Laboratory and field experiments measured urea uptake and assimilation with ¹⁴C⁻ and ¹⁵N-urea and by disappearance of dissolved urea. A modified diacetyl monoxime method was developed, which accurately and precisely determined dissolved urea concentrations in seawater. In the Strait of Georgia, chlorophyll α (chl α) specific uptake rates of ammonium (NH₄⁺) and urea were greatest in stratified water; whereas, chl α specific uptake rates of nitrate (NO₃⁻) were greatest in frontal water. Ammonium and urea regeneration rates were calculated by a mass balance method and the rates were similar. Differences between measurements of particulate nitrogen, dissolved NH₄⁺, NO₃⁻ and urea, and ¹⁵N uptake were used to explain the dominant N transformations in frontal and stratified seawater. Uptake rates measured by ¹⁴C-urea were ca. 1.4 times faster than those determined by ¹⁵N-urea in the Sargasso Sea. Turnover times of urea in the surface-mixed layer were ca. 12 h. Within some seawater samples, phytoplankton utilized urea at rates which approximated the maximum rates of utilization. In a nitrate-sufficient culture of Thaiassiosira pseudonana (clone 3H) (Hustedt) Hasle and Heimdal, urea uptake rates measured by three methods disagreed; whereas, no discrepancies occurred in a nitrate-starved culture. NH₄⁺ was released from cells after urea was taken up and was later reabsorbed. A model of urea uptake and assimilation by T. pseudonana is proposed. An obligate selenium (Se) requirement for growth of T. pseudonana was demonstrated in axenic culture in artificial seawater. The addition of 10⁻⁹ M SeO₃²⁻ to culture medium was sufficient for good growth of this alga; SeO₄²⁻ was only effective at concentrations greater than 10⁻⁷ M. To elucidate the biochemical role of Se in T. pseudonana, cells were cultured in medium containing 10⁻⁹ M Na₂ ⁷⁵SeO₃. Two soluble polypeptides of 21 and 29 kD contained ⁷⁵Se. Glutathione peroxidase was detected on non-denaturing polyacrylamide gels and ⁷⁵Se co-migrated with the enzyme. It was concluded that Se is an essential element for growth of T. pseudonana due, in part, to the presence of the selenoenzyme glutathione peroxidase. / Science, Faculty of / Botany, Department of / Graduate