During parallel sampling of seawater samples in bottles and in free water (1000-2000 m(3) clay ponds), we have measured phytoplankton biomass, Ribulose biphosphate carboxylase (Rubisco), phosphoenol pyruvate carboxylase, and phosphoenol pyruvate carboxykinase activities and major nutrients (ammonium, nitrate, phosphate, silicate). This was done in two ecosystems: one with high grazing pressure due to the presence of oysters and another with low grazing pressure (no oysters). In the ecosystem subjected to high grazing pressure, anaplerotic carbon fixation by phytoplankton in free water was higher in the light period and could represent 25% of total carbon fixation. Incubating samples in bottles led to a major increase in Rubisco activity (80% in 3 h) relative to values measured in free water, a decrease in 5-carboxylases activity (70% in 24 h) due to ammonium exhaustion, as well as disappearance of its diet periodicity. This indicates a contrario that grazers, which are excluded from incubation bottles, drive ecosystems toward heterotrophy in situ by favoring the beta-carboxylation pathway through excretion products such as ammonium. Therefore, incubations in high grazing environments (characterized by a grazing rate near 2 day(-1)) change the way carbon is fixed by unicellular algae within 3 h through a change in the form of nitrogen taken up.