The use of amides and other organic nitrogen sources by the phytoplankton Emiliania huxleyi

Although dissolved organic nitrogen (DON) is beginning to be seen as a potentially important nitrogen source for phytoplankton, much remains to be learned about its components and their utilization. Emiliania huxleyi, a cosmopolitan eukaryotic phytoplankton species abundant in oligotrophic oceans and during blooms in some coastal regions, was screened for use of various DON compounds. Hypoxanthine and other purines support the nickeldependent growth of most E. huxleyi strains. Acetamide and formamide but not longer chain aliphatic amides were found to be excellent nitrogen sources for growth; other phytoplankton were also found to utilize acetamide but not formamide. In E. huxleyi, small amides are transported into the cell followed by degradation to ammonia, possibly by amide-specific enzymes. The related molecules hydroxyurea and thiourea were toxic to the cells and caused an increase in fluorescence consistent with blockage of photosystern II. This fluorescence increase was inhibited by urea and acetamide, suggesting transport of hydroxyurea, thiourea, urea, and acetamide by the same or closely related transporters. Dissolved organic nitrogen (DON) is the major chemical form of N, other than N gas, in marine waters. This pool of N includes a range of compounds whose concentrations have been measured (e.g. amino acids and urea), but is mostly composed of compounds whose identities and concentrations are poorly known (Sharp 1983; Antia et al. 1991; Hopkinson et al. 1993). Because N is thought to control primary productivity in some areas of the world’s oceans, the production rates and biological availability of this pool are clearly important issues. 15N tracer experiments suggest that this pool can be rapidly produced from inorganic N additions and can be available for subsequent utilization (Bronk and Glibert 1991, 1993, 1994; Bronk et al. 1994). The study of the biological utilization of DON can take several approaches. One method is the generation and use of lSN-labeled natural DON for uptake experiments (Bronk and Glibert 1993). A more reductionist approach is the characterization of the turnover times (flux and concentration) of individual known compounds added to natural samples. This second approach is limited by the range of compounds that can be chemically analyzed in seawater or are available as labeled tracers. Part of this approach is also the characterization of the metabolic capabilities of microorganisms. It has been known for some time, for example, that photosynthetic microorganisms do not rely solely on inorganic N sources (a paradigm coming out of an agricultural model; Mills 1989), but are also able to utilize organic forms of nitrogen (reviewed in Antia et al. 1991). However, a large range of organisms and a range of nitrogenous compounds and N moieties remain to be examined as sources of N for phytoplankton and other marine microbes. Just as marine Acknowledgments We thank John Koke for help with media preparation and some biochemical assays and Jackie Collier, Sonya Dyhrman, and two anonymous reviewers for comments on the manuscripts. Funding for this research was provided by the Coastal Margins Program, Dept. of Energy DE-FG03-93ER6 1692. chemists have tested the sensitivity of a range of chemical bond types to oxidation in determining dissolved organic carbon (DOC) and DON concentrations (Hopkinson et al. 1993; Lee and Henrichs 1993), marine biologists could test the biological availability of the same range of chemical moieties. The goal of this work was to examine the abilities of a common phytoplankton, Emiliania huxleyi, to utilize a subset of the potential DON pool, in particular the amides and related corn pounds.