Peatlands Geomicrobiology

Iron Cycling in an Acidic Fen

Dr. Marco Reiche collecting a core from the small diameter peat corer.

Syringes attached to the rhizome samplers to collect groundwater at 5 cm intervals between 0 and 40 cm below the surface.

Peatlands have waterlogged soils with acidic conditions and a peat forming vegetation. High water levels have led to an imbalance between net primary production and decomposition which results in the storage of large carbon pools. Therefore, peatlands have a great impact on global carbon cycling. With the prospect of global climate change and the consequent predicted decreases in annual precipitation in most European regions, lower water levels might turn peatlands into sources of CH₄ and CO₂. The aeration of the upper peat layers (0-40 cm) renews the pool of electron acceptors like nitrate, Fe(III) and sulfate, making them available again for the oxidative processes leading to the release of CO₂. The cycling of carbon is also greatly influenced by the turnover of acetate, an important intermediate in the anaerobic degradation of organic matter. Investigating peatland biogeochemistry and microbiology under altered water level conditions is therefore of great importance. Additionally, the dynamics of soil processes under extreme meteorological boundary conditions are studied within this project as a joint effort with researchers from the University of Bayreuth.

Our study site, the Schlöppnerbrunnen fen, (pH 4.8) is located in the Fichtelgebirge (northern Bavaria, Germany). It is part of the Lehstenbach Catchment, has an area of 0.8 ha, and has a the peat layer from 40 to 70 cm thick. The soil is Histosol build on granite bedrock. The fen receives high amounts of iron(II) via groundwater flow (up to 160 µM). This Fe(II) is oxidized in oxygenated surface layers to Fe(III) oxides, which are then available as an alternative electron acceptor for the oxidation of organic matter. The formation of an oxic- anoxic interface with a putative cycling of Fe has been observed due to the abundance of both Fe(II) oxidizers and Fe(III) reducers.

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In our previous work we investigated the impact of drying and rewetting of peat soil. We determined the effect of altering the water table on mineralization processes like Fe(II) and CO₂ formation rates, as well as enzymatic activities (Reiche et al., 2009). Our research also aimed to compare microbial Fe(III)-reduction and methanogenesis (Reiche et al., 2008) and to characterize the fen microbial communities. In particular, we were interested in characterizing microaerophilic Fe(II)-oxidizing bacteria (FeOB) and we successfully isolated an acid-tolerant FeOB, which competes with chemical oxidation at pH 4.0-5.5 (Lüdecke et al. 2010).

In our ongoing research, we are investigating how peatland FeOB can be important to carbon cycling by providing biogenic Fe(III) for Fe(III)-reducing bacteria, thereby suppressing methanogenesis. In addition, to better understand iron cycling we need to determine whether soluble Fe(III)-complexes, i.e., colloidal or chelated Fe(III), play a role as intermediates and facilitate the oxidation of Fe(II). We are working with Dr. Martial Taillefert (GA Tech) to measurements of Fe(III)-complexes with voltammetric techniques to determine how oxidized Fe is transported from oxic to anoxic zones, driving the iron cycle.

In the work of Anke Hädrich, the fate of acetate during the anaerobic oxidation of organic matter is being investigated. The sources and sinks of acetate were illuminated by a combination of porewater analysis, microcosm studies, stable isotope analysis (in collaboration with Dr. Verena Heuer), and microbial populations involved in acetogenesis were characterized by targeting the fhs functional gene.

Additional methods used in this project include anoxic microcosm experiments, gas chromatography, HPLC, dialyses chambers, FeS probes (to demonstrate the stability of anoxic conditions), cultivation and enrichment methods (microaerophilic Fe(II) oxidizers, Fe(III) reducers), microscopic methods (EDX, REM), XRD, Raman spectroscopy, Mössbauer spectroscopy, and photometric measurements.