Patuxent Wildlife Research Center


by Eleanora I. Robbins, USGS

SLIDE1. The banks of the Anacostia River are the location of a scientific mystery that is shown in this slide. The mystery lies under a mat of blue-green cyanobacteria that spreads out on otherwise unvegetated sand. Under the mat is a zone where all the sand grains are colored red from iron oxide minerals. Each sand grain, like the one in this slide, is colonized by red, rod-shaped iron bacteria. Usually under an algal mat, the sand gets colored black from the precipitation of reduced iron, especially where sulfide is also present. For some unknown reason, the iron under this mat is oxidized.

SLIDE2. The green alga, Mougeotea sp., proliferates at one of the manmade ponds in the park. These green algae are colonized by an iron bacterium having brown holdfasts, Leptothrix discophora. Holdfasts are the attachment structures of these iron bacteria that otherwise are colorless rods. The rods attach themselves to "solid" surfaces like algae. After attachment, they secrete doughnut-shaped holdfasts. If manganese is present in the water, the bacteria can incorporate oxidized manganese onto their holdfasts, thereby precipitating manganese oxide minerals and turning solid surfaces dark brown or black. These bacteria will colonize anything--soda bottles, cobbles, tennis shoes, styrofoam cups, plastic bags, microscope slides.

SLIDE3. The dike wall along the Anacostia River has cracks. Ground water that has no oxygen and carries colorless reduced iron seeps out through the cracks until it reaches the oxygen-rich surface. Iron bacteria take advantage of the presence of water and iron, to speed up the precipitation of oxidized iron and coloring the wall red. Siderocapsa cf. treubii, one of the iron bacteria, look like little spheres in the material that oozes from the wall. The boat-shaped diatoms, which are silica-fixing algae, utilize the silica present in the water.

SLIDE4 At the same ground water seep, one type of iron bacteria that resemble corn cobs (Siderocystis confervarum) is seen colonizing another that looks like empty drinking straws (Leptothrix ochracea).

SLIDE5. Oil-like films are created by bacteria that attach themselves to the air/water surface where sunlight bounces off the films, giving them an oily appearance. At a seep along the river trail, the film was lifted off with a glass microscope slide. The microscope reveals that both an old film (thicker) and a young film (thinner) are full of the rod-shaped, iron bacteria, Leptothrix discophora. The greenish spots surrounded by clear areas are unidentified bacteria that have clear holdfasts.

SLIDE6. Another oil-like film is made up Leptothrix discophora. that have coated their filaments with iron- and manganese-oxides. Typically these bacteria only coat their holdfasts and not their entire filaments with oxides of iron and manganese. So this site is unusual.

SLIDE7. The undersides of water lilies that float on the water of a natural pond near Park Headquarters are colored red from the iron bacteria that colonize this "solid" environment and oxidize the iron from the water. These bacteria are Leptothrix discophora and they have holdfasts that look like red doughnuts.

SLIDE8. Beaverdam Creek is a polluted, manmade ditch that feeds into the Anacostia River from a landfill. Amazingly, there are many organisms that thrive in polluted water, such as this floating mat of organic matter that is filled with boat-shaped diatoms.

SLIDE9. Another group of microorganisms that thrive in this polluted creek are the blue-green bacteria, Oscillatoria sp., and the green algae, Spirogyra sp.

SLIDE10. Roots and rootlets of aquatic plants in unpolluted waters are often colored red due to being colonized by iron bacteria. Scrapings from some red roots in a seep along the river trail yielded the sausage-shaped iron bacterium, Leptothrix cholodnii, which in turn, is colonized by spheres of another iron bacterium, Siderocapsa cf.treubii.

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U.S. Department of the Interior
U.S. Geological Survey
Patuxent Wildlife Research Center
Laurel, MD, USA 20708-4038
Contact: Sam Dreoge
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