Diurnal variability of Synechococcus abundance in Sagami Bay, Japan

Synechococcus, the most dominant picophytoplankton in coastal regions, exhibits diurnal variations in the open ocean. The aim of this study was to assess its short-term population dynamics and cell cycle phases through DNA analysis in a coastal region (Sagami Bay, Japan) by sampling at fixed intervals vertically across the euphotic zone. Day-to-day variations in physical processes such as mixing, induced by wind, and advective events played an important role in structuring Synechococcus population. Compared to sub-surface population, surface cells entered S phase earlier in the day and were arrested and accumulated in the S phase for a longer time suggesting an inhibitory effect by higher irradiance. Increase of G(2)-phase cells during the dark suggests progression of cells through S phase in absence of light. Although the length of G(2)-phase was not known, subsequent increase in G(1)-phase cells in the early morning and late evening suggests occurrence of two cohorts of cells which divided in the dark phase or late evening of the cycle. This study showed that the population responds differently across the euphotic zone depending on the physical structure of the water column and its cell cycle phases are controlled by the vertical irradiance availability.