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Competition for inorganic and organic forms of nitrogen and phosphorous between phytoplankton and bacteria during an Emiliania huxleyi spring bloom

par Løvdal, Trond;Eichner, Christiane;Grossart, Hans Peter;Carbonnel, Vincent ;Chou, Lei ;Martin-Jézéquel, V.;Thingstad, Tron Frede
Référence Biogeosciences, 5, 2, page (371-383)
Publication Publié, 2009

Article révisé par les pairs

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Résumé :

Using 15N and 33P, we measured the turnover of organic and inorganic nitrogen (N) and phosphorus (P) substrates, and the partitioning of N and P from these sources into two size fractions of marine osmotrophs during the course of a phytoplankton bloom in a nutrient manipulated mesocosm. The larger size fraction (>0.8μm), mainly consisting of the coccolithophorid Emiliania huxleyi, but also including an increasing amount of large particle-associated bacteria as the bloom proceeded, dominated uptake of the inorganic forms NH4 +, NO3 -, and PO4 3-. The uptake of N from leucine, and P from ATP and dissolved DNA, was initially dominated by the 0.8-0.2 μm size fraction, but shifted towards dominance by the >0.8μm size fraction as the system turned to an increasing degree of N-deficiency. Normalizing uptake to biomass of phytoplankton and heterotrophic bacteria revealed that organisms in the 0.8-0.2 μm size fraction had higher specific affinity for leucine-N than those in the >0.8 μm size fraction when N was deficient, whereas the opposite was the case for NH4 +. There was no such difference regarding the specific affinity for P substrates. Since heterotrophic bacteria seem to acquire N from organic compounds like leucine more efficiently than phytoplankton, our results suggest different structuring of the microbial food chain in N-limited relative to P-limited environments.