Effects of iron oxide mineralogy on methanogenesis transpiring in soils and sediments

Methane is an important greenhouse gas, possessing a global warming potential 30 times greater than carbon dioxide over a 100 year period. Soils and sediments, particularly within wetland environments, are dominant sources of methane to the Earth's atmosphere; however, the extent and magnitude of soil-sediment methane emissions are difficult to constrain owing to complex biogeochemical-physical controls on methanogenic activity. Methanogenesis may be inhibited in the presence of terminal electron acceptors that provide a greater free energy yield, including sulfate and iron(III)-bearing solids. Of these, Fe(III) solids (dominantly iron (hydr)oxides such as ferrihydrite, goethite, and hematite), possess a range of thermodynamic properties that impart different inhibitory effects on methanogenesis. Also, a recently proposed mechanism that predicts coupled iron reduction-methanogenesis—direct interspecies electron transfer (DIET)—may further convolute predictions of methane evolution in systems containing iron hydr(oxides). It is possible that, under energetically favorable conditions, iron reducers contribute an electron to a (semi)conductive iron oxide; this electron can then be conducted through the mineral and subsequently used by another microbe to produce methane. This research aims to explore the effect of iron chemistry and mineralogy on electron transfer between iron-reducing microbes and methanogens. A number of iron oxide minerals with varying crystallinities and conductivities were inoculated with wetland sediment. Iron and methane were monitored throughout the course of the experiments. Iron in solution was measured using inductively coupled plasma - optical emission spectroscopy (ICP-OES), while HCl-extractable iron was measured using spectrophotometric methods. Headspace methane was measured using a residual gas analyzer (RGA). According to thermodynamic predictions, enhanced methanogenic activity is observed in the presence of crystalline iron oxides relative to ferrihydrite, a less thermodynamically stable solid. Ongoing studies will explore how the semi-conducting properties of iron (hydr)oxides mediate and couple iron(III) reduction and methanogenesis transpiring in soils and sediments.