Use of Inorganic and Organic Nitrogen by Synechococcus spp. and Diatoms on the West Florida Shelf as Measured Using Stable Isotope Probing

The marine nitrogen (N) cycle is a complex network of biological transformations in different N pools. The linkages among these different reservoirs are often poorly understood. Traditional methods for measuring N uptake rely on bulk community properties and cannot provide taxonomic information. ¹⁵N-based stable isotope probing (SIP), however, is a technique that allows detection of uptake of individual N sources by specific microorganisms. In this study we used ¹⁵N SIP methodology to assess the use of different nitrogen substrates by Synechococcus spp. and diatoms on the west Florida shelf. Seawater was incubated in the presence of ¹⁵N-labeled ammonium, nitrate, urea, glutamic acid, and a mixture of 16 amino acids. DNA was extracted and fractionated using CsCl density gradient centrifugation. Quantitative PCR was used to quantify the amounts of Synechococcus and diatom DNA as a function of density, and ¹⁵N tracer techniques were used to measure rates of N uptake by the microbial community. The ammonium, nitrate, urea, and dissolved primary amine uptake rates were 0.077, 0.065, 0.013, and 0.055 μmol N liter⁻¹ h⁻¹, respectively. SIP data indicated that diatoms and Synechococcus spp. actively incorporated N from [¹⁵N]nitrate, [¹⁵N]ammonium, and [¹⁵N]urea. Synechococcus also incorporated nitrogen from [¹⁵N]glutamate and ¹⁵N-amino acids, but no evidence indicating uptake of labeled amino acids by diatoms was detected. These data suggest that N flow in communities containing Synechococcus spp. and diatoms has more plasticity than the new-versus-recycled production paradigm suggests and that these phytoplankters should not be viewed strictly as recycled and new producers, respectively.