Relations between carbon and nitrogen during growth of Nannochloropsis-Oculata (Droop) Hibberd under continuous illumination

Growth of the marine eustigmatophyte Nannochloropsis oculata (Droop) Hibberd was followed through two periods of batch growth from stationary phase until cessation of net C-fixation. Growth continued for two generations after exhaustion of the N-source (100 mu M NH4+), with an increase in the cell C/N mass ratio from 6 to 28. N-deprived cells had a larger mean volume (130%) than N-replete cells. Intracellular concentrations of carotenoids and Chla correlated with cell-N until exhaustion of the N-source, when Chla ml(-1) of culture declined significantly. The contribution of intracellular amino-N to cell-N was less than 3% throughout exponential and stationary phases. The major intracellular amino acids were glutamate (Glu), glutamine (Gin) and alanine; concentrations of tyrosine (on a population basis) increased steadily during N-deprivation. The intracellular concentrations of other amino acids showed fluctuations corresponding with changes in growth rate (e.g. arginine) or cell-N (e.g. glycine). The intracellular ratio of Gln/Glu reached a peak of more than 26 within 3 h of inoculation of stationary phase cells into fresh medium, before falling to around 0.6 during exponential growth, and then remained below 0.1 after NH4+ exhaustion. Of the extracellular amino acids, concentrations of alanine and serine remained relatively constant throughout, while those of glycine, glutamate and of total dissolved free amino acids increased during exponential and early post-exponential phases, before levelling off at 1.4 mu M amino-N. The behaviour of Nannochloropsis oculata is contrasted with the growth and physiology of Isochrysis galbana.