Aquatic Iron Cycling

Oxidation and reduction of iron in acidic environments

Acidic Lignite Mining Lake 77

Lakes formed by lignite mining processes are characterized by low pH, low nutrient status, and high concentrations of Fe(II) and sulfate due to the oxidation of pyrite in the surrounding mine tailings. Within the water body of these acidic lignite mine lakes, iron-rich aggregates are formed during the oxidation of Fe(II). They mainly composed of reactive Fe(III)-(hydro)oxides (e.g. schwertmannite). This so-called ‘iron snow’, precipitates downward to the anoxic sediment, where microbial Fe(III) reduction takes place. This electron-accepting process is coupled to the oxidation of organic matter. The sediment of these acidic lakes is often separated into an upper acidic zone (pH ~3) with large amount of fluffy, reactive Fe(III) and a deeper, slightly acidic zone (pH ~5) with less reactive Fe(III).

The acidic lignite mining-associated Lake 77 is located in the Lusatian mining area in east central Germany near Lauchhammer. The surface area is about 0.8 km² and the maximum depth approaches 7 m at two different locations. The abandoned open mining pit was flooded in 1965 and subsequent acidification was caused by input of Fe(II) and sulfate rich ground water due to pyrite (FeS₂) oxidation in the dump on the northern end of the lake. This continuous input of sulfate and iron into the lake water keeps the pH at approximately 3. There are two distinct mixing regimes in AML 77. The Central-basin shows a dimictic pattern with water column stratified in summer and mixed in winter; however, the water in Northern-basin is meromictic, which means a monimolimnion exists.

Map of the Lake 77 area >>

Due to the selective pressure from the different conditions in the water column, iron snow, and sediments, it can be speculated that the microbial communities inhabiting these habitats may also be very different from each other. Previous research has revealed that Actinobacteria and Acidobacteria were the most prevalent groups in the sediments of a coal mine lake.

Acidobacteria are known to be involved in the Fe-cycling. However, the role of Actinobacteria or other less abundant phylogenetic groups, such as Alicyclobacillus, Acidocella and Dyella in the iron cycle remains unclear. Thus, isolation and characterization of strains from these phylogenetic groups may help us understand the underlying mechanisms.

In our ongoing research at Lake 77, we are currently addressing these questions with a multi-pronged, interdisciplinary approach combining field sampling, culturing, advanced molecular techniques, microscopy, and spectroscopy. In these experiments, we are first examining the microbial oxidation of Fe(II) and the origin and fate of iron-rich, precipitating aggregates (iron snow) in the lake water. Additionally, we are trying to determine what Influence seepage flow has on the activity of Fe(III) reducing processes in the sediment. We are also looking at the diversity of Fe(II)-oxidizing and Fe(III)-reducing prokaryotes in the lake water and sediments. Lastly, we are characterizing the active microbial communities with RNA based techniques and the state-of-art metaproteomics.

Structure and function of acid-tolerant iron-oxidizers and iron-reducers in technical systems

Clogging of pumps caused by iron ochres (iron oxides) are common problems in groundwater wells in areas for surface mining. The mineralogy and density of these iron ochres vary in different wells from fluffy to more cemented iron precipitations. In general, iron bacteria are known to take part in iron mineral formation (biomineralization) especially under acidic conditions where chemical oxidation is low. Thus, there is an urgent need to understand how bacteria contribute to the formation of these different iron precipitates and how we can inhibit especially the formation of cemented precipitates.

As a part of the project Microbiology of Ochre Formation in Technical Systems we are studying the role of iron bacteria in the formation and dissolution of iron ochres in acidic wells (Subproject 2). We will characterize the community structure of different iron ochres, monitor their temporal development, isolate both iron oxidizing and reducing bacteria, and correlate the community structure to geochemical parameters of the clogging material.