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Aquatic Microbial Ecology

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AME 63:89-100 (2011)  -  DOI:

Dissolved organic nitrogen: an important source of nitrogen for the microphytobenthos in sandy sediment

Kristina Sundbäck1,*, Elin Lindehoff2,3, Edna Granéli2

1Department of Marine Ecology, University of Gothenburg, 405 30 Gothenburg, Sweden
2Linnaeus University, Marine Ecology Department, 391 82 Kalmar, Sweden
3Present address: Umeå Marine Science Centre, 910 20 Hörnefors, Sweden

ABSTRACT: To test the hypothesis that dissolved organic nitrogen (DON) is important for sustaining primary production by the microphytobenthos (MPB) in nitrogen-limited conditions, we measured the uptake of 15N-labelled urea, the amino acids glycine (GLY) and glutamic acid (GLU), and nitrate and ammonium under simulated in situ light and temperature conditions. Microphytobenthic primary production and chlorophyll a (chl a) were also measured. The MPB was dominated by diatoms attached to sand grains, with cyanobacteria making up ~30% of the biomass. Activities of the hydrolytic ectoenzymes leucine aminopeptidase (AMA), alkaline phosphatase (APA) and β-glucosidase (GLA) in filter-fractionated sediment showed that the microbenthic community was strongly deficient in nitrogen (N), with the bacterial fraction (<1 µm) also limited in phosphorus. Uptake of DON (urea + GLU + GLY) accounted for ~50 to 65% of the uptake of 15N-labelled substrates, with a higher proportion of DON uptake at low substrate concentrations (≤2 µM). Except for nitrate, the kinetics fitted a linear model. The calculated relative preference index (RPI), based on pore water concentrations, suggested that the order of preference of the microbenthic community was NH4+ > urea > GLU > NO3 > GLY. Using a prokaryotic inhibitor (chloramphenicol), and theoretical calculations of algal uptake based on C:chl a ratios, it was estimated that the ‘algal’ uptake of nitrogen accounted for ~55 to 90% of DON uptake. Uptake rates were, however, estimated to cover only 26 to 50% of the nitrogen demand of the MPB, suggesting that pore water concentrations of nitrogen were not sufficient to meet the microalgal demand in early summer and that, in sandy sediments of microtidal waters, the MPB may often be severely limited in nitrogen.

KEY WORDS: Microphytobenthos · Nitrogen · DON · Urea · Amino acids · Ectoenzymes

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Cite this article as: Sundbäck K, Lindehoff E, Granéli E (2011) Dissolved organic nitrogen: an important source of nitrogen for the microphytobenthos in sandy sediment. Aquat Microb Ecol 63:89-100.

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