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Report

Teaser, summary, work performed and final results

Periodic Reporting for period 1 - OILY MICROCOSM (Mechanistic Microscale Approach to the Microbial Degradation of Oil-Droplets in Subsea Crude Oil Releases)

Teaser

Summary

In the aftermath of natural or accidental releases of crude oil in the sea, part of the oil ends up in clouds of droplets that travel along with underwater sea currents and disperse deep into the oceans. The droplets may be created either at the sea surface during the breakup of floating oil layers by sea waves, or at the seafloor during the extrusion of crude oil from natural cracks and broken wellheads. In spite of the frequent and extended contamination of marine waters with hydrocarbons from surface and deep-sea oil spills, including massive spills like the Ixtoc I (1979), Exxon Valdez (1989), Prestige (2002) and many others, the occurrence and significance of underwater droplet clouds was only discovered during the recent Deepwater Horizon event (2010).

Since then, numerous studies have demonstrated that excessive amounts of dispersed oil droplets in seawater disturb the established dynamics of the local ecosystem (e.g., carbon cycle, marine microbiome structure, micronutrient and oxygen depletion, marine snow blooms). On top of that, when microsized oil droplets are ingested by fish and other marine animals, not only they pose an imminent risk of toxicity to the animals but also might go up the food chain and end on the plate of humans. At present, there are no practical means for the collection or in situ treatment of oil droplets in vast bodies of marine waters and, thus, the lifetime of underwater droplet clouds is determined by natural attenuation processes, mainly dissolution into the seawater and biodegradation by oil-eating microbial communities.

It is therefore imperative to understand and quantify the physical and biological mechanisms that rule the fate of dispersed oil droplets in marine waters and, upon that knowledge, build technologies that will enable the mitigation of pertinent adverse effects. The overarching scope of the OILY MICROCOSM project is to obtain an improved understanding of the fundamental microscale mechanisms that underpin the biodegradation of droplet clouds by microbes at both the single-droplet and droplet-population levels through a creative combination of microfluidics, biochemical analyses and computational modeling.

Work performed

The research activities during the outgoing phase of the project were focused on oil biodegradation at the single-droplet level of observation.

We have developed a novel theoretical model for estimating how long individual oil microdroplets may persist in the seawater while being consumed by oil-eating microbes. A compound particle of the core-shell type is used to represent an oil microdroplet successively surrounded by an ultrathin skin of oleophilic microbes (a couple micrometers) and another biofilm layer of finite thickness (tenths of micrometers). The model accounts for three major biodegradation modes (i.e., interfacial uptake, bioreaction in the biofilm layer, and bioreaction in the bulk aqueous phase), as well as for the effects of oxygen limitation (hypoxia) and multiple oil components with varying toxicity and bioavailability. This compound particle model provides estimates for the shrinking rate, size evolution and residence time of oil microdroplets in a water column as functions of key system parameters (drifting velocity, microbial kinetics, diffusivity, solubility, density). An interesting point raised by the theoretical analysis is that biofilms with high concentration of fast oil-eating microbes and lipophilic biopolymers would be ideal for oil biodegradation applications, as such biofilms are expected to retain the solubilized oil until complete degradation instead of releasing it into the water column.

We have also carried out in vitro experiments on the biodegradation of hexadecane droplets by Marinobacter sp. microbes in synthetic saltwater, using standard flask microcosms and novel microfluidic devices. The fluidic devices are constructed using microfabrication techniques and enable the generation, entrapment, long-term incubation and microscopic imaging of oil droplets while they undergo microbial degradation. In flask microcosm experiments, the Marinobacter microbes exhibit a very strong emulsification activity against hexadecane, which is accentuated by even minute amounts of soluble carbon sources (e.g., peptone). Qualitative visualization experiments have demonstrated that these microbes also form thick biofilms on the surface of hexadecane droplets (see also cover image).

Final results

\" Current theoretical models for the fate of oil droplets in marine waters account only for the biodegradation via direct interfacial uptake and neglect any effects resulting from the formation of biofilms around the droplets or the limitation in the oil consumption rate that may be caused by low oxygen availability. The compound particle model developed in this project is the first and simplest possible model that accounts for (some) biofilm and hypoxia effects on the biodegradation of oil droplets. With regard to hypoxia effects, the anaerobic biodegradation of certain hydrocarbons is also feasible but very slow. Areas with low concentration of dissolved oxygen are known as \"\"dead zones\"\" and exist in marine waters throughout the globe. In those areas, the biodegradation of dispersed oil might slow down or not even take place at all. Therefore, it is expected that the compound particle model will eventually be incorporated in macroscale models that track the fate of underwater droplet clouds at the ocean level of observation. Until the end of the project, we shall further relax certain hypotheses of the compound particle model and also combine it with a droplet cloud simulator that accounts for droplet-droplet and droplet-marine snow interactions (collision, clustering, breakup).

Previous microscale experiments of oil biodegradation were based on closed systems (batch bioreactors) and/or were conducted without fluid flow. The millifluidic platform devised in this project is not subject to these two important limitations. During the return phase of the project, this platform will be used to investigate the biodegradation of crude oil droplets by different marine microbes.
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