Title: Vertical segregation of two cell-cycle phases of the calcifying freshwater phytoflagellate<i>Phacotus lenticularis</i>(Chlorophyta)
Abstract: Specific responses of cells from different cell-cycle phases to environmental variations strongly impact their ecology and productivity. We quantified responses of both the motile, calcified growing cell phase (G1 phase) and the calcite shell forming, non-motile mitotic phase (S, G2 and M phase) of the freshwater phytoflagellate Phacotus lenticularis to changes in depth-specific physicochemical parameters and meteorological parameters over two growing seasons. Growing (GCs) and mitotic cells (MCs) segregated vertically during peak development. GCs accumulated within the thermocline, whereas MCs aggregated in the epilimnion down to 4 m depth, which corresponded to the maximum depth of mixing. GCs and MCs differed in their preference with respect to pH and water temperature, but not to water density. Interestingly, pH and temperature variations in the epilimnion, where MCs aggregate and calcify, explained up to 83% of the variance of GC density at a time lag of one and more days in a daily sampling campaign. Among the meteorological variables we tested, rainfall and air temperature had an additional positive effect on GC density. The results suggest that Phacotus cells migrate upwards prior to loss of motility to layers that reduce sinking but likely enhance calcification rates, which translates into a response of GC density.