Chapter 8 



Nitrogen Fixing Organisms in the Sea* 



Mary Belle Allen 



N. 



itrogen in nature goes through a series of transformations 

 from one chemical form into another, as summarized in Figure 

 1. The studies of Rakestraw and associates ( 12, 13) and of Watson 

 (Chapt. 7) have demonstrated all of the steps of the nitrogen 

 cycle, with the exception of the fixation of molecular nitrogen, in 

 marine ecosystems. Biological fixation of nitrogen in the sea has 

 been assumed by some (5, 8), but generally neglected by those 

 who have been concerned with nutrient budgets in the oceans 

 (2, 6, 4). The only replenishments of the nitrogen supply these 

 authors have envisaged have been runoff from land and non- 

 biological fixation during thunderstorms. The figures for over-all 

 nitrogen balance, however, are sufficiently uncertain that the 

 question of biological nitrogen fixation in the sea cannot be an- 

 swered pro or con by reference to them. 



This question can be divided into two parts : ( a ) do niti'Ogen 

 fixing organisms occur in the sea, and, ( b ) if so, are they actively 

 fixing nitrogen there? The first part of this question can be an- 

 swered with an unequivocal yes. Nitrogen fixing marine microbes 

 have been known for a number of years, since the sulfate reducing 

 bacteria studied by Sisler and ZoBell (10) were found to carry 

 out this reaction. However, these bacteria are strict anaerobes, 

 whereas most of the marine enxironment is aerobic. A search 

 has therefore been made for other potential biological sources 

 of combined nitrogen in the sea. Because of the low concentra- 

 tion of organic matter in sea water, attention was first focused 

 on autotrophic microorganisms, especially the blue-green algae. 

 Ecologists have known for a number of years (11) that in 

 many places the upper region of the intertidal zone is dominated 



*Supported in part by Conract Nonr-3015(00) with the Office of Naval 

 Research. 



85 



