CHEMOSYNTHETIC ASSIMILATION OF CARBON DIOXIDE 361 



not always produce precisely the same results, for different plants may react 

 differently, and various influences may aid in modifying the character of the 

 response. 



/?. Chemosynthetic Assimilation of Carbon Dioxide. 



SECTION 63. 



Our knowledge of the nitrate and nitrite bacteria is almost entirely 

 due to Winogradsky's researches 1 . These organisms have the power of 

 assimilating carbon dioxide chemosynthetically by means of energy de- 

 rived from the oxidation of ammonia into nitrites or nitrites into nitrates. 

 The whole of the organic food of these bacteria is obtained in this manner 

 (Sect. 50), and hence they can develop in a fluid free from all organic 

 substances provided the necessary inorganic salts are present. The nutrient 

 fluid used by Winogradsky contained i gramme of potassium phosphate, 

 0-5 grm. magnesium phosphate, and 0-5 to i grm. basic magnesium car- 

 bonate to 1,000 grm. of water. To this solution 0-2 per cent, of ammonium 

 phosphate must be added for the culture of nitrite bacteria, whereas 

 a similar amount of potassium or ammonium nitrites must be supplied 

 and renewed from time to time to cultivate nitrate micro-organisms. 



Winogradsky has shown that nitrite-bacteria (Nitromonas, Nitrococcus] 

 oxidize ammonia to nitrous acid, and proceed no further even when fully 

 supplied with oxygen, whereas nitrate-bacteria (Nitrobacter} oxidize nitrites 

 to nitrates, and thereby obtain energy for their growth and development. 

 Both forms are always present in ordinary soil, and if well aerated no 

 marked accumulation of nitrites ever occurs (Sect. 28). The nitro-bacteria 

 are strongly aerobic and consume oxygen in great abundance 2 , while since 

 they cannot develop in the absence of oxygen, any formation of nitrites that 

 may occur under such conditions must be due to the reduction of pre- 

 existent nitrates by other bacteria (Sect. 102). 



The nitro-bacteria are of great importance in the economy of nature by 

 providing a continual supply of nitrates to the soil, and at the same time 

 they obtain the energy for the chemosynthetic assimilation of carbon 

 dioxide 3 , and hence are able to grow and form organic substance when 

 this gas affords the sole source of carbon. When carbonates are present 



1 Winogradsky, I. Ann. d. 1'Inst. Pasteur, 1890, T. iv, p. 213; II. p. 2;;; III. p. 710; 

 IV. 1891, T.V, p. 92; V. p. 577; VI. Archiv. p. 1. sci. biol. a 1'Inst. imp. a St.-Pe'tersbourg, 1892, 

 T. i, p. 87 ; VII. Centralbl. f. Bact., 1896, Abth. ii, Bd. n, p. 415. 



" For measurements of the oxygen consumption, see Godlewski, Anzeiger d. Akad. d. Wiss. in 

 Krakau, 1892 and 1895. 



3 This energy is indirectly derived from the sun (see Sect. 5i\ 



