Date of Award

Summer 1999

Document Type


Degree Name

Doctor of Philosophy (PhD)


Ocean/Earth/Atmos Sciences

Committee Director

George T. F. Wong

Committee Member

William M. Dunstan

Committee Member

Louis A. Codispoti

Committee Member

John R. Donat


The present method for the determination of new production (NP) by measuring the uptake of added 15NO3 suffers from a number of limitations. In an attempt to improve this situation, this research examined the possibility of estimating 15NO3– uptake by measuring the activity of nitrate reductase (NRA). In addition, because it has long been suspected that the biological reduction of IO3 to I may be mediated by nitrate reductase (NR), this research investigated the ability of NR to catalyze the reduction of iodate to iodide.

An improved method for the determination of NRA was developed. The sensitivity of this method is about five times higher than methods used in previous studies. NRA and 15NO3 uptake (NU) were determined over a wide range of nitrate concentrations in the East China Sea and in the adjoining Kuroshio Current in May, 1996. In light and nitrate-replete waters ([NO3]>1 μM, %PAR > 10%, PAR: photosynthetically active radiance), NRA was linearly related to NP so that NRA may be used for estimating NP. A high ratio of NU/NRA was found in nitrate-depleted ([NO3] < 1 μM) and light-replete (%PAR > 10%) conditions. The high ratio of NU/NRA data might have been caused by an overestimation of NU due to the stimulation of the addition of 15NO3 to nitrate-deficient water. This result revealed that NRA may be a reliable index for estimating new production in oligotrophic waters. In comparing NU in the literature with our NU values in different hydrographic realms, the NU values estimated from NRA fall well with recently reported values in similar types of waters.

NRA and iodine speciation were measured in the East China Sea in May, 1996. The results suggest that the reduction of iodate to iodide in the upwelling areas is caused by the enzyme NR. In the process of iodate reduction, the depletion of iodate and the enrichment of iodide relative to the composition of the source water of a surface water mass represent an integration of NRA through the residence time of the water mass.

A method for estimating the reduction of iodate to iodide by NR by using 125IO3 was developed. The reduction of IO3 to I by NR was observed in the cultures of S. costatum and in natural phytoplankton assemblages. The rates were 0.008 to 0.019 n mol I μg chl a–1 h–1 in natural samples. The iodate reduction rate was linearly related to NRA, suggesting that iodate reduction may be coupled to nitrate reduction.