Characterization and DNA Stable-Isotope Probing of Methanotrophic Bioaerosols

The growth and activity of bacteria have been extensively studied in nearly every environment on Earth, but there have been limited studies focusing on the air. Suspended bacteria (outside of water droplets) may stay in the atmosphere for time frames that could allow for growth on volatile compounds, including the potent greenhouse gas methane. We investigated the ability of aerosolized methanotrophic bacteria to grow on methane in the airborne state in rotating gas-phase bioreactors. The physical half-life of the aerial bacterium-sized particles was 3 days. To assess the potential for airborne growth, gas-phase bioreactors containing the aerosolized cultures were amended with 1,500 ppmv ¹³CH₄ or ¹²CH₄. Three of seven experiments demonstrated ¹³C incorporation into DNA, indicating growth in air. Bacteria associated with the genera Methylocystis and Methylocaldum were detected in ¹³C-DNA fractions, thus indicating that they were synthesizing new DNA, suggesting growth in air. We conclude that methanotrophs outside of water droplets in the air can potentially grow under certain conditions. Based on our data, humidity seems to be a major limitation to bacterial growth in air. Furthermore, low biomass levels can pose problems for detecting ¹³C-DNA synthesis in our experimental system.