AME 48:35-46 (2007)  -  doi:10.3354/ame048035

ABSTRACT: We investigated the incorporation of nitrogen from amino acids and urea by the microbial community in intertidal surface sediment, focusing on the relative contributions of bacteria versus algae to total microbial nitrogen incorporation and the (un)coupled incorporation of carbon from these organic substrates. Dual-labeled (¹⁵N and ¹³C) urea and an amino acid mixture were added to surface sediment from 2 intertidal mudflats in the Scheldt Estuary (The Netherlands), and ¹⁵N and ¹³C were subsequently traced into bulk sediment, total hydrolysable amino acids (THAAs) and the bacterial biomarker d-alanine (d-Ala) over a 24 h incubation period. All added ¹⁵N from urea and the amino acids was incorporated into microbial biomass within 24 h, with relatively rapid incorporation of ¹⁵N from the amino acids. The bacterial contribution to total microbial ¹⁵N incorporation (derived from ¹⁵N incorporation into d-Ala) was large during the first 1 to 2 h of incubation, but small to negligible after 24 h for both substrates, indicating that total ¹⁵N incorporation was dominated by benthic microalgae (diatoms) that also dominated total microbial biomass in the sediment. Comparison of ¹⁵N versus ¹³C incorporation into total microbial biomass (THAAs) after 24 h showed strong preferential incorporation of urea-N over urea-C. Incorporation of nitrogen and carbon from the amino acids was partially uncoupled, indicating that a large fraction (≥50%) of amino acid-N was taken up as NH₄⁺ resulting from extracellular amino acid oxidation.

KEY WORDS: Nitrogen uptake · Urea · Amino acids · Bacteria · Benthic microalgae · Sediment · Hydrolysable amino acids · D-alanine