Anthropogenic activities resulted in major shifts in nutrient inputs to coastal seas, which may have alteredthe biomolecule composition of phytoplankton because of different C : N : P requirements for biosynthesis.In order to understand the effects of N- and P-limitation on the allocation of photosynthetically fixed C, wedirectly measured seasonal and spatial dynamics of amino acid, fatty acid and carbohydrate concentrationsand biosynthesis rates in the phytoplankton of the North Sea using a novel 13C-tracer approach. Nutrientlimitation and season had large effects on composition and biosynthesis rates of all biochemical classes, forinstance the contribution of protein derived amino acids varied up to fourfold in concentration and up toeightfold in biosynthesis rates. Nutrient specific effects and the prevailing nutrient limitations were identifiedby short-term (24–72 h) nutrient addition experiments. Addition of the growth-limiting nutrient increasedamino acid synthesis while storage compound synthesis decreased concurrently. The strongest effect wasdetected in N-limited flagellates, where amino acid synthesis increased up to fivefold within 24 h upon reliefof nutrient limitation. P-limitation was only detected in diatom-dominated near-coast stations, and aminoacid synthesis responded much slower, likely because of the necessary preceding synthesis of ribosomes. Thismetabolic elasticity of phytoplankton in response to nutrient availability will have major consequences fortheir nutritional value within the food web and subsequently the carrying capacity of coastal ecosystems.
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