Utilization of dissolved inorganic carbon (DIC) and the response of the marine flagellate Isochrysis galbana to carbon nitrogen stress

The growth of the marine flagellate Isochrysis galbana was followed in batc h cultures at four concentrations of dissolved inorganic carbon (DIC), from C- and N-replete lag phase into C- and/or N-deplete stationary phase. Orga nic buffers were omitted from the growth medium, and culture pH was maintai ned at 8.30+/-0.05 by the addition of acid or alkali. The responses of the flagellate to N stress included an increase in the C:N ratio, and decreases in the ratios of glutamine (Gln): glutamate (Glu) and Chl a:C, and the cel l Chl a quota. Conversely, the responses to C stress included a decrease in the C: N ratio, and increases in the ratios of Gln:Glu and Chl a:C, and th e cell Chi a quota. The relationship between carbon-specific growth rate (C -mu), and the concentration of extracellular DIC, [DIC](ext), exhibited Mic haelis-Menten type kinetics with a half saturation constant, K-G(DIC), of 8 1 mu M. Comparative studies of the diatom Phaeodactylum tricornutum showed similar results, although the value of K-G(DIC) was lower ae 30 mu M.