The feasibilities of (i) liberating free energy from dissimilative iron reduction and (ii) coupling oxidative phosphorylation to electron transport to Fe(III) are sensitive to the aqueous chemistry of iron. The addition of ligands, such as nitrilotriacetic acid (NTA), to solution significantly impacts both the overall thermodynamics and kinetics of dissimilative iron reduction. The overall free-energy change due to electron transfer from glucose or lactate ion to Fe(III) is negative, but when Fe(III) is presented as an iron oxide there may be insufficient free energy in the transformations to permit coupled ATP generation. A systematic investigation of iron-reduction kinetics as a function of Fe(III) speciation indicated that in Pseudomonas sp. 200 (i) iron-reduction rate was functionally related to the concentrations of individual iron species and (ii) direct contact between Fe(III) and the electron-transport chain (ferrireductase) was required for electron transfer. Iron reduction in the absence of microbial activity was negligible. The addition of equimolar quantities of NTA enormously accelerated the initial rate of microbial iron reduction, and the calculated concentration of Fe(NTA)(OH)22- correlated strongly with measured iron-reduction rates.
When Fe(III) was provided as an iron oxide, the overall reduction rate was much slower, though still dependent upon the concentration of NTA added to solution. Primary factors controlling mineral dissolution and Fe(III) reduction were mineral surface area (or concentration of high-energy surface sites), ligand concentration, and cell number. Saturation kinetics were evident, as indicated by the following relationship governing reductive dissolution of hematite:
d[Fe(II)] / dt = Vmax(I)Km(NTA)Vmax(II)[NTA] / Km(NTA) + [NTA] • [Fe(III)] / Km(Fe) + [Fe(III)]
where Vmax(I) = 2.8 x 10-5 M•hr-1
Vmax(II) = 6.3 x 10-4 M•hr-1
Kmax(NTA) = 1.2 x 10-3 M
Kmax(Fe) = 1.0 x 10-1 M (as Fe)
NTA = nitrilotriacetic acid
[Fe(III)] = volume concentration of hematite (as Fe).
Experiments involving oxide/microorganism separation indicated that cell/mineral contact was essential to reductive dissolution of goethite.
Specific respiratory inhibitors were utilized to identify elements of electron transport chains involved in reduction of molecular oxygen and Fe(III) and to compare transport-chain compositions of cells grown under high- versus limited-O2 conditions. Pseudomonas sp. 200 expressed both a constitutive (cytochrome o) and an inducible (cytochrome d) cytochrome oxidase. Induction of the alternate transport pathway resulted from growth at low oxygen tension (<0.01 atm.). Induced cells were capable of O2 utilization at moderately increased rates. Pseudomonas sp. 200 also expressed a constitutive and an inducible ferrireductase. Growth at low oxygen tension resulted in acceleration of the overall rate of dissimilative iron reduction by a factor of 6 to 8, but iron reduction appeared to be uncoupled from oxidative phosphorylation. Maximum rates of electron transfer in induced cells were independent of the identity of the electron acceptor indicating a common rate-limiting step. Dissimilative iron reduction occurred via an abbreviated electron transport chain in both the induced and uninduced cases. Electron-transport-chain compositions for the induced and uninduced cases are postulated.
