Chapter XI — 151 — The Nitrogen Cycle 



Urea-decomposing bacteria were found by Bavendamm (1932) to be 

 widely distributed in water and mud around the Bahama Islands. He be- 

 lieved that the activities of such bacteria promote the precipitation of 

 CaCOs: 



(lSrHa)2C0 -I- 2 H2O + CaS04 = CaCOs + (NH4)2SO« 



Bertel (1935) attributed the high />H values in water immediately over- 

 lying the ooze to ammonia produced by urea-decomposing bacteria. 

 RuBENTSCHiK (1925) found urea-decomposing bacteria in all samples 

 taken from the Odessa limans. 



Throughout the euphotic zone, which is populated by urea-excreting 

 animals, ZoBell and Feltham (1935) found from i to 10 urea-decompos- 

 ing bacteria per ml. of sea water. Surface mud, which is also populated 

 by urea-excreting animals, was found to contain from 10 to 1000 urea- 

 decomposing bacteria per gram. Some of these bacteria obtain their 

 nitrogen requirements from urea without decomposing any beyond their 

 needs and others ferment urea with the liberation of excess ammonia. 

 Bacteria in the latter category were able to liberate enough ammonia in 

 sea water enriched with urea to cause a reaction as alkaline as pH 9.7. 



Bacterial oxidation of ammonia: — Cooper (1937^) points out that 

 the oxidation of ammonia to nitrite is an exothermic reaction which is ac- 

 companied by a decrease in thermodynamic potential or free energy of 

 59,400 gram calories at 25° C: 



NH4+ + OH- -f 3/2 02(gas) = H+ -f NO2- -I- 2 H2O -f- 59,400 cal. 



This indicates that the reaction will proceed from left to right in the pres- 

 ence of an appropriate catalyst or activator. The energy of activation 

 may be provided by photic, chemical, or biological agents. 



A limited amount of photochemical oxidation of ammonia may occur 

 in the topmost few centimeters of sea water but, owing to the rapid ab- 

 sorption of ultraviolet radiations, will be of no importance below a depth 

 of one meter. Purely chemical catalysis of the reaction has not been 

 demonstrated under conditions which exist in the sea. Therefore, it is 

 generally believed that bacteria are primarily responsible for the oxidation 

 of ammonia to nitrite in the sea. 



Autotrophic organisms responsible for the oxidation of ammonia to 

 nitrite, namely the Nitrosomonas, have been found in the sea by many 

 investigators. However, failure to find them universally distributed in 

 the sea, and failure to find specific marine Nitrosomonas species, leaves a 

 large gap in our knowledge of the nitrogen cycle. Nitrification appears to 

 be a much more common phenomenon in the sea than can be accounted 

 for by the nitrifying bacteria which have been demonstrated. This dis- 

 crepancy may be due to the inadequacy of the experimental methods 

 which have been employed to demonstrate nitrifying bacteria. 



Most investigators have employed the conventional media employed 

 by soil microbiologists to demonstrate nitrifiers in the soil. Essentially, 

 such media consist of physiologically balanced mineral salts solutions en- 

 riched with an ammonium salt and buffered with CaCOa or MgCOa. Sea 

 water serves as the mineral solution for marine nitrifiers. 



Vernon (1898) demonstrated the presence of nitrifying bacteria in the 

 Gulf of Naples. Brandt (1902) found them in two out of three samples 

 of mud from the Kiel inlet, but he was unable to demonstrate such bac- 



