Résumé
The influence of dissolved inorganic and organic nitrogen (DIN and DON) enrichments on pools of enzymes responsible for CO
2 fixation by the Calvin–Benson (Rubisco) and β-carboxylation pathways (β-carboxylases) were studied in a natural plankton assemblage. The plankton community from a coastal Mediterranean lagoon were incubated in situ for 24 h with initially ammonium, nitrate and DON (taurine) enrichments and compared to a control without any enrichment. An increase of small picophytoplankton and diatom biomass was observed in the enriched inorganic nitrogen treatments 7–10 h after the initial enrichment. Phytoplankton biomass decreased in the control and under taurine enrichment suggesting an inorganic nitrogen limitation of phytoplankton growth. Nevertheless, the ratio of particulate carbon (C) versus chlorophyll
a concentrations decreased throughout the experiment, pointing out the increasing importance of the autotroph community in all the carboys. The pool of Rubisco enzyme per biomass generally decreased in all the incubation carboys. In contrast, an increase in the pool of β-carboxylase enzymes normalised to the particulate carbon, chlorophyll and cell concentrations was observed at the end of the experiment under inorganic N-deficient conditions, in the control and with the addition of taurine. Under these latter conditions, the anaplerotic carbon fixation by β-carboxylation may potentially exceed photosynthetic carbon fixation by Rubisco in phytoplankton cells. A shift towards heterotrophy for natural phytoplankton cells using organic sources of nitrogen is proposed to explain the increase of the enzymatic pool of the β-carboxylation pathway.