Abstract
In tropical iron ore regions, biologically mediated reduction of crystalline iron oxides drives ongoing iron cycling that contributes to the stability of surface duricrusts. This represents a biotechnological opportunity with respect to post-mining rehabilitation attempts, requiring re-formation of these duricrusts. However, cultivated dissimilatory iron reducing bacteria typically reduce crystalline iron oxides quite poorly. A glucose-fermenting microbial consortium capable of reducing at least 27 mmol/L goethite was enriched from an iron duricrust region. Metagenome analysis led to the recovery of a metagenome assembled genome (MAG) of an iron reducer belonging to the alphaproteobacterial genus Telmatospirillum. This is the first report of iron reduction within the Telmatospirillum and the first reported genome of an iron-reducing, neutrophilic member of the Alphaproteobacteria. The Telmatospirillum MAG encodes putative metal transfer reductases (MtrA, MtrB) and a novel, multi-heme outer membrane cytochrome for extracellular electron transfer. In the presence of goethite, short chain fatty acid production shifted significantly in favor of acetate rather than propionate, indicating goethite is a hydrogen sink in the culture. Therefore, the presence of fermentative bacteria likely promotes iron reduction via hydrogen production. Stimulating microbial fermentation has potential to drive reduction of crystalline iron oxides, the rate limiting step for iron duricrust re-formation.
Highlights
Microbial iron reduction was discovered in the late 1800s (Allison and Scarseth, 1942)
Goethite and canga-reducing cultures prepared from various iron duricrust-associated samples collected in Western Australia and Brazil were initially enriched using a mixture of lactate, acetate, and glucose as per Lentini et al (2012)
Compared to when the electron donor was lactate alone, glucose-oxidizing cultures produced maximum ferrous iron much faster (6–12 days compared to 40–70 days) and to higher concentration (e.g., ∼15 mM compared to ∼5 mM, data not shown)
Summary
Microbial iron reduction was discovered in the late 1800s (Allison and Scarseth, 1942). Microorganisms capable of coupling the complete oxidation of organic compounds (or hydrogen) to reduction of iron oxides are known as dissimilatory iron reducers. They occur in a wide variety of natural ecosystems including sediments, soils, and groundwater ecosystems, and are significant contributors to global biogeochemical iron cycling. Lentini et al (2012) proposed that fermentative processes are likely more significant than dissimilatory processes for microbial reduction of crystalline iron oxides. Consistent with this hypothesis, fermentative bacteria have been implicated
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