead to a more robust, hydrophobic bubble biofilm that kept the bubbles inflated for extended periods (several weeks, and in some cases even months).

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