Incorporation of nitrogen from amino acids and urea by benthic microbes: role of bacteria versus algae and coupled incorporation of carbon

Abstract

We investigated the incorporation of nitrogen from amino acids and urea by the micro- bial community in intertidal surface sediment, focusing on the relative contributions of bacteria ver- sus algae to total microbial nitrogen incorporation and the (un)coupled incorporation of carbon from these organic substrates. Dual-labeled ( 15 N and 13 C) urea and an amino acid mixture were added to surface sediment from 2 intertidal mudflats in the Scheldt Estuary (The Netherlands), and 15 N and 13 C were subsequently traced into bulk sediment, total hydrolysable amino acids (THAAs) and the bac- terial biomarker D-alanine (D-Ala) over a 24 h incubation period. All added 15 N from urea and the amino acids was incorporated into microbial biomass within 24 h, with relatively rapid incorporation of 15 N from the amino acids. The bacterial contribution to total microbial 15 N incorporation (derived from 15 N incorporation into D-Ala) was large during the first 1 to 2 h of incubation, but small to negli- gible after 24 h for both substrates, indicating that total 15 N incorporation was dominated by benthic microalgae (diatoms) that also dominated total microbial biomass in the sediment. Comparison of 15 N versus 13 C 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 +