How to Train Your Shewanella

Katherine Mateos, Carleton College

The Approach: In my previous post, I introduced my project, investigating the role of Antarctic bacterium, Shewanella BF02, in the cycling of volatile organic sulfur compounds (VOSCs). 

Sterile technique in action Photo Credit: Peter Lee

The first order of business in this effort is keeping the Shewanella alive and happy. In order to do this in the lab, I make a liquid (known in the biology world as “medium”) for the Shewanella to live in. Our medium is designed to resemble Blood Falls in chemical makeup. In particular, it is very salty, and contains iron and sulfate. I am also careful to remove all the dissolved oxygen in the medium, since the Blood Falls water has very little oxygen. In my medium, I am also careful to keep out any bacteria other than my Shewanella. Since microbes are everywhere, including in the air, on my skin, and on the lab bench, I  use a special set of techniques to avoid unwanted bacteria from infecting my samples. 

Membrane Inlet Mass Spectrometer

Once we have a perfect mix of chemicals for Shewanella, I also add my target organic sulfur compounds. Because I want to see if Shewanella changes these added compounds, I keep track of them using a technique called isotope labeling. Isotope labeling is a clever trick, where the target compounds are tagged with atoms that are the tiniest bit heavier than the ones that we usually see. If Shewanella make the labeled compounds into the VOSC products that I am interested in, those products will also have the same tag, making it easy to identify them.

To identify the tiny differences in mass between tagged and untagged molecules, I use a piece of equipment called a mass spectrometer. A mass spectrometer works kind of like a scale and can determine the mass of each molecule. This allows me to detect isotopically labeled VOSC products. If I see isotopically labeled products, I can be pretty sure that the Shewanella are cycling the labeled compound that I added to their medium. 

Thank you to my mentor, Dr. Peter A. Lee, and our collaborators, Dr. Jill Mikucki and Abigail Jarratt, for their guidance in the research process. This project is supported by the Fort Johnson REU Program, NSF DBI-1757899.

Shewanella: Sneaky Sulfur Cyclers?

Katherine Mateos, Carleton College

Like Shewanella, I too thrive at cold temperatures!

Life can survive almost anywhere! From hot pools on volcanoes to beneath ice-cold glaciers, pretty much all of the inhabitants in these hostile environments are so small that you cannot see them with your bare eye. These extremophiles, as they are often called, include tiny single-celled microbes—bacteria and archaea. By studying tiny microbes we can answer big questions: How did life begin on Earth?  How can we find life on other planets? How will our planet respond to its changing climate?

Outflow of Blood Falls on the Taylor Glacier. Image credit: Dr. Jill Mikucki.

This summer, I am working with one of these extremophiles, a type of bacteria separated out from a sample from Blood Falls, Antarctica. This lake is a pool of brine (very salty water) covered by more than 150 feet of ice from the Taylor Glacier. Blood Falls gets its name from the bright red stain that the brine leaves on the Taylor glacier as it leaks out from beneath the glacier. As you would expect, this location is cold and dark, but the chemicals in the brine are what truly make this ecosystem extreme. For one, Blood Falls is super salty, over twice as salty as the ocean. Most water has oxygen trapped within it, but Blood Falls has very little. Two important chemicals are also found in unusually high quantities: iron and sulfur.

Electron Microscope image of Shewanella BF02. Image credit: Bruce Boles.

The bacteria that I am studying makes good use of the iron in this environment. Like a battery produces energy from a variety of chemical reactions, Shewanella (strain BF02) gets most of its energy by harnessing the energy that is released when one chemical form of iron changes to another. However, there might be another source of energy Shewanella can live off of—perhaps a chemical that contains sulfur. Sulfur is one of the most common elements on earth, found in pesticides, foods, and in humans. Sulfur can form compounds with other common elements including hydrogen, carbon, and oxygen. Some of these chemicals, known as volatile organic sulfur compounds (VOSCs), easily evaporate into our atmosphere and affect our environment. We want to know if the Shewanella are creating these VOSCs, and if they do, what chemicals the Shewanella turn into VOSCs.

The strain of Shewanella that I am studying is from an extreme ecosystem but similar Shewanella are found throughout many ocean ecosystems. We can treat Blood Falls as a model to learn about the way that our oceans will affect our environment.  Even though Shewanella are too small to see with your bare eyes, figuring out what compounds they break down can help us understand the future of the environment around the world.

Thank you to my mentor, Dr. Peter A. Lee, and our collaborators, Dr. Jill Mikucki and Abigail Jarratt, for their guidance in the research process. This project is supported by the Fort Johnson REU Program, NSF DBI-1757899.

References

Mikucki, J. A. et al. A contemporary microbially maintained subglacial ferrous ‘ocean’. Science 324, 397–400 (2009).

Sievert, S. M., Kiene, R. P. & Schulz-Vogt, H. N. The sulfur cycle. Oceanography 20, 117–123 (2007).