Stable nitrogen isotopes in essential versus non-essential amino acids of different plankton size fractions†

Abstract

The stable nitrogen isotope values (δ15N) of the essential amino acid (EAA) leucine and the δ15N values of six non-essential amino acids (NEAAs) from plankton size fractions from the South China Sea (SCS) were analysed. Data from the SCS were collected during two cruises in July 2003 and 2004 onboard of RV Nghien Cuu Bien. The δ15N values of alanine, aspartic acid, glutamic acid and leucine increased with size at all sites. The δ15N of glycine did not increase with size, the δ15N of tyrosine increased with size only at offshore stations and the δ15N of proline increased with size only at inshore stations. We found highly significant correlations between the δ15N ratios of leucine to the δ15N ratios of glutamic acid, proline, alanine, tyrosine and aspartic acid at oligotrophic sites of enhanced nitrogen fixation. In contrast thereto these correlations were less distinct or absent at more eutrophic sites of low nitrogen fixation. A comparison with an independent data set from the tropical North Atlantic revealed intriguing similar patterns. We interpret these patterns as result of the connected metabolism of EAA and NEAA in zooplankton at sites of nitrogen limitation. †Revised version of a paper presented at the 27th Annual Meeting of the German Association for Stable Isotope Research (GASIR), 4–6, October, 2006, Freiberg, Germany.