Following yesterday’s posting about a pond with a strange orange substance beneath shallow cloudy water, I received a most interesting comment suggesting that the orange colouring might not be decomposing algae as I had initially thought but the result of bacterial activity. Linda Grashoff has a special interest in iron-oxidising bacteria and has written many posts on the subject in her WordPress blog Romancing Reality, and she has written a beautiful and lavishly illustrated book about this fascinating topic. It is called “They Breathe Iron – Artistic and Scientific Encounters with an Ancient Life Form“. She said:
Your “something russet and mysterious” is indeed iron that has been oxidized and precipitated out of the water by the iron bacteria. The patches of pale blue film on top of the water—shown in the first and last photograph—are created by Leptothrix discophora, one of the iron-oxidizing bacteria. Other iron bacteria also oxidize iron in the water and are probably present along with the L. discophora on the edges of this pond. The L. discophora bacteria live at the air/water interface, with one end of their rod-shaped bodies in the air and the other end in the water. My guess is that the film keeps the tiny bodies oriented. As the microbes reproduce, they shove parts of the film over and under other parts, so that the film becomes thicker. Various film thicknesses produce various colors by light-wave interference, often resulting in the appearance of an oil slick.
This made me think about another occasion when I had encountered a strange little stream issuing from rocks at Yachats in Oregon on the west coast of America. It possessed a distinct iridescent film, and flowed across green photosynthesising organic matter that in places was coated orange. Bubbles of oxygen were trapped within the ?algae and beneath the film on the surface of the water. The rocks on the beach were rich in iron. The small stream must have been a good example of the activity of iron-oxidising bacteria in action.
This post shows a few photographs from that site – and more will follow tomorrow.
