Chapter XI — 155 — The Nitrogen Cycle 



tury, Brandt (1929) finally retracted part of it by attributing the lack of 

 nitrate in surface tropical waters to thermal stratification. However, he 

 maintained that denitrifying bacteria destroy the surplus nitrogen com- 

 pounds in the ocean, thereby providing the balance in Nature. From 

 their own observations Waksman ct al. (19336) concluded that, although 

 denitrifying bacteria are present in the sea, the activities of such bacteria 

 are so limited under marine conditions as to render them in most instances 

 of little importance in limiting the nitrate supply of the sea. Lack of an 

 available energ}^ source was found to be the chief limiting factor. 



The problem of bacterial denitrilication in the sea was attacked with 

 renewed vigor after Drew (191 i) credited marine denitrifiers with the 

 ability to promote the precipitation of CaCOs in tropical seas (page 100). 

 According to Bavendamm (1932), Drew's (191 2) Bacterium {Pseudo- 

 monas) calcis is closely related to Bad. bauri, Bad. granii, and Bad. 

 feiteli, denitrifiers which Parlandt (191 i) isolated from the Baltic Sea. 

 It is also related to Bad. halticum and Bad. ornatum, marine denitrifiers 

 studied by Feitel (1903), and to Bad. russelli and Bad. brandti, marine 

 denitrifiers described by Issatchenko and Rostow^zew (191 i). Other 

 marine denitrifiers named as new species include Bad. triviale, Bad. 

 re pens, and Bad. henseni isolated by Gran (1901), Bact. adinopelte and 

 Bad. lobatum isolated by Baur (1902), and Bad. helgolanicum isolated by 

 Nadson (1903). Unfortunately none of these denitrifiers is completely 

 described in the available literature. 



MoLiscH (1925) described Pseudomonas calciprecipitans and Kalan- 

 tarian and Petrossian (1932) described Flavobaderium sewanense, both 

 of which were believed to be marine denitrifiers capable of promoting the 

 precipitation of CaCOs. Lloyd (193 16) discussed the mechanism of 

 denitrification and described B. {Vibrio) costatus which she isolated from 

 the sea. 



About half of the bacteria found in the sea are capable of reducing 

 nitrate to nitrite in sea water enriched with organic matter and nitrate. 

 Thirty-four of the sixty species of marine bacteria studied by ZoBell and 

 Upham (1944) reduced nitrate to nitrite in enrichment media. Most of 

 their Bacillus species, Achromobader stenohalis, Pseudomonas hypolhermis, 

 Ps. aestumarina, Ps. marinopersica, Serratia pelagia, Vibrio hyphalus, and 

 V. marinagilis are examples of extremely active nitrate reducers. Pseu- 

 domonas azotogena and Ps. perfedojnarinus are the only organisms among 

 the sixty species described by ZoBell and Upham which reduced nitrate 

 to free nitrogen. 



Although denitrifiers are widely distributed in the sea, particularly in 

 bottom deposits, it is estimated from the aforementioned pure-culture 

 studies, as well as from ecological surveys, that fewer than five per cent 

 of the bacterial species in the sea are endowed with the ability to liberate 

 free nitrogen from nitrate or nitrite in the presence of an abundance of 

 organic matter. Except in certain highly localized environments, there 

 appears to be insufficient organic matter in the sea to provide for the 

 activity of denitrifiers. 



Gran (1901) classified marine bacteria in four categories according to 

 their action on nitrate and nitrite iX^") Both nitrate and nitrite reduced to 

 nitrogen, (2) Nitrate reduced to nitrite and ammonia, (j) Nitrite but not 

 nitrate reduced, and {4) Neither nitrate nor nitrite reduced. Several 

 other types of nitrate- and nitrite-reducing bacteria are now recognized. 

 Throughout the foregoing discussion the term denitrification is applied 

 exclusively to the liberation of free nitrogen. 



