Growth Rate Relationships to Physiological Indices of Nutrient Status in Marine Diatoms

The growth of two species of marine diatom, Thalassiosira weissflogii (Grunow) and Thalassiosira pseudonana (Hustedt), was followed in batch cultures at four concentrations of dissolved inorganic carbon from N‐ and C‐replete lag phase into N‐ and/or C‐deplete stationary phase. Results describe the relationship between carbon‐specific growth rate (μC) and chl a:carbon (chl a:C) and glutamine:glutamate (gln:glu) ratios with changes in the cells' nutritional status (N:C), during the utilization of either NO3− or NH4+. The use of the gln:glu ratio as an index of N:C requires further clarification. For both species and N sources, N stress resulted in a decrease in μC, chl a:C, and N:C relative to μCmax values, whereas C stress resulted in a decrease in μC and an increase in chl a:C and N:C relative to μCmax values. Both species attained a chl a:C ratio of approximately 15 μg·g−1 at μCmax using either N source. However, this value was not necessarily an indicator of maximal growth rate. NC colimitation resulted in decreased μC to values less than 20% of μCmax with only minor changes in chl a:C and N:C relative to μCmax values. Chl a:C results suggest a similarity between the light stress and C stress responses of marine diatoms. The potential for C stress in the marine environment needs to be addressed.