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Picophytoplankton dynamics in the equatorial Pacific: diel cycling from flow-cytometer observations

1997; Elsevier BV; Volume: 320; Issue: 11 Linguagem: Francês

10.1016/s0764-4469(97)80878-5

1768-3238

Jean Blanchot, Jean‐Michel André, Claudie Navarette, Jacques Neveux,

Microbial Community Ecology and Physiology

Au cours de la campagne Flupac en octobre-novembre 1994 dans l'océan Pacifique, nous avons étudié les variations circadiennes du picophytoplancton à 0 °–150 °W pendant 7 j. Un suivi haute fréquence des variations d'abondance, conduit sur 24 h en surface, permet de démontrer que les deux groupes majeurs du picophytoplancton, les Prochlorococcus et les picoeucaryotes, évoluent de manière synchrone et parallèle. Les cellules se divisent au maximum une seule fois, entre la fin de l'après-midi et le milieu de la nuit. Ce cycle circadien est observable sur toute la zone euphotique pour les deux groupes. En surface, les variations d'abondances permettent d'estimer un taux de croissance d'environ 0,6 j1. L'augmentation du nombre de cellules qui résulte de la division est progressivement résorbée avant que ne démarre un nouveau cycle de division. La diffusion (FS) aux petits angles des cellules des deux groupes décroît au moment de la division cellulaire. Elle augmente régulièrement durant la phase de croissance liée à l'activité photosynthétique. The French JGOFS cruise FLUPAC was conducted in October–November 1994 in the western and central equatorial Pacific Ocean. During a 7-day time series at 150 °W, in addition to conventional sampling (four times per day from discrete depths between 0 and 150 m), a high frequency (hourly) experiment was performed by continuously pumping water at 5 m below the surface over 24 h. Flow cytometric measurements allowed us to recognise and to follow separately the two major components of equatorial picophytoplankton, the Prochlorococcus and the picoeukaryotes. The hourly surface experiment confirmed the synchrony of Prochlorococcus cell division and showed that picoeukaryotes exhibited a similar behaviour. The main consequence is that the maximum potential growth rate of picophytoplankton is one doubling per day for both cell groups. The vertical profiles indicated that the diel cycling extends throughout the surface layer for both algal groups. The cells were observed to divide daily from late afternoon to the middle of the night, and then to disappear, probably as a result of grazing. In the surface layer, variations of abundance allowed us to estimate a growth rate of about 0.6 day1. Mean cell light scattering parFS as measured by the cytometer indicated a decrease in cell size concurrent with cell division and an increase during the photosynthetic growth phase.