You may have noticed orange or rust-colored, filamentous, gelatinous muck in places along the bottom or banks of Left Hand Creek, looking something like this:
At times the orange muck may be accompanied by an oily-looking, rainbow-colored sheen floating on the surface of the water:
With visual effects such as these, it would be understandable to assume that some sort of nasty pollutant has contaminated the creek.  Fortunately, however, what we are witnessing is a common natural phenomenon that occurs in perfectly healthy creeks.  As you might imagine, there is an interesting micro-bio-geo-chemical story behind the disconcerting colors and textures.  The rusty substance is just that:  rust, or iron oxide (ferric oxide or Fe2O3).  It is an easily observable part of an interesting, mostly hidden cycle of iron reduction and re-oxidation mediated by two different types of iron bacteria.  The gelatinous mass is a large colony of iron-oxidizing bacteria.  The thing that separates iron-oxidizing bacteria from other types of bacteria is the way they get their energy.

Most bacteria, like people, get their energy from eating (and oxidizing) reduced organic material, which we know as food.  Iron-oxidizing bacteria, however, have evolved to get their energy from oxidizing something else:  reduced iron, or ferrous iron.  They don’t get much energy from oxidizing reduced iron, but they get enough to live and thrive in certain habitats.  Those habitats occur in a special transition zone where groundwater containing no dissolved oxygen, but some dissolved iron, comes to the surface.  The source of the iron can be naturally occurring iron-containing sediments.  The iron from the sediments, which is usually the oxidized or ferric form, which is insoluble, gets into the water underground when another type of iron bacteria, iron-reducing bacteria, eat organic matter in the water.  Since there is no oxygen in the groundwater to support aerobic respiration, these iron-reducing bacteria use something other than oxygen to oxidize the organic matter they eat:  ferric iron.  The result is reduced, ferrous iron, which is soluble in water.  The groundwater carries this soluble ferrous iron to places where the groundwater comes to the surface in a spring or a creek bottom, and there it serves up the ferrous iron to the iron-oxidizing bacteria waiting to feed on it.  The iron-oxidizing bacteria combine the ferrous iron from the groundwater with oxygen from the atmosphere and the creek, and produce iron oxide or rust, which again is insoluble, and makes the orange gunk.  There are several species of iron-oxidizing bacteria, from genera including Sphaerotilus, Leptothrix. Thiobacillus, and Leptospirillum.  The rainbow-colored, oily-looking sheen that often accompanies the bacteria is not oil, but is a biofilm produced by the bacteria.  You can verify that it is not oil by touching it with a twig.  If it were oil, the sheen would stick to the twig in an oily mess.  But these biofilms, when touched with a twig, tend to break up into little plates and dissipate away from the twig.

So do these bacteria and biofilms cause any harm?  The answer is basically no.  They are a natural part of a healthy ecosystem.  The bacterial colonies can impart a musty or unpleasant taste or odor to the water, but they are not toxic.  If these bacteria get into a well they can cause problems by clogging the well screen, pumps, and pipes, requiring disinfection of the well.  The bacterial colonies in the creek can alter the physical habitat of the creek bottom, but again are not toxic to people or aquatic organisms.  So the next time you notice the orange goo and the oily-looking sheen, don’t bother calling in a pollution report, and just enjoy observing this interesting part of the iron cycle.  If you’d like a brochure about this topic, you can visit this website:

https://www.marincounty.org/~/media/files/departments/pw/mcstoppp/ironoxidizingbacteria_printshopfinal_31715.pdf?la=en