These photos supplement the article "Response of microorganisms to gradients", pp 151-156 in: Druger, M. & Anderson, O.R., eds., "Explore the World Using Protozoa". National Science Teachers Association & Society of Protozoologists, 1996)
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Last updated: 3 July 1997
This is the basic setup, with agar based medium in the bottom and sample water over top, with cotton or synthetic wool plug to allow a cross gradient of oxygen and nutrients.
This close up shows a layer of microorganisms at the Redox transition zone (a), and a cloud of anaerobic bacteria just above the agar (b).
The layer at the redox transistion from the previous image is actually two layers: a bacterial layer (a) and a ciliate protist layer (b). Apparently the bacteria are able to tolerate slightly lower oxygen concentrations than the protists.
Methylene Blue at 2 mg/ml final concentration, is colorless at an Eo of +11 mV., pH 7.0, 30 degrees C (Jacob, 1970. Redox Potential in Norris & Ribbons, "Methods in Microbiology", AP press NY 2:92-123.) indicates the Redox transtion zone relative to the microbial layers.