The objective of this dissertation is to investigate and compare the Fe-cycling microbial communities in Fe(III)-rich aggregates and sediments at different pH conditions in an acidic lignite mine lake. The water body of the Lake 77 was separated by a bank rising from the bottom of the lake, forming two basins that differ in stratification patterns and pelagic boundary conditions. Biogenic iron-rich particles (iron snow) formed at steeply opposing gradients of oxygen and Fe(II) within the redoxcline of both basins and were highly colonized by microbial cells. The iron snow was different from other naturally formed particles. It had a diameter of up to 380 µm and a high sedimentation velocity (~2 m h-1). The organic carbon content of iron snow was below 11% and the iron content more than 35%, primarily in the form of schwertmannite (>91% mineral content). Microbial Fe-cycling appeared to be the dominant metabolism in the iron snow. The proteins retrieved from metaproteomic analyses were likely involved in primary production, motility and metabolism of Fe-cycling microbes. Based on these results, we proposed a three-stage developmental model of iron snow in these waters. Similar microbial communities were found in iron snow and sediment surface at same locations by DGGE. The microbial communities in sediments were investigated using cultivation-independent and -dependent approaches. 117 isolates were obtained, including 27 putative new species. Twenty one strains could oxidize and reduce iron, and most of the acidophilic Fe reducers preferred microoxic conditions for Fe(III) reduction. The results strongly suggested that iron snow acted as carriers bringing iron, organic carbon and living microorganisms from the water column to the lake sediment. In addition, pH might be the major driving force shaping the microbial communities responsible for Fe-cycling in the iron snow and sediments.