Resource limitation and the biochemical composition of marine phytoplankton

The aim of the work presented in this thesis was to determine the effects of nutrient availability on the biomolecule composition and synthesis in North Sea phytoplankton. Nutrient availability in coastal seas has changed substantially over the past decades due to anthropogenic activities. Riverine nutrient loads and consequently nutrient ratios of nitrogen to phosphorus (N:P) shifted. Phytoplankton at the base of the food web is first and foremost affected by these changes and responds by adapting its biomolecule composition because the main biomolecules like carbohydrates, amino acids, fatty acids and RNA/DNA bases have different C:N:P requirements for the synthesis.
Using a combination of ¹³C stable isotope tracers and methods in gas and liquid chromatography/isotope ratio mass spectrometry were used to determine concentrations and biosynthesis rates of individual biomolecules and assemble a biomolecule based carbon budget. This allowed quantifying the effects of nutrient limitation on the biomolecule composition of primary producers and evaluating changes in their nutritional value. Furthermore, a first study was conducted to determine the adaptation mechanisms used by herbivorous zooplankton to cope with mismatching requirements in their food. Overall, phytoplankton communities were flexible in adapting their biomolecule composition and biosynthesis to prevailing nutrient availabilities. Species and nutrient specific mechanisms were contributing factors and quick adaptations occurred when nutrient situations change. The consequent a lack of essential compounds translated up the food chain forcing zooplankton to employ various physiological response mechanisms.