Stable carbon isotopic analysis of amino acids in a simplified food chain consisting of the green algaChlorellaspp., the calanoid copepodCalanus sinicus, and the Japanese anchovy (Engraulisjaponicus)

Abstract

Japanese anchovy (Engraulis japonicus Temminck and Schlegel, 1846) is a keystone species in the food web of the Yellow Sea and East China Sea ecosystem. To study nutrient flow at intermediate and lower trophic levels, a controlled feeding experiment was carried out on a simplified food chain of the green alga Chlorella Beijerinck, 1890 – the calanoid copepod Calanus sinicus Brodsky, 1962 – E. japonicus. For the whole organism, bulk δ13C was found to be enriched with escalating trophic level, although the intertrophic gap in δ13C was slightly lower than the commonly reported 1.5‰ per increase in level. Furthermore, 15 amino acids (AAs) were detected in the studied organisms. Among them, the δ13C values of 12 AAs were determined and were found to exhibit diverse patterns of variation. The δ13C levels of essential AAs changed very little and were highly correlated across trophic levels, indicating that they underwent little trophic fractionation and were mainly ingested by the consumers from the proteins in their food. In contrast, the δ13C values of nonessential AAs differed greatly across trophic levels, indicating that the nonessential AAs in consumers were synthesized de novo from food proteins. The fractionation pattern of nonessential AAs may depend on the carbon pool and the AA composition of the organism at the lower trophic level.