Crystalline iron oxide mineral (magnetite) accelerates methane production from petroleum hydrocarbon biodegradation

Methane (CH₄) emissions are a factor in climate change; in addition, CH₄ production may affect reclamation of fluid fine tailings (FFT) in tailings ponds, and end-pit lakes (EPLs). In laboratory cultures, we investigated the effect of crystalline iron mineral (magnetite) on CH₄ production from the biodegradation of hydrocarbons added to FFT collected from methanogenically more and less active sites in a demonstration EPL. Magnetite enhanced CH₄ production from both sites, having a greater effect in more active FFT, where it increased the CH₄ production rate as much as 48% (from 6.67 μmol d^-1 to 9.87 μmol d^-1) compared to FFT without magnetite. Correspondingly, magnetite hastened biodegradation of hydrocarbons (monoaromatics, n-alkanes and iso-alkanes), with a pronounced effect on o-xylene, ethylbenzene, m/p-xylenes, n-octane, n-nonane, and 2-methyloctane, where biodegradation rates increased by 46, 117, 11, 45, 28 and 37%, respectively, compared to FFT without magnetite. Little Fe^II was produced, suggesting that magnetite is not being used as an electron acceptor but rather functions as a conduit for electron transfer. Thus, magnetite may be a suitable amendment to enhance bioremediation of anaerobic environments contaminated with hydrocarbons. Importantly, our observations imply that magnetite may increase CH₄ emissions from terrestrial ecosystems, thus affecting carbon budget estimations.