Air currents provide a rapid means of dispersal for live microbes on very large scales, with the potential to spread diseases, influence microbial population composition over hundreds of kilometers, and impact atmospheric phenomena. While the field of aerobiology has explored the distribution and diversity of microbes found in the air, our understanding of microbial interactions between the atmosphere and aquatic systems is severely lacking. Specifically, the dynamics of microbial fluxes into the atmosphere are poorly understood. The mechanisms by which marine microbes leave the ocean give us our first avenue for investigating this microbial flux: above the ocean, microbial aerosolisation arises largely from bursting bubbles. Bubbles are produced in the ocean by crashing waves and these bubbles capture microbial cells on their surfaces as they rise through the water. These cells are subsequently trapped in micro-droplets ejected into the air when the bubbles burst on the ocean surface. This ‘scavenging’ of microbes by a rising bubble is key, as it can lead to a thousand-fold increase in the microbial content of aerosols. Understanding the microbial characteristics which lead to capture by bubbles is therefore fundamental to characterizing the ocean-atmosphere microbial flux. This would further support aerobiology models and our understanding of disease outbreaks, as well as contribute to elements of atmospheric research like cloud formation physics, given the very effective ice nucleation activity of some microbes. Finally, the separation of microbes from their suspending medium by rising bubbles, or ‘flotation’, could offer a powerful technique for the growing industry of microbial biosynthesis. These broad impacts demonstrate that the value of this work extends beyond understanding the nature and abundance of microbial aerosols. This project aims to combine microfluidic techniques with microscopic imaging to address the present knowledge gap, by performing the first direct observations of microbial collection by bubbles at the microscale, thus providing a major step forward in our understanding of the interaction between bubbles and microbes. Specifically, the objectives of this project are to investigate two hypotheses.
First, we investigate whether microbial motility (or ability to swim) increases microbial collection by rising bubbles. Indeed motility sets microbes starkly apart from inert particles, likely promoting collection by increasing the likelihood of encountering bubbles, and also influencing attachment as swimming appendages modify the cell surface properties. Second, we investigate whether starvation of microbes increases microbial collection by rising bubbles. Starving bacteria modify their surface properties and size, which may make them ‘stickier’ and hence enhance collection by bubbles –this might promote dispersal and escape from nutrient poor areas. Aerosolisation could then represent an active stage in the life cycle and ecological dynamics of bacteria in oceans.
To investigate these hypotheses, this project’s goal is to develop a novel microfluidic flow channel containing a pinned bubble, and use advanced optical microscopy to quantify collection rates for a range of microbes on a single bubble in flow. By bringing together pioneering quantitative microscopy and characterisation of flow regimes and microbial surface properties, the proposed research will determine the role of key factors influencing microbial collection by bubbles and develop new mathematical models for microbial aerosolisation.
