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

The growth of the marine flagellate Isochrysis galbana was followed in batch cultures at four concentrations of dissolved inorganic carbon (DIC), from C- and N-replete lag phase into C- and/or N-deplete stationary phase. Organic buffers were omitted from the growth medium, and culture pH was maintained 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 cell 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 the cell Chl a quota. The relationship between carbon-specific growth rate (C-μ), and the concentration of extracellular DIC, [DIC] , exhibited Michaelis-Menten type kinetics with a half saturation constant, K , of 81 μM. Comparative studies of the diatom Phaeodactylum tricornutum showed similar results, although the value of K was lower at 30 μM.