Nitrogen metabolism of aquatic organisms: I. The assimilation and formation of urea in Ochromonas malhamensis

Abstract

Although the energy metabolism of the phytoflagellate Ochromonas malhamensis has been studied extensively, little is known about its nitrogen metabolism. Chromatographic analyses and assays of enzymic activities in intact cells suggest that the pathways leading to the formation of urea in O. malhamensis cells grown in the complete nutrient medium are: (a) urate → allantoin → allantoic acid → urea; and (b) arginine → urea. O. malhamensis cells, grown in a complete nutrient medium, secrete urea and have very low urease activity. When these cells are starved for 24 hr, their urease activity increases fivefold. On refeediug, urease activity returns to the original low level. Cells grow rapidly when ammonium salts are used as their only nitrogen source. When the organism is grown in a medium with urea as its only source of nitrogen, there is a 7- to 8-day lag before the cells multiply exponentially. At the end of the logarithmic growth phase, the urease activity of cells grown in the urea assimilation medium is 44 times higher than the level of urease of cells grown in the complete nutrient medium. When cells grown in the complete medium are transferred to the urea assimilation medium for 24 hr, their urease activity increases tenfold. Since urea is a normal catabolite of the cells, the appearance of urease in O. malhamensis when grown in a starvation medium might indicate that in the absence of amino nitrogen, urea nitrogen can be reutilized. The greatly increased urease activity in the urea assimilation medium may be due to environmental conditions or to a gene-controlled enzyme induction.