Nitrate reductase is known to be the enzyme regulating the
reduction of nitrate to nitrite in plants. This reduction is the first
and rate-limiting step in the transformation of inorganic nitrate to
a cellular nitrogen form. Since this reduction process is essential
to phytoplankton growth when nitrate is the only available nitrogen
source, the species with the highest capability to reduce nitrate,
or the highest nitrate reductase activity, should be the species most
favored to dominate a phytoplankton population under nitrate-limiting
conditions. To study this hypothesis, the nitrate reductase activities
of two species were studied; these species are co-occurring yet dominate
under different conditions. Thalassiosira nordenskioldii is the
dominant species in Auke Bay, Alaska in early spring when nitrate
levels are decreasing from 15 μM to 1 μM, and the temperature is
about 5 C. Skeletonema costatum is dominant in mid-summer when
nitrate levels are below 1 μM and the temperature is above 12 C.
The results show that at 15 C, Skeletonema has a higher enzyme
activity at all nitrate levels than does Thalassiosira, which is consistent
with the hypothesis. In addition, Thalassiosira shows a higher
enzyme activity at 10 C, nearer its optimal temperature for growth,
than at 15 C, suggesting that temperature affects species succession
through its influence on enzyme activity. The applicability of
Michaelis-Menten kinetics to this reduction reaction, based on the
enzyme activities measured for these two species, is doubtful but
inconclusive. The results also have implications for such concepts
as nitrate-limited growth and the Redfield model relating nutrient,
O₂ and CO₂ changes in the ocean. / Graduation date: 1971
| Identifer | oai:union.ndltd.org:ORGSU/oai:ir.library.oregonstate.edu:1957/29288 |
| Date | 16 November 1970 |
| Creators | Roelofs, Nancy Ann |
| Contributors | Park, P. Kilho |
| Source Sets | Oregon State University |
| Language | en_US |
| Detected Language | English |
| Type | Thesis/Dissertation |
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