From Ecological Stoichiometry to Biochemical Composition: Variation in N and P Supply Alters Key Biosynthetic Rates in Marine Phytoplankton

One of the major challenges in ecological stoichiometry is to establish howenvironmental changes in resource availability may affect both the biochemicalcomposition of organisms and the species composition of communities. This is apressing issue in many coastal waters, where anthropogenic activities have causedlarge changes in riverine nutrient inputs. Here we investigate variation in the biochemicalcomposition and synthesis of amino acids, fatty acids (FA), and carbohydrates inmixed phytoplankton communities sampled from the North Sea. The communities werecultured in chemostats supplied with different concentrations of dissolved inorganicnitrogen (DIN) and phosphorus (DIP) to establish four different types of resourcelimitations. Diatoms dominated under N-limited, NCP limited and P-limited conditions.Cyanobacteria became dominant in one of the N-limited chemostats and green algaedominated in the one P-limited chemostat and under light-limited conditions. Changesin nutrient availability directly affected amino acid content, which was lowest under Nand NCP limitation, higher under P-limitation and highest when light was the limitingfactor. Storage carbohydrate content showed the opposite trend and storage FAcontent seemed to be co-dependent on community composition. The synthesis ofessential amino acids was affected under N and NCP limitation, as the transformationfrom non-essential to essential amino acids decreased at DIN:DIP ? 6. The simplecommunity structure and clearly identifiable nutrient limitations confirm and clarifyprevious field findings in the North Sea. Our results show that different phytoplanktongroups are capable of adapting their key biosynthetic rates and hence their biochemicalcomposition to different degrees when experiencing shifts in nutrient availability. This willhave implications for phytoplankton growth, community structure, and the nutritionalquality of phytoplankton as food for higher trophic levels