Influence of nitrogen enrichment on size-fractionated in vitro carboxylase activities of phytoplankton from Thau Lagoon (Coastal Mediterranean Lagoon, France)

Abstract

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.

Heterotrophic Bacteria Show Weak Competition for Nitrogen in Mediterranean Coastal Waters (Thau Lagoon) in Autumn

“…5). This may be explained by the very high biomass specific carbon production of small picoeukaryote cells previously measured in this coastal lagoon [9] which were dominated by the small picoeukaryote Ostreococcus tauri (-green flagellate‖ pigment markers) throughout the year [36].…”

Phytoplankton seasonal dynamics in a Mediterranean coastal lagoon: emphasis on the picoeukaryote community

“…Different fractions of whole water were mixed with filtered water to obtain five serial dilutions (0.15, 0.25, 0.45, 0.75 and 1.0 unfiltered water) in duplicate 1L PC bottles. Each bottle received a nutrient enrichment based on K medium (Keller et al, 1987) resulting in a final nitrate concentration of 10 µM, which had been previously verified to be sufficient to satisfy phytoplankton nitrogen demands over 24 h (Fouilland et al, 2002). All other nutrients contained in K medium were added in stoichiometric proportions to the original K composition.…”

Spatio-temporal distribution of the dominant Diatom and Dinoflagellate species in the Bay of Tunis (SW Mediterranean Sea)

“…Human activities have significantly increased the input of nitrate and phosphate nutrients into coastal waters, while the silicate concentration has remained constant or even decreased in terrestrial freshwater discharges. This has affected the distribution of diatoms, FOUILLAND et al (2002), demonstrated an increase of small picophytoplankton and diatom biomass in the enriched inorganic nitrogen treatments in a Mediterranean lagoon. EGGE (1998) reported that diatoms were not able to dominate when phosphate was deficient, although silicate and nitrate were in excess.…”