Diel periodicity in Synechococcus populations and grazing by heterotrophic nanoflagellates: Analysis of food vacuole contents

The relationship between heterotrophic nanoflagellates (HFLAG) and the autotrophic prokaryote Synechococcus was examined in the bay of Villefranche (NW Mediterranean) in June 1998. We determined Synechococcus concentrations, % of dividing Synechococcus cells, concentrations of HFLAG, and numbers of Synechococcus cells inside HFLAG food vacuoles in samples taken every 2-3 h over 62 h (n = 28). Synechococcus cells in division ranged from 10 to 30%, with peak values in late afternoon-early evening hours. Numbers of Synechococcus in HFLAG food vacuoles (0.06-0.34 Synechococcus HFLAG(-1)) were not related to concentrations df total Synechococcus, or nondividing Synechococcus but were negatively related to % of dividing Synechococcus. The data suggest there may be a relationship between vulnerability to predation and average Synechococcus cell size. HFLAG community grazing pressure, estimated from food vacuole content and HFLAG abundances, ranged from about 0.2% Synechococcus stock removed h(-1) at midnight to 1% Synechococcus stock removed h(-1) at noon.