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Three diatoms from the Sargasso Sea and two diatoms and a haptophyte flagellate from an inshore area near Woods IIolc exhibited similar growth rates on urea, nitrate, and ammonia. A coccolithophore Emiliana (Coccolithus) huxleyi from the Sargasso Sea did not grow on urea. The half-saturation constant (K) for urea determined for one inshore diatom, Stephanopyxis costata ( Skeletonema costatum), was 8.5 pg-atom urea-N liter-l. At the urea concentrations of its habitats, the calculated division rate for this species, growing on urea as the sole nitrogen source, was 2.2 days, similar to that of diatoms growing in inshore habitats. It appears, from data on natural urea concentrations and urea uptake rates by S. costata, that urea can be ,a significant N source for at least one common inshore phytoplankter. According to growth rate studies, at least three inshore and three offshore marine phytoplankton can grow as rapidly on urea as on nitrate or ammonia |
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