AME 42:175-186 (2006)  -  doi:10.3354/ame042175

ABSTRACT: Although it is established that labile fractions of dissolved organic nitrogen (DON), such as dissolved free amino acids (DFAA), can be utilized by microalgae, few studies have considered their quantitative importance for microphytobenthos (MPB). Mixed MPB communities, from 1 and 20 m water depth and treated in 2 ways (semi-natural and isolated), and axenic diatom cultures (Cylindrotheca fusiformis) were examined for the uptake of DFAA. Uptake was measured using naturally low concentrations (<1 µmol l^–1) of a mixture of ³H-labeled DFAA (³H-AA) under different light conditions and varying levels of dissolved inorganic nitrogen (DIN). Prokaryotic and eukaryotic uptake were differentiated using chloramphenicol. Biomass (chl a) and production (¹⁴C uptake) were measured as well. ³H-AA was taken up during the incubations in all experiments: 20 to 40% in the culture experiments, and 2 to 18% in the mixed MPB experiments. Of the total adsorption-corrected uptake, prokaryotic uptake accounted for 45 to 85% (0.3 to 2.2 µmol N m^–2 h^–1), while eukaryotic uptake accounted for 15 to 55% (0.3 to 0.7 µmol N m^–2 h^–1). Short-term (hours) light changes had no effect on ³H-AA uptake, and ammonium availability lowered ³H-AA uptake only marginally. Shade-adapted MPB communities (20 m) took up ³H-AA more efficiently than light-adapted communities (1 m). The estimated contribution from DFAA to the microalgal N-demand was 6 to 12 and 55 to 100% (1 and 20 m sediment, respectively). Our findings suggest that DON, specifically DFAA, may be an important source of N for MPB communities, particularly where DIN and light are limiting. Moreover, tentative estimations suggest that DFAA can provide MPB communities with a considerable portion of their total N-demand (10 to 100%).

KEY WORDS: Dissolved free amino acids · DFAA · Nitrogen · Sediment · Microphytobenthos · DON