Journal of Plankton Research Vol.23 no.6 pp.623-632, 2001
© Oxford University Press 2001
Size-fractionated Carboxylase Activities During a 32 h Cycle at 30 m Depth in the North-western Mediterranean Sea After an Episodic Wind Event
1 Present Addresses: Dunstaffnage Marine Laboratory, P.O. Box 3, Oban, Argyll, Pa34 4ad, Uk,
2 Laboratoire Modèles En Biologie Cellulaire, Umr Cnrs 7628, Observatoire Océanologique, 66551 Banyuls-Sur-Mer, France,
3 Université Montpellier Ii, Umr Cnrs 5554, Isem, Cc 061, Paléoenvironnements Et Palynologie, Place Eugène Bataillon, 34095 Montpellier Cedex 05, France
Using size-fractionation filtration (1 µm), we associated carboxylase activities (Rubisco, ß-carboxylases) and chlorophyll measurements with cell enumeration by flow cytometry at a permanent site of the central Ligurian Sea in the north-western Mediterranean Sea (73°25'N–7°51' E). The analyses were carried out over a day/night cycle (at 30 m depth) following a strong wind event, during the transition period from spring mesotrophic to summer oligotrophic conditions. The highest values of Rubisco activity and ß-carboxylase activity per chlorophyll a (Chl a) for >1 µm cells were observed during the light period of the cycle, reaching 18.9 and 4.3 nmol CO2 (µg Chl a)–1 h–1, respectively. This higher activity is assumed to be correlated with a dominance of nanoflagellates in the phytoplankton community. Such phytoplankton species generally had higher ß-carboxylase activity, expressed as a percentage of Rubisco activity (the ßC/R ratio), than diatoms. Using flow cytometry analysis to enumerate those cells <1 µm in size, we followed the values of Rubisco activity and pigment content expressed per cell, for picophytoplankton cells. The photoautotrophic activity, measured as the in vitro Rubisco activity for small picoeukaryote cells, was higher than for cyanobacteria cells with lower apparent cell size. These results suggested an optimum of CO2 assimilation reached by the pico- and nano-phytoplankton in accordance with the cell size and growth rates from previous observations in the literature.