Variability in amino acid composition of alpine crustacean zooplankton and its relationship with nitrogen-15 fractionation

M Ventura,J Catalan

doi:10.1093/plankt/fbq066

Abstract

Amino acids (AAs) are critical biochemical compounds for living organisms. Because of the limited capacity for their de novo synthesis in many animals, the nutritional value of food largely depends on its AA composition relative to the animal's requirements. To improve present knowledge on AA variability in freshwater crustaceans, we studied the inter- and intraspecific variability in three contrasting species from an oligotrophic alpine lake (Daphnia pulicaria, cladocera; Cyclops abyssorum, cyclopoid copepod and Diaptomus cyaneus, calanoid copepod). Inter-species differences were larger than intraspecific variation, confirming a non-strict homeostasis in freshwater crustacean zooplankton. The intraspecific variability differed for each species: in Daphnia, it was mainly related with ontogenetic changes rather than reproduction; in Cyclops, both factors were equally important; and reproduction was the most relevant in Diaptomus. Reproduction changes were associated with serine and phenylalanine in the three species, while the AAs responsible for ontogenetic changes differed in each species. There were no gender differences in AA composition in any of the two copepod species. Free AAs formed a very low percentage of total AA pool (<2.7%). Taking advantage of the fact that Daphnia is the main prey for Cyclops in the lake studied, we further investigated to what extent the AA composition is related with Cyclops-Daphnia nitrogen stable isotope fractionation. Only those AAs that are both essential and are not trans-aminated during protein synthesis had a significant correlation with nitrogen stable isotope fractionation, supporting the hypothesis that an AA imbalance can be responsible for a variable nitrogen stable isotope fractionation.

Highlights

Amino acids (AAs) are critical biochemical compounds for living organisms (Strayer, 1988)
Stoichiometry studies focusing on elements, mainly phosphorus, and carbon and ev nitrogen or fatty acids, mainly highly unsaturated iew fatty acids (HUFA) such as eicosapentanoic acid (EPA) or docosahexanoic acid (DHA)
This suggestion is based on two main assumptions, namely that the consumer is unable to synthesise some AAs in significant quantities and that it is homeostatic with respect to the composition in essential AAs

Summary

Introduction

Amino acids (AAs) are critical biochemical compounds for living organisms (i.e. as building blocks of proteins, as part of coenzymes, and as precursors for the biosynthesis of some molecules) (Strayer, 1988). Fo animals (D'mello, 1994) and humans (Reeds, 2000), and for aquatic organisms such as farmed fish (Conceição et al, 2003, Ronnestad et al, 2003), rotifers (Boëchat and Adrian, 2006), mussels (Kreeger et al, 1996) and marine copepods (Kleppel and ee Burkart, 1995, Guisande et al, 1999, Guisande et al, 2000) This knowledge contrasts with the few ecological studies focussing on the relative importance of AAs for animal growth and reproduction (Conklin and Provasoli, 1977, Boëchat and Adrian, 2006). The former assumption is supported by numerous studies, including in crustaceans (Claybrook, 1983), whereas the latter has only been partially demonstrated and further studies are required (Cowgill et al, 1986, Anderson et al, 2004)

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