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Iron in the Environment

Have you ever been out hiking in the woods and seen patches of slimy orange goop on the ground or near the water? Maybe there was an opalescent film on the top that looks just like the rainbow sheen of an oil spill. What did you think that was? Pollution? Toxic sludge? Actually, those are communities of naturally occurring, iron-oxidizing bacteria.

We study those bacteria in our lab, and we were thrilled to find them in such abundance at the Bull Run Mountain Conservancy, especially in several places along Fern Hollow Trail. Not a lot is known about these bacteria, and the researchers in our lab are looking at several different aspects of the way they live, what other bacteria they are related to, and even whether similar organisms could once have been on Mars. While we can grow them in our lab, it is always good to have a fresh sample from the environment, something that BRMC has in ample supply.

Iron is the fourth-most abundant mineral in the earth's crust, and it is everywhere. Iron seeps are locations where there is a significant amount of soluble iron in the soil and enough standing water to encourage bacterial growth. The bacteria need oxygen to survive, but not too much, so you will usually find them in the sediment, or under a few centimeters of water, rather than on the surface. Growth of iron-oxidizing bacteria is visible as the orange fluffy or slimy material-we call it iron floc-composed of iron oxides, microorganisms, extracellular structures some of the bacteria produce, and the occasional worm.

Some seeps, such as those in wetlands adjacent to rivers or streams, can be active constantly. Others are terrestrial wetweather seeps, where visible microbial iron oxidation only occurs during months with lots of precipitation. We have found iron seeps in locations as disparate as deep forests and golf courses.

Iron floc can also be found in quiet pockets of faster moving creeks or streams. The bacteria that produce extracellular structures—which frequently take the form of hollow sheaths or spiral stalks—are often the responsible agents in the clogging ("biofouling") of wells, drains, and industrial pipes. There are also marine iron-oxidizers; most of these are found at hydrothermal vents at the bottom of the ocean. Once you know what to look for, iron-oxidizing bacteria are surprisingly easy to find.

All of these locations have circumneutral pH; they aren't too acidic and they aren't too basic. The pH in the sites we sample from typically runs from 5.0 (the same as black coffee) to 7.5 (the same as human tears), and the bacteria will only grow in that pH range. The process of iron oxidation will actually lower the pH of the surrounding medium, and that needs to be taken into account when we try to grow them back in our lab.

When you look at them under the microscope, they come in a variety of shapes and sizes. Some of them look like tiny lima beans, while others look like slender twisted rods. Some can swim with the assistance of one or more flagella, while others stay sedentary and grow in long chains inside of hollow sheaths. The only two things that they all have in common are their need for iron and their need for a little oxygen. The three questions we get asked most often are: 1) Can the bacteria hurt you? The answer is no, but we still don't recommend eating them. And 2) why do they make that orange goop? The bacteria get their energy from iron. In the process, they turn clear, water-soluble ferrous iron into rustcolored insoluble ferric iron that comes out of solution and forms the basis of the goop. Iron oxidation is rust formation, so basically, the bacteria are making rust. The third question is inevitably: "Why do you study them?" The answer is: because so little is known. Because they can do incredible things and we have no idea how or, in some cases, even why. It is something to keep in mind the next time you come across some sludge in the woods.

“Melissa Merrill Floyd is a graduate student at George Mason University's Environmental Science and Public Policy program.”

Photo by: Todd Plaia