Methanogen Productivity and Microbial Community Composition Varies With Iron Oxide Mineralogy

Quantifying the flux of methane from terrestrial environments remains challenging, owing to considerable spatial and temporal variability in emissions. Amongst a myriad of factors, variation in the composition of electron acceptors, including metal (oxyhydr)oxides, may impart controls on methane emission. The purpose of this research is to understand how iron (oxyhydr)oxide minerals with varied physicochemical properties influence microbial methane production and subsequent microbial community development. Incubation experiments, using lake sediment as an inoculum and acetate as a carbon source, were used to understand the influence of one poorly crystalline iron oxide mineral, ferrihydrite, and two well-crystalline minerals, hematite and goethite, on methane production. Iron speciation, headspace methane, and 16S-rRNA sequencing microbial community data were measured over time. Substantial iron reduction only occurred in the presence of ferrihydrite while hematite and goethite had little effect on methane production throughout the incubations. In ferrihydrite experiments the time taken to reach the maximum methane production rate was slower than under other conditions, but methane production, eventually occurred in the presence of ferrihydrite. We suggest that this is due to ferrihydrite transformation into more stable minerals like magnetite and goethite or surface passivation by Fe(II). While all experimental conditions enriched for Methanosarcina, only the presence of ferrihydrite enriched for iron reducing bacteria Geobacter. Additionally, the presence of ferrihydrite continued to influence microbial community development after the onset of methanogenesis, with the dissimilarity between communities growing in ferrihydrite compared to no-Fe-added controls increasing over time. This work improves our understanding of how the presence of different iron oxides influences microbial community composition and methane production in soils and sediments

Oxalate-Promoted Trace Metal Release from Crystalline Iron Oxides under Aerobic Conditions

Iron oxides are common in soils and sediments and bind trace metals, such as nickel, through adsorption and structural incorporation. Recent work has shown that incorporated metals can be released during recrystallization catalyzed by aqueous Fe­(II), which is generally stable only under anoxic conditions. This paper investigates the effects of the organic acid oxalate on the fate of nickel substituted into goethite and hematite to evaluate potential nonredox pathways of iron oxide recrystallization. Nickel-substituted hematite and goethite were synthesized and then reacted with 1 mM oxalate at pH 3–7 for ≤15 days. At all pH values, the presence of oxalate led to substantial nonstoichiometric release of nickel into solution compared to oxalate-free systems, with the extent of release increasing with decreasing pH. The rate of nickel release also increased with increasing oxalate surface coverage, and nonstoichiometric release of nickel was observed even under conditions producing no net mineral dissolution. The results demonstrate that iron oxide recrystallization may be a widespread phenomenon occurring in aerobic soils and aquatic systems, substantially altering contaminant fate and micronutrient availability in the environment