Diel variability of photosynthetic picoplankton in the equatorial Pacific

The diel variability in cell abundance, light scatter, and pigment fluorescence of three autotrophic picoplankton groups ( Prochlorococcus, Synechococcus , picoeukaryotes) measured by flow cytometry was investigated in surface waters of the equatorial Pacific Ocean (5°S, 150°W) during 5 days with about 1 hour temporal resolution. The diel variability of vertical profiles was examined at the same station on days 2 and 4. Prochlorococcus division rate was also estimated from cell cycle measurements. A more limited data set was obtained at a station located in very oligotrophic waters (16°S, 150°W). All three picoplankton populations exhibited very marked diel variability. Cell division was highly synchronized but not phased identically for all three populations: Synechococcus divided first, followed 2 hours later by Prochlorococcus and 7 hours later by picoeukaryotes. Cells grew in size only once the sun had risen, but growth did continue in the dark for a short period. Growth processes occurred in parallel at the top and the bottom of the mixed layer, inducing uniform profiles for cell abundance and scatter. For chlorophyll fluorescence, in contrast, prokaryotes displayed opposite patterns during the light period between surface (decrease due to very strong quenching) and depth (increase). This created steep vertical gradients during the day that vanished at night because of convective mixing. In the top 25 m, strong light intensities (including UV radiation) had very pronounced detrimental effects on prokaryotes, especially on Prochlorococcus , inducing fluorescence quenching, slowed down growth, and retardation of DNA synthesis.