Iron and Water

Sometimes in a swamp or marsh, you can see a metallic sheen on the surface of the water.  It may look like an oil spill but it is usually a molecular scale layer of iron floating on the water’s surface.  You can tell oil on water from iron on water by touching the surface.  If the film breaks up into plates, it is metallic.  

Iridescent layer of iron produced by bacteria floating in a marsh in Clayton County, Georgia.

If the layer swirls and reforms, it is oil.  The basis for the sheen with both oil and iron is thin film iridescence.  This type of iridescence is found not only with oil on water and iron on water but also in soap bubbles.  The thickness of the film determines which colors are reflected, so oil spilled on a wet road or soap bubbles may show all the colors of the rainbow. 

Plates of iron bacterial film broken up by water movement. 

Oil on pavement showing thin film iridescence.

This iridescent layer is the signature of a bacterial ecosystem that runs on different ionic forms of iron.  The sheen appears in still water when there has been no rain for several days.  Rain or water current will break up the layer into plates and they will wash away.  In these swampy places, decomposition of plant material by bacteria and fungi deplete the oxygen in the water.  This anaerobic environment is where a several bacteria use iron in their metabolism.  Animals, plants and most fungi are aerobes and carry out their energy metabolism by removing electrons from organic molecules like sugars.  These electrons go through a bewilderingly complex set of chemical reactions to make cellular energy.  In the last step of this process, the electrons end up on oxygen.  Bacteria involved in making the iron sheen have a similar type of energy metabolism but rather than dumping their electrons on oxygen, they put their electrons on the oxidized form of iron.  This oxidized iron is abundant in the red clay soils of the southeastern Piedmont.  The iron that has gained those electrons is said to be reduced, an odd term since the atoms have actually gained something (electrons). 

Bacterial iron film.

Reduced iron is an energy source used by another group of bacteria, the iron oxidizers.  These bacteria remove the electrons from the reduced iron and regenerate the oxidized form while producing their energy.  Iron oxidizing bacteria live at the top few millimeters of the anaerobic swamp water where a small amount of oxygen is dissolved.  The bacteria deposit the iron produced by their metabolism as iron oxide, also known as rust, at the surface of the water and make the shining layer.  These two groups of bacteria depend on the metabolism of each other to fuel their survival.

If you run across iridescence in a marsh, it probably does not signal a petroleum release but rather a sign of an iron-based microbial ecosystem.