Diel patterns of growth and division in marine picoplankton in culture

The effect of a 12:12-h light:dark (LD) cycle on the phasing of several cel l parameters was explored in a variety of marine picophytoplanktonic strain s. These included the photosynthetic prokaryotes Pro-chlorococcus (strains MED 4, PCC 9511, and SS 120) and Synechococcus (strains ALMO 03, ROS 04, WH 7803, and WH 8103) and five picoeukaryotes (Bathycoccus prasinos Eikrem et Throndsen, Bolidomonas pacifica Guillou et Chretiennot-Dinet, Micromonas p usilla Manton et Parke, Pelagomonas calceolata Andersen et Saunders, and Py cnococcus provasolii Guillard et al.). Flow cytometric analysis was used to determine the relationship between cell light scatter, pigment fluorescenc e, DNA (when possible), and the LD cycle in these organisms. As expected, g rowth and division were tightly coupled to the LD cycle for all of these st rains. For both Prochlorococcus and picoeukaryotes, chi and intracellular c arbon increased throughout the light period as estimated by chi fluorescenc e and light scatter, respectively. In response to cell division, these para meters decreased regularly during the early part of the dark period, a decr ease that either continued throughout the dark period or stopped for the se cond half of the dark period. For Synechococcus, the decrease of chi and sc atter occurred earlier (in the middle of the light period), and for some st rains these cellular parameters remained constant throughout the dark perio d. The timing of division was very similar for all picoeukaryotes and occur red just before the subjective dusk, whereas it was more variable between t he different Prochlorococcus and Synechococcus strains. The burst of divisi on for Prochlorococcus SS 120 and PCC 9511 was recorded at the subjective d usk, whereas the MED 4 strain divided later at night. Synechococcus ALMO 03 , ROS 04, and WH 7803, which have a low phycourobilin to phycoerythrobilin (PUB:PEB) ratio, divided earlier, and their division was restricted to the light period. In contrast, the high PUB:PEB Synechococcus strain WH 8103 di vided preferentially at night. There was a weak linear relationship between the FALS(max):FALS(min) ratio and growth rate calculated from cell counts (r = 0.83, n = 11, P < 0.05). Because of the significance of picoplanktonic populations in marine systems, these results should help to interpret diel variations in oceanic optical properties in regions where picoplankton dom inates.