Paper: Cable Bacteria and the Bioelectrochemical Snorkel: The Natural and Engineered Facets Playing a Role in Hydrocarbons Degradation in Marine Sediments. (2017) Bruna Matturro, Carolina Cruz Viggi, Federico Aulenta and Simona Rossetti. Frontiers in Microbiology. DOI: 10.3389/fmicb.2017.00952
This article is a part of the World Oceans Week series, and highlights oceanic bioremediation.
Sometimes, duct tape doesn’t fix everything
Oil spills are a problem. You’ve seen the heart breaking pictures of ducks and otters covered in shiny slimy black grease. You’ve seen ads for Dawn dish soap, where people use toothbrushes and suds to gently clean up the animals, slowly releasing them back into the wild. We’re not going to dwell on that today. Today, we’re going to learn about how we can use science and things already in the environment to clean up spilled oil.
Clean up this mess!
There are some physical and chemical ways to clean up oil. We mentioned dish soap, which is something called a surfactant. Much like how you use soap to get grease off of pans, you can use it to break up and clean up oil, too. This can make it easier for winds and water currents to disperse the oil, or even make it sink. While it dilutes the oil, it doesn’t actually make it go away. You can also use ‘boomers’ and ‘skimmers’ to clean up oil physically. Oil spills are like cooking oil when you pour it in water – it floats. The boomers are floats that keep the oil on the ocean surface from floating away, then skimmers slurp up the oil. This still isn’t easy though, and if the weather is rough, it isn’t very effective.
Have you been told ‘mother knows best’? Sometimes, mother nature knows best. There are naturally occurring bacteria in the ocean that eat oil. These are often called ‘hydrocarbon degrading bacteria’. These bacteria will take the oil, nohm on it, and eventually the oil gets turned into carbon dioxide. The oil nohmers do this best when they have oxygen. In the open ocean, oxygen usually isn’t a problem to get (with some notable exceptions!). In the ocean’s underlying mud, though, oxygen can disappear quickly since so many bacteria are trying to use it. How then, do you naturally clean up oil that has seeped into mud?
Different scientists have tried different ways to help out our oil eating bacteria. One thing you can do it just leave it alone. There are some bacteria who eat oil, and don’t need oxygen. That takes a while though. You can pump oxygen into the mud, but that doesn’t work very well because oxygen quickly disappears again, reacting with lots of muddy minerals and metals. Scientists noticed a clever thing that some bacteria do though – they can use electrodes instead of oxygen.
Electrodes probably sound familiar, but you may not know exactly what they do. Don’t worry, I had to look this up too. Basically, electrodes are things that energy can flow through. They can be a lot of different things, like metals, and can accept electrons (anodes) or give them (cathodes). Some electrodes have one end that’s an anode, and one that’s a cathode. You might be wondering what this has to do with bacteria. When bacteria (or animals, or us) breathe, they are basically moving electrons between different chemical compounds, like hydrocarbons and oxygen. It turns out, not all bacteria are picky about where they ‘dump’ their electrons, so instead of dumping them on oxygen, they dump them on the anode of an electrode. These were given the awesome name ‘electroactive bacteria’.
Scientists in Italy decided to use these electroactive bacteria to clean up oil in mud. To do this, the scientists set up microcosms, which are almost like little fish tanks for microbes. The scientists added oil to natural sea mud, then put that in these microcosms, put some natural seawater on top, and put graphite electrodes that they dubbed Oil Spill Snorkels half into the mud, leaving half in the seawater. Then, they waited.
After 200 days, the scientists collected samples from the microcosms for microbiological study. They did two main things: microscopy through a technique called CARD-FISH; and DNA sequencing to see what microbes were in the mud and what microbes were attached to the electrodes.
The scientists found some surprising things. First, they found that cable bacteria were growing in the mud. Cable bacteria are awesome, and possibly one of my favorite types of life. Cable bacteria live in mud, and make long chains of themselves (sometimes even centimeters long!), forming almost straw-like cables. Other than making sure they never get lonely, the bacteria do this so that they can breath things, like oxygen, while using other chemicals that are buried deeper in the mud. They are almost forming their own electrode. The scientists figured this out by using CARD-FISH. CARD-FISH stands for Catalyzed Reporter Deposition-Fluorescence In situ Hybridization. This is when you make a ‘reporter’ with piece of DNA that is attached to a fluorescent molecule, and that DNA attaches to a specific piece of DNA in an organism of interest. In this case, the organism of interest was a type of cable bacteria called Desulfobulbaceae. You can then look through a microscope to see the fluorescent part of the reporter, which lets you tell if you have that organism of interest, and in this case, if that organism has formed cables. In this case, the answer to both of these was yes!
Second, the scientists found that the mud had a very different set of microbes living in it than the Oil Spill Snorkels electrodes had living on them. The mud had the cable bacteria we just discussed, plus a lot of microorganisms that can use different kinds of sulfur compounds and can degrade oily hydrocarbons, but don’t need oxygen. The Snorkels had attached to them lots of microorganisms that degrade hydrocarbons, too, but that might be using the Snorkels instead of oxygen to get rid of their unwanted electrons. Then the Snorkel itself can transport electrons to the mud’s surface when there is oxygen in the water. The electrons dumped onto the Snorkel can react with the seawater’s oxygen to maintain the Snorkel’s preferred electron balance. The oxygen gets turned into water – so the by products of the process aren’t toxic! Additionally, this team of scientists calculated how diverse the bacteria community on the Snorkel and in the mud were. They found that the mud community was more diverse, and that the community on the Snorkel was less diverse, indicating specialized bacteria live on the Snorkel.
So overall, what does this all mean? The scientists found two systems in their microcosm Snorkel experiments. They found one that breaks down oil without oxygen or the snorkel, maybe through these cable bacteria that naturally occur in the sediments. They also made the system using the Snorkel, where bacteria eat oil, then take energy from that oil in the form of electrons, and dump them onto the Snorkel.
A New Hope
The Oil Spill Snorkel is a really exciting and nifty invention. Since it’s a single rod electrode made from graphite, like what you find in your pencils, it’s relatively easy to make and non-toxic. Unlike other things used as electrodes, there’s no glass, metal, or membranes, all of which are susceptible to breaking or corroding. It is cheap and doesn’t require much effort to install – in this study, the authors just stuck it in mud!
The goal of this Snorkel and other technology like it, is to work with the natural environment to clean up oil spills. The huge Deep Water Horizon oil spill of 2010 showed us that microbes degrading oil can really help with fixing pollution. Other types of environmental contamination can be cleaned up by microbes as well, and scientists are exploring using those as well. Waste water, trash, toxic compounds – microbes can break these down, and produce relatively safe products in return. This oceanic bioremediation is an exciting field of research, and one that can hopefully clean up the planet!
I am a PhD student studying sediment geomicrobiology at the University of Southern California. My primary research interests lie deep under the sea studying how microorganisms survive in dark environments and how they interact with chemical cycles in sediment and on earth. When I surface from my studies, I enjoy backpacking, trying to mimic my ridiculous dog, and applying my laboratory techniques in the kitchen.