SECT. 1] THE INFLUENCE OF ORGANISMS ON THE COMPOSITION OF SEA-WATER 43 



CH2O represents the carbon compounds of the organic matter and NH3 the 

 nitrogen liberated by its oxidation. 



Oxidation by oxygen : 



CH 2 + 2 ^C0 2 + H 2 



NH3 + 2O2 >HN0 3 + H 2 



Denitritication : 



5CH 2 + 4HN0 3 — >5C0 2 + 2N 2 + 7H 2 



5NH 3 + 3HN0 3 ^4N 2 + 9H 2 



Sulfate reduction : 



2CH 2 + H 2 S0 4 > 2C0 2 + H 2 S + 2H 2 



No oxidation of NH3 



The order and extent to which these steps proceed depends on the free 

 energy available from the respective reactions and on the concentration of the 

 reactants. The free energy decreases in the order oxygen > nitrate > sulfate > 

 carbonate when these serve as hydrogen acceptors (McKinney and Conway, 

 1957). Consequently, oxygen should be utilized first if it is available. With the 

 exhaustion of oxygen, the oxidation of organic matter should continue by the 

 reduction of the nitrate produced while oxygen was available. Following the ex- 

 haustion of the nitrate, oxidation continues by the reduction of sulfate. The 

 free energy available from sulfate reduction is apparently insufficient to 

 oxidize NH3 and, consequently, this product should accumulate during sulfate 

 reduction. Thus the oxidation of organic matter may be expected to proceed 

 in three stages — oxidation by free oxygen, denitrification and sulfate reduction 

 — separated by the exhaustion of free oxygen and of nitrate. In nature, these 

 stages may overlap to some extent. Nitrite is produced and should be present 

 as an intermediate product both of the formation of NO3 and of its reduction. 



The limiting concentrations of oxygen below which denitrification occurs in 

 sea-water are not known. In fresh water nitrate reduction takes place only 

 when the oxygen concentration falls below a few tenths of a milligram per liter, 

 depending on the nitrate concentration (Langley, 1958). Relatively large 

 concentrations of nitrite (up to 2.5 mg atoms N/m 3 ) occur below the thermo- 

 cline in parts of the Pacific where the oxygen is depleted, but only if the oxygen 

 concentration is less than 1.0 ml/1. The presence of nitrite in these places is 

 attributed to the reduction of nitrates as the initial step in denitrification 

 (Brandhorst, 1959). 



Free nitrogen appears to be the final product of denitrification. In the Black 

 Sea, the concentrations of gaseous nitrogen increase with depth and super- 

 saturation occurs in the deeper water (Kriss, 1949). In the anoxic water of the 

 Cariaco trench and of the Dramsfjord, Richards and Benson (1961) have 

 examined the accumulation of free nitrogen in excess of that expected from its 

 solubility by comparing the observed nitrogen/argon ratios with those in sea- 

 water equilibrated with air. The excess quantities of nitrogen found correspond 



