File:Leptothrix discophora Film and Iron Oxide Floc along the Vermilion River.jpg

English: What is important to notice about this photo is the fragmentation of the film, which indicates that this is not an oil spill. But beyond that, what is important to notice is the sheer beauty of the benign bacterial action depicted. (I consider visible, tangible existence not only real but also sacred, and I try to convey my reverence in photographs.)

The following—adapted from my self-published book They Breathe Iron: Artistic and Scientific Encounters with an Ancient Life Form—is what is interesting or important to know about the iron bacteria, in particular Leptothrix discophora:

The reflectivity and rainbow colors that signal L. discophora have complex origins. One contributing factor is the shape of the organism, specifically its “holdfast,” a doughnut-like spherical extension by which each cell (bacteria are one-celled creatures) attaches itself to the air–water interface. Sunlight glancing off a layer of densely packed holdfasts and microbial waste products helps account for the shiny effect. The shiny surface reflects the colors around it—especially green light cast by nearby vegetation and the bright blue of October’s brilliant skies.

Shine and color result perhaps most of all from the optical properties of the thin polymer films that L. discophora colonies produce on water as they oxidize iron. Many substances, including soap bubbles and some organic solvents like gasoline, form thin films that demonstrate the same qualities and the laws of physics responsible for them. Interference is the term that physicists use to describe the way light interacts with thin films layered over other materials to produce color.

When bacteria or physical weathering liberate iron from rocks, the metal enters a process of reduction and oxidation called a redox cycle (“red” from “reduction” and “ox” from “oxidation”). A simplified example of such a cycle goes like this: As river water recedes, films formed by the iron oxidizer L. discophora gradually settle onto the muddy surface along with particles of oxidized iron deposited by the bacterium. In time more mud, deposited by water or wind, covers the films. Then reducing bacteria such as Desulfovibrio living in the mud convert the iron back to its reduced and more water-soluble form. The reduced metal dissolves when the river rises again. Reduced iron at the air–water interface becomes colonized by the oxidizing bacterium L. discophora, and the cycle is complete, only to repeat when the river water recedes once more. It’s an ancient loop, one that goes back at least three and a half billion years.

Most people can easily ignore the iron bacteria, but ignored or not, these creatures set in motion an important process: a cycle from nonliving to living matter and back around again. Iron, like every chemical element that organisms require, participates in a biogeochemical cycle that plays through organisms and the physical media that support them. For iron in the Vermilion River, movement into the living phase of the cycle begins when bacteria release the iron from pyrite- and marcasite-bearing shale and the iron dissolves in water, where plants growing along the river use it as a nutrient. Herbivores eat the plants, and some of the herbivores become food for carnivores. When the plants and animals die, bacteria and fungi feed on their remains, releasing the iron that they accumulated in life back into the soil or water, where living plants or bacteria eventually reabsorb it, continuing the cycle. The bacteria that extract their energy from rock-bound iron in places like the Vermilion River are not only harmless to humans but are also essential to all other forms of life. If bacteria didn’t participate in these miraculous biogeochemical cycles, usable forms of several nutrient metals would be too scarce to sustain other organisms—indeed, to sustain the living world as we know it.

The iron bacteria responsible for the array of splendid colors that we see in the Vermilion River—including those of iron oxide—present rich opportunities for scientific investigation and aesthetic contemplation.