346
Chemical Oceanography, 4th Edition
preferred. The uptake shows a direct hyperbolic behavior that increases to a maximum
rate at a given nutrient concentration. When nitrate is below 0.7 μM, nitrogen- deficient
cells are produced before cell division stops. These cells can take up NH 3 and NO 3
– , but not
NO 2
– , in the dark. Some phytoplankton can utilize amino acids, as can some diatoms with
the help of bacteria. In polluted waters, a significant amount of NO 3
– can be obtained from
organic nitrogen. Urea is also used in restricted coastal and estuarine areas.
The conversion of NO 3
– to amino acids requires the formation of NH 3 :
NO 3
– + 2H + + 2e → NO 2
– + H 2 O
(8.26)
2NO 2
– + 4H + + 4e → N 2 O 2
2− + 2H 2 O
(8.27)
N 2 O 2
2− + 2H + + 2e → NH 3 + H 2 O
(8.28)
where N 2 O 2
2− is hyponitrite, and NH 2 OH is hydroxylamine. The first step is catalyzed by
coenzyme II. The NH 3 is converted to glutamic acid by
HOOC – CO – (CH 2 ) + NH 3 + 2NADPH
→ HOOC – CH(NH) 2 CH 2 CH 2 COOH + 2NADP + H 2 O
(8.29)
Combustion
Atmospheric Nitrogen
(N 2 , NO, NO 2 , N 2 O)
Physical Fixation
Biological Fixation
Upwelling
Nitrate
Deposits
NO 2
NO 3
Assimilation
Organic
Nitrogen
(plants)
Ammonia
Decomposition
Organic Nitrogen
(microorganism)
Organic Nitrogen
(animals)
Loss of NO 3
to Deep Water
Loss of Nitrogen
to Sediments
Figure 8.6
The nitrogen cycle in ocean waters.