The marine phytoflagellate Isochrysis galbana Parke was followed through two consecutive cycles of ammonium-limited growth. There was a significant (48 h) lag in cell division following the resupply of ammonium but no discernible lag in biomass growth, which led to a transient increase in the mean biomass per cell. Cell carbon and Coulter volume were linearly correlated. Net chlorophyll synthesis was closely linked to the absorption of extracellular N, while carotenoid synthesis continued until C fixation ceased. Photosynthetic activity (C fixation per unit chlorophyll) varied significantly with the cellular N:C ratio. A rapid rise in the ratio of glutamine to glutamate (0.1-6) during the first 24 h after nitrogen supply indicated a temporary imbalance in the flux of C and N into biomass synthesis. Intracellular ammonium accounted for less than 4% of total cell N, while free intracellular amino acids accounted for 3-5%. Nevertheless, cell numbers were able to double after the medium became depleted of ammonium, presumably by recycling and redistributing high molecular weight intracellular N.
Keywords: C-N metabolism; Isochrysis galbana; growth dynamics; photosynthetic activity; phytoplankton; pigments.