It doesn't matter how metal you think you are, you'll never be as metal as a bacterium that breaths iron instead of oxygen. If you're a researcher at the University of Wisconsin-Madison, however, you can at least get in on the act by explaining how these microbes function.
Breathing oxygen is so normal, for animals and many bacteria, that we take it for granted. From a chemistry perspective, oxygen is used to soak up electrons. So associated is oxygen with being where chemical reactions dump spare electrons, we call the process oxidation even when another element performs the role.
Where oxygen is scarce, microorganisms have turned to other elements instead, including electron-deficient iron. As Professor Eric Roden said in a statement: "We pass electrons from organic matter to oxygen. Some of these bacteria use iron oxide as their electron acceptor. On the flip side, some other microbes receive electrons donated by other iron compounds. In both cases, the electron transfer is essential to their energy cycles."
Iron-breathing bacteria have been known for a century, and Roden has spent decades improving our sparse knowledge of their workings. Two papers in quick succession have revealed his progress.
In Applied and Environmental Microbiology, Roden announces the “near-complete genome” of a member of the Gallionellaceae family. This, Roden and his co-authors announce, is the primary oxidizer in a sample of freshwater sediment.
However, like all good heavy metal artists, the bacterium doesn't work alone. Lacking the genes to process nitric oxide and nitrous oxide, the paper suggests Gallionellaceae may “partner with flanking populations capable of complete denitrification to avoid toxic metabolite accumulation.” Cooperation would explain why attempts to grow the bacterium in culture have been unsuccessful – it can't survive without the rest of its band.
Microbial cultures turn amorphous iron oxides in Yellowstone sediment (left) into grey siderite in a week. Nathaniel Fortney
To address the failures of culturing, Roden has been working with geneticists to sequence the bacteria's genomes. "Eyebrows rose when we contacted the Biotech Center three or four year ago to discuss sequencing: 'Who are these people from geology, and what are they talking about?' But we stuck with it, and it's turned into a pretty cool collaboration that has allowed us to apply their excellent tools that are more typically applied to biomedical and related microbial issues," he said.
In Geobiology, Roden explores the capacity of Yellowstone National Park bacteria to use Fe3+ (iron atoms depleted of three electrons) for oxidation rather than the better understood process using Fe2+ that's investigated in the other paper.
The team found that some of these bacteria can transport electrons in both directions across their outer membrane. "Bacteria have not only evolved a metabolism that opens niches to use iron as an energy," said study co-author and graduate student Shaomei He. "But these new electron transport mechanisms give them a way to use forms of iron that can't be brought inside the cell."
Roden is interested in iron metabolizers for their role in shaping the Earth's geology. He adds that if life exists on Mars, where iron is common and free oxygen rare, this might be its secret.