Pelagic boundary conditions affect the biological formation of iron‐rich particles (iron snow) and their microbial communities

Abstract

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 μm, showed high sedimentation velocity (∼ 2 m h‐1), and were dominated by the iron mineral schwertmannite. Although the particles were highly colonized by microbial cells (∼ 1010 cells [g dry weight]‐1), 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 Chlorobia‐related 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.”