Bacterial reductive dissolution of crystalline Fe(III) oxide in continuous-flow column reactors.

Bacterial reductive dissolution of synthetic crystalline Fe(III) oxide-coated sand was studied in continuous-flow column reactors in comparison with parallel batch cultures. The cumulative amount of aqueous Fe(II) exported from the columns over a 6-month incubation period corresponded to (95.0 +/- 3.7)% (n = 3) of their original Fe(III) content. Wet-chemical analysis revealed that only (6.5 +/- 3.2)% of the initial Fe(III) content remained in the columns at the end of the experiment. The near-quantitative removal of Fe was visibly evidenced by extensive bleaching of color from the sand in the columns. In contrast to the column reactors, Fe(II) production quickly reached an asymptote in batch cultures, and only (13.0 +/- 2.2)% (n = 3) of the Fe(III) oxide content was reduced. Sustained bacterial-cell growth occurred in the column reactors, leading to the production and export of a quantity of cells 100-fold greater than that added during inoculation. Indirect estimates of cell growth, based on the quantity of Fe(III) reduced, suggest that only an approximate doubling of initial cell abundance was likely to have occurred in the batch cultures. Our results indicate that removal of biogenic Fe(II) via aqueous-phase transport in the column reactors decreased the passivating influence of surface-bound Fe(II) on oxide reduction activity, thereby allowing a dramatic increase in the extent of Fe(III) oxide reduction and associated bacterial growth. These findings have important implications for understanding the fate of organic and inorganic contaminants whose geochemical behavior is linked to Fe(III) oxide reduction.

Reduction of Cr(VI) by a Consortium of Sulfate-Reducing Bacteria (SRB III).

A consortium of bacteria with tolerance to high concentrations of Cr(VI) (up to 2,500 ppm) and other toxic heavy metals has been obtained from metal-refinishing wastewaters in Chengdu, People's Republic of China. This consortium consists of a range of gram-positive and gram-negative rods and has the capacity to reduce Cr(VI) to Cr(III) as amorphous precipitates which are associated with the bacterial surfaces. An endospore-producing, gram-positive rod and a gram-negative rod accumulate the most metallic precipitates, and, over time, 80 to 95% of Cr can be removed from concentrations ranging from 50 to 2,000 ppm (0.96 to 38.45 mM). Kinetic studies revealed a first-order constant for Cr removal of 0.1518 h for an initial concentration of 1,000 ppm (19.3 mM), and the sorption isothermal data could be interpreted by the Freundlich relationship. The sorption was not entirely due to a passive interaction with reactive sites on the bacterial surfaces since gamma-irradiated, killed cells could not immobilize as much metal. When U or Zn was added with the Cr, it was also removed and could even increase the total amount of Cr immobilized. The consortium was tolerant to small amounts of oxygen in the headspace of tubes, but active growth of the bacteria was a requirement for Cr immobilization through Cr(VI) reduction, resulting in the lowering of E(h). Our data suggest that the reduction was via H(2)S. This consortium has been named SRB III, and it may be useful for the bioremediation of fluid metal-refining wastes.

Remobilization of Heavy Metals Retained as Oxyhydroxides or Silicates by Bacillus subtilis Cells.

The stability of heavy metals bound as either silicates or oxyhydroxides by the surfaces of Bacillus subtilis with changes in pH and presence of competing cations and EDTA was studied. Wall-bound silicate was very stable. Of all conditions, a pH of 3.0 (by HNO(3)) promoted the greatest release of Cd (87% [wt/wt]), Pb (77%), or Cu (54%). Bacterial Pb resisted extraction by EDTA, and Cd that was included in bacterial-silicates proved difficult to remobilize.

Proton motive force may regulate cell wall-associated enzymes of Bacillus subtilis.

Bacterial metabolism excretes protons during normal metabolic processes. The protons may be recycled by chemiosmosis, diffuse through the wall into the medium, or bind to cell surface constituents. Calculations by Koch (J. Theor. Biol. 120:73-84, 1986) have suggested that the cell wall of gram-positive bacteria may serve as a reservoir of protons during growth and metabolism, causing the wall to have a relatively low pH. That the cell wall may possess a pH lower than the surrounding medium has now been tested in Bacillus subtilis by several independent experiments. When cultures of B. subtilis were treated with the proton conductors azide and carbonylcyanide m-chlorophenylhydrazone, the cells bound larger amounts of positively charged probes, including the chromium (Cr3+) and uranyl (UO2(2+) ions and were readily agglutinated by cationized ferritin. In contrast, the same proton conductors caused a decrease in the binding of the negatively charged probe chromate (CrO4(2-)). Finally, when levansucrase was induced in cultures by the addition of sucrose, the enzyme was inactive as it traversed the wall during the first 0.7 to 1.0 generation of growth. The composite interpretation of the foregoing observations suggests that the wall is positively charged during metabolism, thereby decreasing its ability to complex with cations while increasing its ability to bind with anions. This may be one reason why some enzymes, such as autolysins, are unable to hydrolyze their substrata until they reach the wall periphery or are in the medium.