FIG. 3.

Growth of Fe-oxidizing bacteria in gradient tubes. Tubes A and B were inoculated with sections of plant roots, tube C was inoculated with a purified enrichment culture of an Fe-oxidizing bacterium, and tube D is an uninoculated control. When the roots were inoculated into gradient tubes, the growth started as discrete foci on the root and continued to form a band of Fe oxides at the oxic-anoxic interface. In the control tube, Fe oxidation occurs throughout the overlaying semisolid gel.