How bacteria help clean up oil spills

Nearly 5 million barrels of oil spilled into the Gulf of Mexico after the 2010 Deepwater Horizon explosion—the largest marine oil spill in history. One of the oil-eating marine bacteria that descended on the site was Alcanivorax borkumensis, also known as Alca, which can survive in the ocean solely from the consumption of hydrocarbons.

Essentially, all bacteria form biofilms—vast communities of the microorganisms that are held together by slimy extracellular substances and, in the case of Alca, that stick to the surfaces of oil droplets. Alca‘s biofilms deform oil drops and eat the hydrocarbons, or they can sometimes transform the drops into larger aggregate structures that can then sink to the seafloor.

Andrew Utada (University of Tsukuba), Jacques Fattaccioli (Pierre-Gilles de Gennes Institute, ENS-PSL), Jean-François Rupprecht (Aix-Marseille University), and their colleagues have now identified how the biofilms form. Using a microfluidic device they developed, the researchers observed and quantified the biofilm formation process in real time, from its beginning with 20–50 cells to complete consumption of the oil droplets.

With confocal microscopy, the team observed two biofilm morphologies, shown in the figure with scale bars of 10 µm. Bacteria that were fed for a day or less before the experiment typically formed spherical biofilms, whereas those on a five-day diet formed a Medusa head of thin, branching biofilm tubes, or dendrites. Unexpectedly, the bacteria that were fed for two to four days switched more than 10 times between a spherical shape and large tubular structures.

The oil-eating bacteria secrete biosurfactant molecules that, like soap, lower the surface tension of the oil–water interface. That makes it easier for Alca to deform the oil drops into the dendritic shape.

Looking at 2D confocal image slices of the oil drops revealed a new clue about how the tubes form. Between two and four hours, when the tubes first start to grow, the researchers identified cellular orientations that look somewhat similar to the petals of a flower.

In liquid-crystal theory, that pattern is known as the +1 charge topological defect. It’s important because as more bacterial cells divide and add to the population, the biofilm buckles the water–drop interface at the defects. After tubes then form, the cells align along their long axes.

Those oil-filled tubes increase the oil drop’s surface area and thus accelerate its consumption. The bacteria of the spherical biofilm morphology ate 90% of the oil volume after about 72 hours, whereas the bacteria with the dendritic morphology took just 20 hours to eat the same volume. (M. Prasad et al., Science 381, 748, 2023 .)