CARBON AND NITROGEN-SOURCES OF PLANKTONIC BACTERIA IN LAKE CONSTANCE STUDIED BY THE COMPOSITION AND ISOTOPE-DILUTION OF INTRACELLULAR AMINO-ACIDS

We investigated the physiological response of planktonic bacteria to varying C and N sources in mesotrophic Lake Constance (Bodensee) by studying the amino acid composition and the isotope dilution (ID) of [H-3] amino acids in the intracellular pool. Bacteria predominantly utilized amino acids for growth from the phytoplankton spring bloom in May until August. During this period the amino acid composition of the pool was rather similar to the composition of dissolved free amino acids (DFAA) in the water. Serine always was the dominant DFAA and also dominated the intracellular pool from May to August. From January until April, however, the intracellular pool was dominated by glutamate, suggesting that during this period the bacterioplankton utilized NH4+ as a major source of N. During the spring bloom and in June, the ID of the most abundant amino acids was 1 (no dilution) to 4. Glutamate, however, exhibited a much higher ID (10-47), suggesting that NH4+ was utilized by the bacterioplankton during these months. In August, however, ID of all amino acids was high and was not reduced by increasing the concentration of DFAA, indicating that the bacteria in this period predominantly utilized dissolved combined amino acids. The ID of leucine when added at saturating concentration (30 nM was 1.5-3. Incorporation rates of [H-3]DFAA as a fraction of bacterial production were consistent with the results of ID measurements.