The Assimilation of Nitrate-nitrogen by Nitrogen-starved Cells of Platymonas (Tetraselmis) striata (Prasinophyceae)

Nitrate refeeding of nitrogen-starved cells of Platymonas striataresulted in approximately a doubling of average cellular nitrogenwithin 24 h. All the nitrate-nitrogen removed from the culturemedium could be accounted for as non-nitrate nitrogen withincells. Thus no significantly sized nitrate pool existed in Platymonasstriata and no assimilated nitrogen was lost from the cellsto the medium over the 48 h period studied. The slight fallin average cellular nitrogen which occurred from 24 to 36 hcould be attributed to cell division. The majority (70–80per cent) of the assimilated nitrate was recovered in the trichloroaceticacid (TCA)-insoluble fraction. There was some increase in thepercentage of nitrogen found in the TCA-soluble fraction duringthe period of most rapid nitrate assimilation (0–24 h).This presumably reflects an inability of the cells to assimilatelow-molecular-weight metabolic intermediates into macromoleculesat the same rate at which they were being formed. The majorityof the TCA-soluble fraction could be accounted for in termsof amino acids, purine and pyrimidine bases and ammonia. Cellswith adequate nitrogen nutrition seemed to maintain amino acidand purine + pyrimidine base nitrogen pools of about 0.8–0.9and 0.3–0.4 pg per average cell respectively. Algal amino acids, algal purine and pyrimidine bases, algal ammonia