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10 August 2010 The influence of trophic complexity on preferential uptake of dissolved inorganic and organic nitrogen: a laboratory microcosm experiment
Melody J. Bernot, Erika C. Martin, Randall J. Bernot
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N is a critical element for all living things, including the flora and fauna in aquatic ecosystems. However, N exists in many forms, and some organisms might preferentially use one compound over another. We used 15N tracers in laboratory microcosm experiments to measure preferential uptake of 4 different N compounds (NH4-N, NO3-N, glycine, methionine) by filamentous green algae (Cladophora glomerata; autotrophs), microbes in stream sediment (heterotrophs), and freshwater snails (Physa acuta; consumers) in response to varying trophic complexity. Seven trophic complexity treatments were applied: 3 types of single-organism treatments (filamentous algae, sediment microbes, or snails), three types of 2-organism treatments (filamentous algae sediment microbes, sediment microbes snails, filamentous algae snails), and 1 treatment consisting of all organisms (filamentous algae sediment microbes snails). We combined trophic complexity treatments (n  =  7) with 1 of 5 15N tracers: control (no 15N), 15NH4, 15NO3, 15N-glycine, and 15N-methionine (enriched to ~3000‰). Each trophic complexity treatment was replicated 4 times for each 15N tracer treatment to yield a total of 140 microcosms. We incubated microcosms for 5 d after which organisms were rinsed, dried, ground, and analyzed for 15N content. Overall, uptake of 15NH4 was greater than uptake of any other N form for all organisms. However, trophic complexity changed the relative uptake rates of different N forms. Cladophora preferentially used 15NH4 when alone, but when in the presence of heterotrophic competitors or consumers, Cladophora 15NO3 uptake doubled. Cladophora uptake of 15N-glycine and 15N-methionine decreased by ½ in the presence of other organisms. In contrast, when sediment microbes were exposed to Cladophora or Physa, 15NH4 and 15N-methionine uptake rates increased. The N form incorporated by Physa was dependent on the food source present (Cladophora vs sediment microbes). The highest N uptake rates for all N forms were associated with the presence of Cladophora. These data indicate that N uptake is a plastic trait, and organisms can alter use of N forms in response to the presence of competitors or consumers.

Melody J. Bernot, Erika C. Martin, and Randall J. Bernot "The influence of trophic complexity on preferential uptake of dissolved inorganic and organic nitrogen: a laboratory microcosm experiment," Journal of the North American Benthological Society 29(4), 1199-1211, (10 August 2010).
Received: 15 October 2009; Accepted: 1 June 2010; Published: 10 August 2010

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