Studies of Carbon and Nitrogen Metabolism by Three Marine Phytoplankton Species in Nitrate‐limited Continuous Culture1,2

ABSTRACT Characteristics of carbon production, excretion and dark respiration, and nitrate uptake kinetics were studied using continuous culture techniques for Thalassiosira allenii Takano , Monorhrysis lutheri Droop and Dunaliella tertiolccta Butcher. Fur T. allenii. the ratio of dark C loss to daytime net C production varied between 0.1 and 0.2 over a growth rate range from ca. 0.005 to 0.06 h ‐1 . For M. lutheri and D. tertiolecta. this same ratio varied belween 0.2 and 0.3 between growth rates of ca. 0.005 and 0.025 h ‐1 , but declined at higher growth rates when the dark nitrate uptake capacity of the cells was exceeded by the pumping rate. Carbon excretion rates averaged less than 1.5% of daytime net C production rates. Productivity indices showed little correlation with growth rate, due to the significant poisitive correlation between chl a :C ratios and growth rate. Chlorophyll a: C ratios for T. allenii were less than 0.01 al growth rates less than 0.03 h ‐1 , and appoached zero at zero growth rate. Dark nitrate maximum uptake rates for M. lutheri, D. tertiolecta and T. allenii averaged 23, 64 and 120%, respectively, of light nitrate maximum uptake rates. Excretion of nitrite was observed during most nitrate uptake experiments. This excretion reduced net uptake of nitrate spikes in the dark for M. lutheri and D. tertiolecta by 79 and 23%, respectively .