Chapter XI — 157 — The Nitrogen Cycle 



out enrichment with organic matter, filtered sea water docs not support 

 the growth of nitrogcn-fixing bacteria. 



There are numerous accounts of the occurrence of both aerobic and 

 anaerobic nitrogen-fixing bacteria in fresh-water lakes, but whether they 

 are indigenous species which are functional in lakes or are adventitious 

 forms from the soil is another unsolved problem. 



It has been definitely established that some species of blue-green algae 

 fix atmospheric nitrogen. Pure cultures of Nostoc punctiforme and Ana- 

 baena variabilis isolated by Drewes (1928) fixed 2 to 3 mgm. of nitrogen 

 in 50 days in 250 ml. of medium. Allison et al. (1937) worked with pure 

 cultures of Nostoc muscorum which fLxed as much as 18 mgm. of nitrogen 

 per 100 ml. of medium in 85 days. In discussing the role of algae in the 

 nitrogen cycle of soil, Stokes (1940) relates that De demonstrated the 

 ability of Anabaena variabilis, A. gelatinosa, and A. naviculoides to fix 

 nitrogen. The ability of these blue-green algae from the soil to fix nitrogen 

 suggests the possible importance of algae as nitrogen fixers in the sea. 



