Shipboard cultures of natural seawater enriched with nitrate, ammonium, or urea were used to compare different techniques for measuring nitrogen assimilation by phytoplankton and to measure the rates of uptake of these nutrients when more than one was present. Increases in particulate nitrogen, decreases of nitrogenous nutrients, and the uptake by phytoplankton (using the 15N‐isotope technique) balanced rather well and for the nitrate‐enriched culture were in good agreement with the estimate of nitrogen assimilation from nitrite reductase activity. Glutamic dehydrogenase activity was present with both NADH and NADPH in all three cultures; either singly or as a sum it agreed poorly with ammonium uptake estimated by the 15N‐isotope technique. The presence of ammonium almost totally suppressed the uptake of nitrate and urea over a wide range of concentrations. The suppression of nitrate uptake in the presence of urea was nearly the same as the suppression of urea uptake in the presence of nitrate. Comparisons between the isotopic and enzymatic techniques were also made in seawater samples. Nitrite reductase activity was usually much greater than the rate of nitrate uptake estimated by the 15N‐isotope technique. Comparisons with chlorophyll a and ATP analysis indicate that chlorophyll‐containing cells were primarily responsible for the nitrite reductase activity. The rate of pyridine nucleotide oxidation with both NADII and NADPII was more consistent with the ATP than with the chlorophyll a content of seawater samples, suggesting that this enzymatic activity was not restricted to the phytoplankton.
No takes yet. Share an insight, caveat, or question.
McCarthy et al. (1972) studied this question.