Lovley and Phillips 1986

Lovley DR, Phillips EJP. 1986. Organic matter mineralization with reduction of ferric iron in anaerobic sediments. Applied and Environmental Microbiology 51: 683-689.

These authors examined the reduction of iron, both supplemented and already present, in sediments collected from the bottom of the Potomac river across a salinity gradient from freshwater to brackish estuarine. A previous hypothesis in the literature suggested that observed decreases in methane production in the presence of trivalent iron, Fe(III) were caused by Fe(III) being toxic to methanogenic prokaryotes. This hypothesis was disproved in this study, and the results of this study suggest instead that methanogens are outcompeted by iron-reducing species because of the greater thermodynamic benefits of Fe(III) reduction compared to methane production. Van Bodegom et al. (2004) reported instead that rather than competition, Fe(III) directly inhibits methanogenesis, though I think this is due to metabolic switching within individual cells, not interactions between distinct methanogenic and iron-reducing populations.

The other major finding of this paper is that the form of the Fe(III) in the environment has a major effect on Fe(III) reduction. Amorphous ferric oxyhydroxides are reduced much more readily than are crystalline forms. These authors do not speculate on the mechanism underlying this difference, though I suspect surface area exerts a major controlling influence.

These authors did not measure Fe(III) forms in native sediments, rather they added amorphous Fe(III) and measured Fe(II) after incubation. Thus, while my own studies of the ratios of amorphous to crystalline Fe(III) are not assisted by these techniques, these authors do provide a clear and apparently fairly simple protocol for the measurement of Fe(II), involving extraction by HCl and reaction with a molecule that turns purple when complexed with Fe(II) allowing measurement of Fe(II) amounts from the absorbance spectrum of the resulting solution. I need to learn more about Fe(III) biogeochemistry before deciding to pursue such an analysis; I suspect one fate of Fe(III) is to cycle through an organism and be returned to the oxidized state, rather than shuttling directly from Fe(III) to Fe(II).