How microorganisms degrade oil droplets in the marine environment
Although the fate of crude oil in aquatic ecosystems has been extensively studied over several decades, the occurrence and significance of underwater droplet clouds was only discovered following the Deepwater Horizon accident in 2010. At present, there is no in situ treatment based on the characteristics of the oil droplets in underwater plumes. The EU-funded OILY MICROCOSM project addressed this knowledge gap by investigating the fundamental microscale mechanisms that control the biodegradation of crude oil droplets by marine microbes. “This was achieved through a creative combination of microfluidics, advanced imaging and computational modelling,” explains project coordinator Nicolas Kalogerakis. The MSC fellow, George Kapellos, collaborated with the Massachusetts Institute of Technology in the United States (Prof. Patrick Doyle’s lab), employing a toolbox of microfabrication techniques, including lithography and 3D printing to develop novel microfluidic devices. These devices enabled scientists to observe under the microscope how the oil droplets interact with marine microbes over a period of several weeks.
Role of microorganisms
Project partners used advanced imaging methods like multiprobe fluorescence and confocal microscopy to examine the morphology of biofilm-coated hydrocarbon droplets and posited a new hypothesis regarding the mechanism responsible for their formation. “The biofilm-coated oil droplet grows as highly active microbes accumulate close to the oil-biofilm interface. As they multiply and secrete biopolymers, they force the surrounding biomass outwards,” notes Kalogerakis. Scientists witnessed this phenomenon for the first time as a shrinking oil droplet was consumed by the hydrocarbon-degrading microbes. “We observed that under certain conditions specific marine microbes are able to colonise the surface of hydrocarbon droplets and form thick wrinkled biofilms,” he reveals. Researchers found, to their surprise, that microbial biofilms could successfully encapsulate even large millimetre-sized droplets, which has never been reported before for either natural or artificial systems. Now, the OILY MICROCOSM team aims to discover whether these huge biofilm-coated droplets occur in natural marine waters after an oil spill.
Answers from models
The project also developed a new mathematical model to predict the biodegradation rate and residence time of oil microdroplets in a water column. The model considers the effects of the drift velocity, the microbial kinetics, and the availability of oxygen, essential minerals and multiple oil components. “We found that deficiency in oxygen or other water-derived nutrients, like nitrogen and phosphorus, limits the droplet biodegradation rate by driving highly active microbes away from the oily surface,” points out Kalogerakis. Furthermore, the mathematical model also provides a valuable tool in decision-making. For example, the use of emulsifiers to disperse oil spills should be avoided in ‘dead zones’ with very low oxygen availability. OILY MICROCOSM also paved the way to a better understanding of the biodegradation of hydrocarbons at the single-droplet level. It also made a valuable contribution to the data analysis from experiments at very high pressures (> 250 bar) that resemble potential hydrocarbon deep-sea releases in the eastern Mediterranean Sea. Hence, project work and outcomes will contribute significantly to our understanding of the oil biodegradation process and will drive future research efforts, key enabling technologies and decision-making tools for the mitigation of adverse effects from marine oil spills. This research was undertaken with the support of the Marie Skłodowska-Curie programme.
OILY MICROCOSM, biodegradation, oil spill, crude oil, marine microbes, microfluidic, mathematical model, droplet cloud, biopolymer, microdroplet