Graduate Thesis Or Dissertation
 

Nitrate reductase activity as a factor influencing the seasonal succession of marine phytoplankton

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  • 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.
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