Diel variability of photosynthetic picoplankton in the equatorial Pacific

The diel variability in cell abundance, light scatter, and pigment fluoresc ence of three autotrophic picoplankton groups (Prochlorococcus, Synechococc us, picoeukaryotes) measured by flow cytometry was investigated in surface waters of the equatorial Pacific Ocean (5 degrees S, 150 degrees W) during 5 days with about 1 hour temporal resolution. The diel variability of verti cal profiles was examined at the same station on days 2 and 4. Prochlorococ cus division rate was also estimated from cell cycle measurements. A more l imited data set was obtained at a station located in very oligotrophic wate rs (16 degrees S, 150 degrees W). All three picoplankton populations exhibi ted very marked diel variability. Cell division was highly synchronized but not phased identically for all three populations: Synechococcus divided fi rst, followed 2 hours;later by Prochlorococcus and 7 hours later by picoeuk aryotes. Cells grew in size only once the sun had risen, but growth did con tinue 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 fo r cell abundance and scatter. For chlorophyll fluorescence, in contrast, pr okaryotes displayed opposite patterns during the light period between surfa ce (decrease due to very strong quenching) and depth (increase). This creat ed steep vertical gradients during the day that vanished at night because o f convective mixing. In the top 25 m, strong light intensities (including U V radiation) had very pronounced detrimental effects on prokaryotes, especi ally on Prochlorococcus, inducing fluorescence quenching, slowed down growt h, and retardation of DNA synthesis.