In the latter vials, the resazurin was decolorized to a point just below the zone of Fe(III) oxide precipitation. Because both

resazurin and Fe2+ are rapidly oxidized by O2 at neutral pH and Fe3+ quickly precipitates in the absence of a chelator, the point of resazurin decolorization and Fe(III) oxide precipitation roughly corresponds to the depth of O2 penetration. The resazurin in the third, Na2S-containing vial (vial 2C) never became decolorized, suggesting that the incorrect amount of sulfide was inadvertently added to this vial (see Fig. S1). Figure 3 shows the results of cell enumerations in the upper 10 mL PD0325901 manufacturer of the gradient cultures for each of the three treatments after 8 days of incubation. No cells were observed in the lower 5 mL of the upper layer. With the exception of vial 2C, in which resazurin did not become decolorized, there was no significant difference in cell numbers in fully oxic vials lacking a reductant or in gradient vials containing sulfide in the lower layer. All of these vials contained between 1.8 × 108 and 2.3 × 108 cells. Since 3.7 × 107 cells were added in the inoculum, cells underwent two to three doublings following

addition to the vials. The relatively slight increase in cell numbers (equivalent to two PF-02341066 supplier to three doublings) likely resulted from the consumption of trace organics in the agarose, metabolism of intracellular storage products, or cells in the inoculum that were in the process of division. In all vials that contained Fe(II) in the lower layer, however, cell numbers were approximately one order of magnitude greater and ranged from 1.2 × 109 to 1.6 × 109 in the upper 10 mL of medium. Microscopic observations showed that these cells were highly concentrated in a thin layer selleck screening library at or just below the lower layer of oxide precipitation. To explore the vertical distribution of cells in the redox gradient, cells were also enumerated in vertically sampled aliquots of the upper layer in an additional iron-oxidizing, gradient-culture replicate. As shown in Fig. 4, cell numbers were

the highest (∼5 × 108 mL−1) at a depth of 5 mm below the surface. This depth approximately corresponded to the lower border of the oxide precipitation layer immediately above the decolorized resazurin. At samples collected below this depth, the cell numbers decreased by approximately one order of magnitude with each 5-mm depth interval. Strain M1 was able to grow organotrophically on 5 mM acetate using either O2 or NO3− as an electron acceptor. On solid MG medium, colonies arose more rapidly and were larger when plates were incubated under reduced-O2 conditions than when incubated at ambient O2 concentrations. M1 was unable to couple the oxidation of lactate or acetate to the reduction of Fe(III) citrate or Fe(III)–NTA. Cultures grown under organotrophic NO3−-reducing conditions or in Fe(II)-oxidizing gradient cultures did not exhibit magnetotaxis.