Humic acid reduction by propionibacterium freudenreichii and other fermenting bacteria

Abstract

Iron-reducing bacteria have been reported to reduce humic acids and low-molecular-weight quinones with electrons from acetate or hydrogen oxidation. Due to the rapid chemical reaction of amorphous ferric iron with the reduced reaction products, humic acids and low-molecular-weight redox mediators may play an important role in biological iron reduction. Since many anaerobic bacteria that are not able to reduce amorphous ferric iron directly are known to transfer electrons to other external acceptors, such as ferricyanide, 2,6-anthraquinone disulfonate (AQDS), or molecular oxygen, we tested several physiologically different species of fermenting bacteria to determine their abilities to reduce humic acids. Propionibacterium freudenreichii, Lactococcus lactis, and Enterococcus cecorum all shifted their fermentation patterns towards more oxidized products when humic acids were present; P. freudenreichii even oxidized propionate to acetate under these conditions. When amorphous ferric iron was added to reoxidize the electron acceptor, humic acids were found to be equally effective when they were added in substoichiometric amounts. These findings indicate that in addition to iron-reducing bacteria, fermenting bacteria are also capable of channeling electrons from anaerobic oxidations via humic acids towards iron reduction. This information needs to be considered in future studies of electron flow in soils and sediments.

FIG. 1

Typical time courses of lactate fermentation by P. freudenreichii in the absence (A) and in the presence (B) of humic acids (17 mg ml⁻¹). All assay mixtures contained lactate (□) and 0.05% yeast extract; propionate (•) and acetate (○) were the only products detected. When amorphous ferric iron (40 mM) was added in the presence of humic acids (C), ferrous iron (▾) was formed almost stoichiometrically. The constant background level of ferrous iron resulting from the chemical reaction with humic acids (3.1 mM) was subtracted. For reproducibility and exact stoichiometries, see Table 1.

FIG. 2

Influence of humic acid concentrations on the fermentation product pattern of P. freudenreichii. Cells were grown on lactate (3 mM) and yeast extract (0.05%); propionate (•) and acetate (○) were the only products detected. The specific electron uptake by the humic acids (□) was calculated from the amount of electrons recovered in the fermentation products.