Organic Matter Mineralization with Reduction of Ferric Iron in Anaerobic Sediments

TLDR

The study investigated the potential for ferric iron reduction using fermentable substrates, fermentation products, and complex organic matter as electron donors in freshwater and brackish sediments from the Potomac River Estuary. Enrichment experiments with these sediments examined iron reduction by adding glucose, hematite, or amorphous ferric oxyhydroxide as electron acceptors, and measured the resulting metabolic pathways. Glucose enrichment with hematite produced only minor iron reduction, but replacing hematite with amorphous ferric oxyhydroxide increased iron reduction fifty‑fold and allowed fermentation products to be metabolized concurrently; various organic substrates stimulated iron reduction in inoculated sediments, while ferric iron inhibited methane production in a manner linked to its electron‑accepting capacity, with acetate or hydrogen alleviating this inhibition and shifting electron flow from methanogenesis to iron reduction, demonstrating that in anaerobic sediments amorphous ferric oxyhydroxides can dominate organic matter decomposition by outcompeting methanogens.

Abstract

The potential for ferric iron reduction with fermentable substrates, fermentation products, and complex organic matter as electron donors was investigated with sediments from freshwater and brackish water sites in the Potomac River Estuary. In enrichments with glucose and hematite, iron reduction was a minor pathway for electron flow, and fermentation products accumulated. The substitution of amorphous ferric oxyhydroxide for hematite in glucose enrichments increased iron reduction 50-fold because the fermentation products could also be metabolized with concomitant iron reduction. Acetate, hydrogen, propionate, butyrate, ethanol, methanol, and trimethylamine stimulated the reduction of amorphous ferric oxyhydroxide in enrichments inoculated with sediments but not in uninoculated or heat-killed controls. The addition of ferric iron inhibited methane production in sediments. The degree of inhibition of methane production by various forms of ferric iron was related to the effectiveness of these ferric compounds as electron acceptors for the metabolism of acetate. The addition of acetate or hydrogen relieved the inhibition of methane production by ferric iron. The decrease of electron equivalents proceeding to methane in sediments supplemented with amorphous ferric oxyhydroxides was compensated for by a corresponding increase of electron equivalents in ferrous iron. These results indicate that iron reduction can outcompete methanogenic food chains for sediment organic matter. Thus, when amorphous ferric oxyhydroxides are available in anaerobic sediments, the transfer of electrons from organic matter to ferric iron can be a major pathway for organic matter decomposition.