Carbonate Mineral Formation in the Vicinity of Blooming Algae in a Shallow Lake

Minerals of the sediments of shallow lakes are in continuous interaction with the biota. In order to understand the role of algae in producing carbonate minerals in shallow Lake Balaton, we used electron microscopy to study the nanoscale processes of mineral formation and transformation in association with algal blooms. In the immediate vicinity of photosynthesizing cells, the first phase to precipitate was amorphous calcium carbonate (ACC). Also associated with some blooms were spindle-shaped calcite particles that invariably contained nm-scale flakes of clay minerals belonging to the smectite group. In contrast, carbonate particles collected from the sediment and from suspended matter during non-bloom periods contained only Mg-bearing calcite particles and minor protodolomite, both associated with smectite flakes. By comparing these observations with the results of laboratory carbonate precipitation experiments, we interpret the distinct carbonate phases as representing stages of a complex process: (1) algal photosynthesis creates high supersaturation near the cells, resulting in the fast nucleation of ACC; (2) ACC particles attach to nm-scale smectite flakes where they instantly transform into calcite through partial dissolution and reprecipitation. Step (2) can result either in calcite spindles, or aggregate-looking, Mg-bearing calcite particles that are typical for Lake Balaton sediments. Spindles form exclusively near algae, for reasons not yet known. (3) Further aging of Mg-bearing calcite by dissolution/reprecipitation produces protodolomite. The entire process appears to take place within hours to days; thus, the typical Lake Balaton sediment contains mainly Mg-bearing calcite and some protodolomite particles. Significantly, the above multistep process of carbonate formation involves several dissolution and reprecipitation cycles, and in most stages clay minerals play crucial roles.