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 hav
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