Chapter 7 



Autotrophic Nitrification in 

 the Ocean''' 



Stanley W. Watson 



Ni 



itrification is one of the most important but least understood 

 phases of the nitrogen cycle in the oceans. Our knowledge is 

 limited to shallow coastal waters, where the existence of the 

 process has been demonstrated, as reviewed by Carey (2) and 

 ZoBell (8). Although nitrification has been worked on by more 

 than 12 investigators, only Thomsen (6) has succeeded in cul- 

 turing nitrifying bacteria from a marine habitat, but he could 

 find these only from areas between the shore and 1,200 m 

 from land. In open ocean waters verification of nitrification by 

 indigenous populations is entirely lacking, altliough we know 

 that nitrate must be regenerated because it is constantly used in 

 the euphotic zone as a source of nitrogen for marine plants. 



Veiy little is known about the site of nitrification in the 

 oceans. Carey (2), after finding active nitrifying populations in 

 sediments but not in surface waters, concluded that nitrification 

 occurs primarily in the bottom sediments and that the nitrates 

 are returned to the photic zone by mixing. Her hypothesis may 

 apply to shallow coastal waters, but it does not appear realistic 

 for oceanic waters. If such a hypothesis is applied to the open 

 sea, the maximum nitrate concentrations would be expected just 

 above the ocean floor. In reality, the maximum concentration 

 exists at mid-depths between 250 and 800 m (5). 



It would also seem probable that organisms similar to those 

 responsible for nitrification on land would play a role in the 



* Contribution No. 1200 from the Woods Hole Oceanographic Institution. This 

 investigation was supported in part by research grants 1.3397 and 8339 from 

 the National Science Foundation and Contract AT( 30-1 ) -1918 under the 

 U. S. Atomic Energy Commission. 



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