Abstract
Different metabolic pathways and requirements can lead to imbalances in the biochemical composition, e.g., between prokaryotes and eukaryotes and between heterotrophic and autotrophic organisms. In addition, heterotrophs cannot synthesize many of the required biochemical constituents themselves but rely in their provision on the feeding environment. If imbalances of essential biochemicals between heterotrophic consumers and their prey are large, those biochemicals can become limiting. The importance of essential ω3‐poly unsaturated fatty acids (PUFAs) for heterotrophic organisms (zooplankton, fish) is well recognized. Especially the conversion of C~18~‐PUFAs to C~20~ or C~22~ highly unsaturated fatty acids (HUFAs) are critical for heterotrophic protists and metazooplankton, and storage may be an adaptation to help overcome limitation. The ecological relevance of cholesterol, essential for arthropods, is still an open question as vertebrates and fungi may be important sources in aquatic systems. Less is known about essential amino acids being limiting. However protein limitation seems to be rare in plankton unless detritus contributes largely to the food. One ecological importance of autotrophs lies in the synthesis of essential biomolecules, promoting growth of heterotrophs. However, vitamin B~12~ may preferentially originate from prokaryotes (bacteria, cyanobacteria), even limiting growth of certain eukaryotic algae. Measuring the relevant biochemicals, its environmental forcing and cycling (conservation versus alteration in the food web), going beyond food species affiliation and carbon as a unit, but taking the overall food concentration into account will enhance the characterization of C‐cycling and energy flux, irrespectively of the “classical food chain” or “microbial loop”, with implication to human nutrition. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)