ZoBell — 154 — Marine Microbiology 



Suitably equipped bacteria may oxidize nitrite to nitrate as a direct 

 source of energy. Nitrohacter species in the soil are endowed with this 

 ability. Nitrohacter-Y^ke. organisms have been demonstrated in the sea. 



Nitrate-forming bacteria are somewhat more difficult to demonstrate 

 than nitrite-forming bacteria (Waksman et al., 19336). Therefore it is 

 not surprising that nitrate formers have been demonstrated in the sea 

 only infrequently and that the status of our knowledge of them is very 

 fragmentary. Most of the investigators, whose observations on nitri- 

 fiers are summarized in the preceding section, looked for nitrate formers 

 as well as nitrite formers, usually with little success. Nitrate-forming 

 organisms have been found near shore and in shallow bottom deposits, 

 but only rarely in the open sea. 



Carey (1938) reports that nitrate production was slower than nitrite 

 production in special media inoculated with various marine materials. In 

 most deep-sea mud cores and plankton tows, no nitrate formers were 

 found, although nitrite formers were usually present. Nitrate formation 

 was observed to follow the formation of ammonia and nitrite in the exper- 

 iments on the decomposition of marine plankton of von Brand et al. 

 (1937), as illustrated by Figure 11 on page 153. Attempts to isolate the 

 organisms responsible for nitrate formation were unsuccessful. 



Although, in the light of circumstantial evidence, it is tacitly assumed 

 by many oceanographers that nitrate is formed by bacteria at the bottom 

 of the sea, and thence carried into the photosynthetic zone, there is no 

 conclusive evidence bearing on either the mode or place of formation of 

 nitrate in the sea. The problem is complicated by the dynamic nature of 

 the marine environment. In the photosynthetic zone, nitrate may be 

 utilized by plants as fast as it is produced, thereby precluding its accumu- 

 lation. Elsewhere in the sea, nitrate may be reduced by bacteria. 



Reduction of nitrate and nitrite : — The reduction of nitrate or nitrite 

 is an endothermic reaction and therefore is thermodynamically possible 

 only when the required energy is forthcoming from an accompanying 

 exothermic reaction. The sea abounds in bacteria which can obtain the 

 necessary energy for the reduction of nitrate or nitrite from the oxidation 

 of organic matter. However, inasmuch as there is relatively little readily 

 available organic matter in the sea, the extent to which nitrate or nitrite 

 may be reduced is highly problematical. Denitrifiers resembhng Tliio- 

 hacillus denitrificans, which obtains energy for the reduction of nitrate by 

 oxidizing sulfur, have been found in coastal waters by various workers, 

 but evidence of their activity in marine environments is lacking. 



Most of the pioneer workers including Russell (1893), Fischer 

 (1894a), Vernon (1898), Gazert (19066), and Graf (1909) demonstrated 

 the presence in the sea of bacteria capable of reducing nitrate. Impetus 

 was given to investigations by the hypothesis, advanced by Brandt 

 (1899), that the activity of denitrifying bacteria, or those which reduce 

 nitrate to free nitrogen, destroy nitrate in tropical seas and hence prevent 

 maximum phytoplankton growth. After finding that only near shore, 

 where the organic and nitrate content of the water may be high, is there 

 evidence of denitrification in the sea. Gran (1901) concluded that 

 Brandt's hypothesis is untenable. The extensive literature on this con- 

 troversial subject has been reviewed by Waksman et al. (19336) and 

 Benecke (1933). 



Although he continued to defend his hypothesis for a quarter of a cen- 



