DIEL IN SITU PICOPHYTOPLANKTON CELL DEATH CYCLES COUPLED W ITHCELL DIVISION

摘要

The diel variability in picophytoplankton cell deathwas analyzed by quantifying the proportion of deadcyanobacteria Prochlorococcus and Synechococcus cellsalong several in situ diel cycles in the open Mediterranean Sea. During the diel cycle, total cell abundance varied on average 2.8 +-0.6 and 2.6 +-0.4 timesfor Synechococcus and Prochlorococcus populations,respectively. Increasing percentages of dead cells ofProchlorococcus and Synechococcus were observed during the course of the day reaching the highest valuesaround dusk and decreasing as the night progressed,indicating a clear pattern of diel variation in the cellmortality of both cyanobacteria. Diel cycles of celldivision were also monitored. The maximum percentage of dead cells (Max % DC) and the G2 + Mphase of the cell division occurred within a period of2 h for Synechoccoccus and 4.5 h for Prochlorococcus,and the lowest fraction of dead cells occurred atearly morning, when the maximum number of cellsin G1 phase were also observed. The G1 maximumcorresponded with the maximal increase in newlydivided cells (minimum % dead cells), and the subsequent exposure of healthy daughter cells to environmental stresses during the day resulted in theprogressive increase in dying cells, with the loss ofthese cells from the population when cell divisiontakes place. The discovery of diel patterns in celldeath observed revealed the intense dynamics of picocyanobacterial populations in nature.