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

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Abstract
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]ext , exhibited Michaelis-Menten type kinetics with a half saturation constant, KG(DIC) , of 81 μM. Comparative studies of the diatom Phaeodactylum tricornutum showed similar results, although the value of KG(DIC) was lower at 30 μM.

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