Autotrophic Nitrification in the Ocean 83 



ments, since nitrifying bacteria can be cultured from both. In 

 deep waters the distribution of nitrate and cultural studies indi- 

 cate that the most probable site of nitrification is witliin the water 

 column rather than the sediments as suggested by Carey ( 2 ) . 



The specific organisms responsible for nitrification in the 

 oceans have been a mystery. The nitrifying bacteria cultured by 

 Thomsen (6) were found less than 1200 m from shore and 

 moi-phologically and physiologically resembled terrestrial species 

 of Nitrosomonas and Nitrohacter. Thomsen was probably cultur- 

 ing terrestrial bacteria which had eitlier adapted to the marine 

 environment or had been recently transported into the ocean. 



Since the organisms cultured from open ocean water do not 

 resemble terrestrial nitrifying bacteria, it appears that a distinct 

 population of oceanic nitrifying microorganisms does exist. Until 

 these organisms are isolated in pure culture it is impossible to 

 determine which are responsible for nitrification. 



My observations of simultaneous increase of nitrites and 

 nitrates in sea water enriched with ammonia differ from the 

 observations of Von Brand, Rakestraw and Renn (7), who found 

 that nitrates did not accumulate until nitrites declined. My ex- 

 periments differ from theirs since I periodically added ammonia, 

 whereas they followed the changes which occuiTed with time 

 after initial enrichment with killed phytoplankton. In nature both 

 processes could occur simultaneously. 



My observations of the effect of pH on oxygen uptake by 

 Nitrosomonas europoea (Fig. 5) differ from those of Engle and 

 Alexander ( 3 ) ( Fig. 4 ) . In my experiments at very low concen- 

 trations of ammonia, oxygen consumption decreased rapidly 

 above pH 8, whereas Engle and Alexander (3) reported that 

 ammonia was oxidized at approximately the same rate from pH 

 6.8 to 9. They did observe a slight decrease in oxygen consump- 

 tion followed by an increase between pH 7.6 and 8.6. Engle re- 

 cently stated (personal communication) that they used a phos- 

 phate buffer below pH 8 and Tris above pH 8. In my experiments 

 the pH was adjusted continuously. However, 0.05 M KjHP04 

 was present in all media used. Thus the differences between my 

 observations and those of Engle and Alexander may be accounted 

 for by differences in technique. 



