Microbial Fe(II) oxidation by Sideroxydans lithotrophicus strain ES-1 in the presence of humic acids

Controlled laboratory experiments were combined with field measurements to better understand the interactions between dissolved organic carbon and reduced iron in organic rich peatlands. Addition of peat-derived humic acid extract (HA) to Sideroxydans lithotrophicus strain ES- 1 liquid cultures led to 2.7 times higher microbial Fe(II) oxidation and initially inhibited chemical Fe(II) oxidation. Raman spectroscopy identified similar biotic and abiotic Fe (III) (oxyhydroxides irrespective of HA amendment, but minerals formed with HA were smaller. Voltammetric measurements within agarose-stabilized gradient-tube strain ES-1 cultures confirmed the catalytic effect of HA on microbial Fe (II) oxidation but also demonstrated that HA promotes aggregation of Fe (III) products. In situ voltammetry in an acidic iron-rich fen revealed the existence of a gap between oxygen penetration and iron reduction that may reflect the presence of Fe (II)-oxidizing microorganisms. The highest abundance of Fe(II) oxidizers Sideroxydans (4.9 x107 gene copies gww and Gallionella (1.5 x107 gene copies gww in the upper 20 cm of peat soil coincided with small sized minerals resembling nanoparticulate ferrihydrite or goethite (~10 nm). Our results suggest that microbially-mediated Fe (II) oxidation dominates in the presence of organics leading to the formation of nano-sized biogenic Fe (III) (oxyhydr) oxides that are important to Fe and C cycling.