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

Using N-15 and P-33, 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 (> 0.8 mu m), 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 PO43-. The uptake of N from leucine, and P from ATP and dissolved DNA, was initially dominated by the 0.8-0.2 mu m size fraction, but shifted towards dominance by the > 0.8 mu m 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 mu m size fraction had higher specific affinity for leucine-N than those in the > 0.8 mu m 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.