Spektroskopie / Bildgebung

We studied the formation of iron-rich particles at steeply opposing gradients of oxygen and Fe(II) within the redoxcline of an acidic lignite mine lake (pH 2.9). Particles formed had a diameter of up to 380 mm, showed high sedimentation velocity (, 2 m h21), and were dominated by the iron mineral schwertmannite. Although the particles were highly colonized by microbial cells (, 1010 cells [g dry weight]21), the organic carbon content was below 11%. Bathymetry and the inflow of less acidic, Fe(II)-rich groundwater into the northern basin of the lake results in two distinct mixing regimes in the same lake. The anoxic monimolimnion of the northern basin had higher pH, Fe(II), dissolved organic carbon, and CO2 values compared with the more central basin. Particles formed in the northern basin differed in color, were smaller, had higher organic carbon contents, but were still dominated by schwertmannite. Microcosm incubations revealed the dominance of microbial Fe(II) oxidation. Comparison of bacterial clone libraries suggested that pH was a major driving force, shaping the microbial communities responsible for the oxidation of Fe(II) in both basins. Acidophilic Ferrovum spp. and Chlorobiarelated bacteria were present in the central basin, whereas neutrophilic Sideroxydans spp. dominated the northern basin. Snow-like particles had a high sinking velocity and acted as a carrier for organic carbon, microorganisms, trace metals, and Fe(III) to the sediment. Because these particles are fundamentally different from organic-rich ‘‘snows’’ from lakes, rivers, and oceans, we propose a new term, ‘‘iron snow.’’ Oxic–anoxic interfaces are hotspots for the cycling of elements because they provide continuously favorable conditions for both biotic and abiotic redox reactions. In aquatic ecosystems, these interfaces may appear as pelagic boundaries by separating an upper oxic and a lower anoxic water body, yielding permanently or temporally stratified conditions. These boundaries, or redoxclines, may occur in the water column of marine (Jørgensen et al. 1991; Taylor et al. 2001; Pimenov and Neretin 2006) and of freshwater bodies (Boehrer and Schultze 2008; Casamayor et al. 2008). Within such redoxclines, opposing gradients of oxygen and more reduced components, i.e., Fe(II), S22, and CH4, may establish. While the oxidation of S22 (Jørgensen et al. 1991; Lu¨ thy et al. 2000) and CH4 (Rudd et al. 1974; Liu et al. 1996; Be´dard and Knowles 1997) at redoxclines has been extensively investigated, less information is available about the oxidation of Fe(II). Some evidence for the cycling of iron at pelagic boundaries exists (Campbell and Torgersen 1980; Boehrer and Schultze 2006; Dı´ez et al. 2007), and the formation of Fe(III)-minerals in acidic aquatic environments has been linked to a microbial oxidation of Fe(II) in the water.