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Allen Milligan


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Allen Milligan | PhD Candidate

Allen defended his Ph.D. thesis in December 1998. He is now conducting postdoctoral work with François Morel and may be contacted at the following address:

Geosciences Department
Guyot Hall
Princeton University
Princeton, NJ 08544

Office Phone: (609) 258-2612
Office Fax: (609) 258-5242
email: allenm@Princeton.EDU

PhD Thesis

The Effect of Iron Limitation on Nitrogen Metabolism of Marine Phytoplankton

Iron limitation is believed to be the ultimate controlling nutrient of primary production in the high nutrient low chlorophyll (HNLC) areas of the North Pacific. It has been hypothesized that iron limitation affects the rate of production by reducing the activity of the nitrate assimilation system in large phytoplankton while small phytoplankton utilize ammonium and are not iron-limited. Some support for this hypothesis has been collected from open ocean sites, but no direct physiological evidence has been collected for phytoplankton grown under defined conditions.

The two enzymes responsible for reduction of nitrate to ammonium, nitrate reductase and nitrite reductase contain iron in their prosthetic groups. Nitrate reductase (NR) contains one atom of iron and nitrite reductase (NiR) contains five atoms of iron and has a requirement for ferredoxin (an iron-containing electron donor).

Despite the higher requirement of NiR for iron, little work has been done on this enzyme in relation to iron limitation. In this work it is hypothesized that under iron limitation, nitrate reduction is limited by nitrite reductase. The activities of both nitrate and nitrite reductases have been measured in cultures of Thalassiosira weissflogii grown under iron replete and iron limited conditions.

Under iron replete conditions NR activity was rate limiting to nitrate assimilation and was roughly two orders of magnitude lower than NiR (Fig. 1). Under iron limiting conditions NR activity decreased by 20% and NiR activity decreased by two orders of magnitude with a concomitant increase in the excretion rate of nitrite (Fig. 2), indicating that NiR became rate limiting to nitrogen assimilation.

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