In hardwater lakes, calcite precipitation is a major biogeochemical process coupling carbon cycling with primary production. While recent studies refined the magnitude and physico-chemical conditions underlying calcite precipitation at fine scales, the mechanisms supporting the nucleation of calcite are poorly described. In the pelagic realm of deep oligotrophic lakes, calcite nucleation has often been associated with autotrophic picoplankton, however, direct observations remain scarce. Here, we focused on Lake Geneva, the largest hardwater lake of western Europe, and combined depth-resolved high-frequency and discrete sample data to investigate the coupled dynamics of calcite precipitation and picoplankton populations. The precipitation of calcite during periods of lake thermal stratification and high productivity coincided with peaks in the abundance of distinct picoplankton populations that were characterized and quantified based on specific autofluorescence signatures using spectral flow cytometry. The vertical distribution of picoplankton over time evidenced maximum abundances of the dominant phycoerythrin (PE) rich population at depths of enhanced water column stability. Moreover, cell sorting of this PE-rich population combined with direct imaging by scanning electron microscopy enabled original observations of the close association of calcite crystals with picoplankton cells. These results provide refined mechanistic insights into the calcite precipitation process and confirm its tight link with the dynamics of picoplankton communities from Lake Geneva.
Faculty of Geosciences and Environment
Institute of Earth Surface Dynamics
Geopolis – Office 3227
University of Lausanne
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