Dual Role of Humic Substances As Electron Donors and Shuttles for Dissimilatory Iron Oxide Reduction.

It is well known that dissimilatory iron oxide reducing bacteria can use the quinone moieties in humic substances as an electron shuttle, but little is known about the capacity for humic substances to serve as a source of electron donor for iron oxide reduction.  We therefore investigated the potential for humic acid or humin preparations from a peat soil to serve as electron donors for reduction of synthetic hematite in the presence or absence of glucose.   Measurements of Fe(II) production revealed that the humic substances  were able to serve as electron donors for iron oxide reduction.  The most extensive reduction occurred when glucose and humic acid were present together, which suggests that the humic materials served as an electron shuttle that promoted iron oxide reduction. Illumina MiSeq metagenomic libraries from the microbial communities that arose in the reactors revealed a large increase in the number of glycoside hydrolase 5 gene family (cellulase) copies in reactors containing humic acid.  A similar increase in copy numbers of other genes involved in polymeric organic carbon degradation was observed. Genomic searches for the Porin-Cytochrome Complex (PCC) involved in extracellular electron transfer of iron reducing bacteria resulted in many matches, including hits in a Geothrix sp. that was not previously known to possess PCC.  These findings demonstrate novel linkages between iron reducing bacteria and cycling of subsurface organic carbon, and specifically reveal that humic materials in soils and sediments can serve as both electron donors and shuttles that promote organic carbon oxidation via iron oxide reduction.