A Survey of Free Amino Acids in Copepod Populations

Abstract

The distribution of free amino acids (FAA) in natural copepod populations was studied in six copepods. The species studied included Diaptomus sp., Eurytemora affinis, Aeartia tonsa, Acartia clausi, Pseudoealanus minutus, and Calanus fimnarchieus. All six species were collected from Rhode Island and neighboring waters at times of abundance. Collectively, they represent a chain of species populations that span a salinity gradient from fresh water (Diaptomus sp.) to the oceanic environment (Calanus finmarchicus). Each member of this chain has a higher optimum salinity for propagation. FAA were analyzed by ion-exchange chromatography. Nineteen common FAA were present in all the species, with the exception of Eurytemora affinis, which had no free histidine. Both the fresh and brackish water copepods, Diaptomus sp. and E. affinis, were characterized by the predominance of alanine. In addition, the basic amino acids, arginine and lysine, constituted a large fraction of the total free amino acid content in the Diaptomus sp. Glycine was the most abundant FAA in the higher salinity species -- A. tonsa, A. clausi, P. minutus, and C. finmarchieus. Proline, alanine, taurine, glutamic acid, arginine, lysine, and threonine were also abundant. Collectively, these eight FAA constituted over 60% of the total FAA content in all the species studied. Optimum salinities for the six species plotted against the respective FAA concentrations of glycine, taurine, proline, arginine, alanine, and glutamic acid resulted in a curvilinear relationship. The optimum salinity-total FAA relationship was also curvilinear. The fresh water copepod Diaptomus sp. had the lowest total FAA levels of all the six species. Increasingly higher total FAA concentrations occurred in E. affinis, A. tonsa, A. clausi, and P. minutus, species with successively higher optimum salinities. Thus, a linear relationship between increasing optimum salinities and FAA concentrations in the corresponding species was obtained, with the highest FAA levels occurring in A. clausi and P. minutus. However, in the last species in the chain C. finmarchicus which is oceanic and has the highest optimum salinity, the total FAA level was actually less than in P. minutus and A. clausi. As a result, the relationship dropped off and became curvilinear. The congeneric species in Narragansett Bay --- A. tonsa and A. clausi --- differed not only in their concentrations of taurine, glutamic acid, glycine, and alanine but also in their total FAA concentrations. Aeartia tonsa had lower levels of these FAA (as well as lower total FAA) than A. clausi. This further