While doing my stream keeper observations last month, I noticed a reddish material in the water just below the gravel bar.
It looked like it had started on the bank and spread into the water, but alternatively might have just floated up to the bank. As a stream keeper this is just the sort of thing I am supposed to watch out for, but I didn’t know what it was. I guessed it might be some sort of bacteria, but what kind? This is a closer view.
After some online searching, my best guess is that this is a form of iron-oxidizing bacteria, which I had never heard of. As an example of how just noticing something leads you to learning something completely new, this led me to doing a lot of reading. So be prepared for some biochemistry.
We normally think of biologic energy as coming from two sources. Photosynthetic organisms (plants, algae, cyanobacteria) use the energy from the sun to synthesize carbohydrates, which they subsequently metabolize for energy for their own processes. Other higher organisms such as ourselves and other vertebrates and invertebrates must consume plants or other animals to obtain the carbohydrates and other organic molecules which are metabolized for energy. Plants and animals use the same metabolic pathway, called the electron transport chain, to generate energy. It involves separating an electron from a proton and using the energy created to energize the cell. A carbohydrate source is used to produce the electrons. Oxygen ultimately accepts the electrons and protons and is converted to water. We call this aerobic metabolism. For a more detailed explanation click here.
In the bacterial world things are more complicated. While many bacteria consume carbohydrate and metabolize it aerobically, some bacteria can generate their own energy by means other than photosynthesis. One group is referred to as chemolithotrophs. The “chemo” refers to generation of energy from chemical compounds rather than light, and “litho” refers to the source of the electron being from an inorganic source such as iron, sulphur, etc. Chemolithotrophic iron-oxidizing bacteria get their electron from the enzymatic oxidation of Fe++ to Fe+++ and can use this energy to synthesize carbohydrates for metabolism. Chemolithotrophic bacteria are often found in very harsh environments such as deep-sea vents or lava fields.
The bacteria I am seeing, however, is probably a chemoheterotroph. These bacteria can use chemical sources for energy, but cannot synthesize their own carbohydrate. Chemoheterotrophic iron-oxidizing bacteria live in iron-rich soil and consume and metabolize carbohydrate like other bacteria. However, if there are enough bacteria of any kind in the soil, the oxygen levels may be depleted before the organic material is all metabolized. Chemoheterotrophic iron-oxidizing bacteria enzymatically combine Fe+++ oxide (rust) from the soil with water to form Fe++ hydroxide plus free oxygen which can be used to metabolize the carbohydrates. If the bacteria are leached into oxygen-rich water, then the Fe++ hydroxide is oxidized back to Fe+++ oxide which stains the bacteria colony a rust red.
Iron-oxidizing bacteria in streams are harmless and a normal part of nature. Pennsylvania soils are iron-rich as witnessed by the numerous historical iron furnaces such as the Hopewell Furnace nearby. Iron-oxidizing bacteria are often seen in the water after rain which leaches them out of the soil into the streams. If they leach into moving water, they just wash away, but if they leach into stagnant water, as above, then they accumulate as a rusty-looking scum. In evolution, they probably descend from the earliest forms of life which used inorganic molecules such as iron or sulphur as a source of energy before photosynthesis evolved. Evolution is a horse race, and we see most abundantly around us the winners of the race. But there were many other horses in the race, and some of them are still here.
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