Abstract
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. (15)N-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 (15)N 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 (15)N-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 (15)N 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 micromol N liter(-1) h(-1), respectively. SIP data indicated that diatoms and Synechococcus spp. actively incorporated N from [(15)N]nitrate, [(15)N]ammonium, and [(15)N]urea. Synechococcus also incorporated nitrogen from [(15)N]glutamate and (15)N-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.