The biochemistry of the nitrifying organisms. 2. The free-energy efficiency of Nitrosomonas

The free-energy efficiencies of a number of auto- trophic micro-organisms have been calculated by Baas- Becking & Parks (1927) as the ratio between the free energy consumed in carbon dioxide assimilation and the free energy released by the primary inorganic oxidation. These calculations were based on the results of determinations, published by various workers, of the amounts of organic carbon found in initially inorganic media in which various autotrophs had oxidized measured amounts of substrates. In the case of NitroWomona8, which obtains all the free energy needed for carbon dioxide assimilation from the free energy released by the oxidation ofammonia to nitrite, the calculation runs as follows: The reduction of 1 g. of carbon from carbon dioxide to the oxidation level at which carbon is found in cellular constituents requires approximately 10 000 cal. of free energy. The oxidation of 1 g. of nitrogen from ammonia to nitrite releases about 4 700 cal. of free energy. ( The exact figure depends on the pH of the medium, concentrations of reactants, and other relevant factors.) The free energy efficiency of Nitro8omona8 is therefore co x 1 000 x 100%, where C.=g. N,1 4 700 carbon assimilated by the culture and N,, = g. nitrite N formed by the culture. Baas-Becking & Parks (1927) used the value of 1/35, obtained by Winogradsky (1890b), for the ratio C,/N,, and thus concluded that the free-energy efficiency of Nitro-