Partition nitrogen uptake in phytoplankton and bacteria using bactericidal agents and light-dependent incubations

Background and purpose: The partition of the total nitrogen uptake by phytoplankton and bacteria provides basic information on the nutrient cycling in the marine environment and on the plankton community structure. In this work, the combined utilisation of light-dependent incubations and of a treatment with a bactericidal agent was tested as a method to evaluate the nitrogen (nitrate, ammonium and urea) uptakes by phytoplankton, heterotrophic bacteria and photosynthetic bacteria. Results presented in this study are the first estimate of the partition nitrogen uptake in the Gulf of Trieste (Northern Adriatic Sea). Material and methods: Two nitrogen uptake experiments were carried out in the Gulf of Trieste, in August and October 2003, in conditions of high and low primary production, respectively. Nitrate, ammonium and urea uptakes were evaluated in plankton samples using the 15N-tracer technique. Samples were incubated at different irradiance levels and in the presence of a bactericidal agent. Analysis of inorganic nutrients, urea and dissolved organic carbon were carried out during experiments. Results and conclusions: Data showed that light is an important limiting factor in the marine environment for nitrogen uptake, which should be particularly considered in coastal turbid waters. Bacteria significantly contributed to ammonium and urea uptakes, whereas nitrate uptake by bacteria was negligible in both cases. The results of the fractionation experiments indicated that phytoplankton, heterotrophic and photosynthetic bacteria contributed to the total nitrogen uptake respectively by 64%, 7% and 29% in August and by 88%, 1% and 11% in October 2003. The applicability of this technique and the assumptions inherent in the interpretation of the experimental results are discussed.