22 THE ROYAL SOCIETY OF CANADA 



Denitrification 



The term "denitrification" is used by Drew to cover both the 

 partial reduction of nitrates to nitrites and ammonia and their 

 complete reduction to nitrogen gas. The same use of one term with 

 two meanings occurs commonly in discussions of the same processes 

 under the influence of soil bacteria and even in connection with the 

 activities of pathological bacteria in culture fluids. It would simplify 

 matters very considerably if the production of nitrites or ammonia 

 were described as "partial reduction" and the term "denitrification" 

 were reserved for the reaction resulting in the complete elimination of 

 nitrogen. I shall use the terms in these respective senses in what 

 follows. 



The two processes are clearly of very different significance in 

 their application to the question of the supply of available nitrogen 

 in the sea, for, if the nitrates present are only partially reduced, the 

 compounds formed are still available for plant assimilation (whether 

 or not they have to be reoxidized before being suitable for that purpose) 

 whereas if denitrification takes place the nitrogen is completely lost 

 to the cycle of marine life. 



The power of bringing about partial reduction of nitrates is 

 very generally possessed by bacteria found on land; that of causing 

 denitrification is comparatively uncommon. It is probable that this is 

 also the case in the sea and that sea-water from at, or near, the surface 

 will be generally found to possess the power of inducing partial re- 

 duction, whilst that of causing denitrification occurs less commonly. 

 It is unlikely that the temperature at which growth takes place will 

 affect the nature of the end product, other conditions being equal. 

 Bacteria which can only partially reduce at one temperature are not 

 likely to be able to denitrify at another, nor, according to temperature, 

 are denitrifying species likely to sometimes carry the reaction only as 

 far as nitrite or ammonia. The rate of reaction will, of course, be 

 affected by the temperature, but, provided growth can take place and 

 given sufficient time, the end product will probably depend in the first 

 place on the specific nature of the bacteria present and, in the second 

 place, on the composition of the culture medium. 



Looking at Drew's results from this point of view it is apparent 

 that he found very little evidence of denitrifying species of bacteria 

 in temperate seas. He records it in fact only in the case of samples 

 drawn from the English Channel in 1909 and here "uniformly consistent 

 results were not obtained, as in some of the cultures complete denitri- 

 fication was never obtained even after several months." {loc. cit. p. 24). 



