Heterotrophic Assimilation and Occurrence of Dissolved Free Amino Acids in a Shallow Estuary

Abstract

Seasonal changes in concentrations of dissolved free amino acids (DFAA), heterotrophic assimilation of the 5 most abundant free amino acids, and release of DFAA from natural populations of phytoplankton were investigated in a shallow estuary on the east coast of Jutland, Denmark. Maximum DFAA concentrations (700 to 900 nM; up to 2471 nM was measured) occurred in spring and autumn, lower concentrations (down to 200 nM) in summer and wlnter. The occurrence of DFAA was partly reflected in chlorophyll a concentrations. Serine, glutamic acid, glycine, ornithine, and alanine were the 5 dominant free amino acids. Heterotrophic assimilation of these anlino acids was determined in 3month intervals at high and low tides, and uptake rates of 0.3 to 23 pg C I-' h-' were measured. Serine was preferentially absorbed by microorganisms. Generally, higher uptake rates were found at low tides than at high tldes -apparently due to higher numbers of bacteria at low tide, but also influenced by DFAA concentration. A reduced assimilation rate occurred in winter when both temperature and bacterial densities were low. Mean turnover tunes of the 5 dominant amino acids ranged from 2 h in September to 41 h in December. Primary productivity varied from 2.1 (Mar.) to 21 kg C 1-' h -' (Sept ). Heterotrophic assimilation of the 5 amino acids (in terms of carbon) comprised from 10 to 19% (Sept. and Dec.) and 163 to 238% (Mar. and Jun.) of the primary productivity, witnessing that phytoplankton primary production could not always have been the main contributor of DFAA in Kysing Fjord. Extracellular organic carbon (EOC) released from natural phytoplankton ranged from 8.1 to 42.0% of total primary production in situ, and from 15.4 to 27.2% in the laboratory.