Size-fractionated carboxylase activities during a 32 h cycle at 30 m depthin the north-western Mediterranean Sea after an episodic wind event

Using size fractionation filtration (1 mum), we associated carboxylase acti vities (Rubisco, beta -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 degrees 25 'N-7 degrees 51 ' E). The analyses were carried out over a day/night Cycle (at 30 m depth) fo llowing a strong wind event, during the transition period from spring mesot rophic to summer oligotrophic conditions. The highest values of Rubisco act ivity and beta -carboxylase activity per chlorophyll a (Chl a) for >1 mum c ells were observed during the light period of the cycle, reaching 18.9 and 4.3 nmol CO2 (mug Chl a)(-1) h(-1), respectively. This higher activity is a ssumed to be correlated with a dominance of nanoflagellates in the phytopla nkton community. Such phytoplankton species generally had higher beta -carb oxylase activity, expressed as a percentage of Rubisco activity (the betaC/ R ratio), than diatoms. Using ooze, cytometry analysis to enumerate those c ells <1 mum in size, we followed the values of Rubisco activity and pigment content expressed per cell, for picophytoplankton cells. The photoautotrop hic activity, measured as the in vitro Rubisco activity for small picoeukar yote cells, was higher than for cyanobacteria cells with lower apparent cel l size. These results suggested an optimum of CO2 assimilation reached by t he pico- and nano phytoplankton in accordance with the cell size and growth rates from previous observations in the literature.