550 RESPIRATION AND FERMENTATION 



The same metabolic products may frequently arise in a variety of ways : 

 thus nitrites may also be formed by the oxidation of ammonia (Sect. 63) 

 and sulphuretted hydrogen may not only be produced by proteid-decomposition, 

 but also by the reduction of sulphates, and perhaps by the formation and decom- 

 position of sulphides in plants fed with precipitated sulphur 1 . The manner in 

 which sulphates are reduced is doubtful, and it is possible that in this case the 

 sulphuretted hydrogen results from the decomposition of an intermediate product. 

 Certain bacteria always produce this gas under normal cultural conditions, but in 

 other cases it is formed, as in Saccharomyces^ only under very special nutritive 

 conditions. 



The liberation of nitrogen (sometimes accompanied by a little nitrous oxide) 

 from nitrates by certain bacteria (Bacillus denitrificans, &c.) is a phenomenon of 

 reduction, and the greater part or almost the whole of the nitrogen of potassium 

 nitrate may be liberated with such rapidity as to cause a foam to appear on the 

 surface of the fermenting fluid 2 . The process is apparently inhibited by the 

 presence of an abundance of oxygen, and hence it is possible that these organisms, 

 which are facultative anaerobes either alone or when associated with other bacteria, 

 may utilize the oxygen of nitrates in the absence of atmospheric oxygen. Organisms 

 may exist which can make similar use of nitrous oxide, although Phanerogams, &c. 

 have not this power 3 . The liberation of nitrogen by the bacteria mentioned takes 

 place in the absence of ammonia, but in other cases it may be liberated by the 

 decomposition of ammonium nitrite 4 , and this may be the reason for the slight 

 production of nitrogen which usually accompanies the oxidation of ammonia to 

 nitrites by nitro-bacteria r> . (On the circulation of nitrogen, cf. Sect. 68.) 



The reducing and enzymatic actions which may result from vital 

 activity do not form essential factors in respiratory metabolism, and it is 

 indeed not impossible that all fermentative activity is only the result of 

 metabolism, in so far as the latter produces the enzyme responsible for the 

 changes in question. Even when fermentation ceases coincidently with 

 the death of the ferment-organism, this may simply be because the exciting 

 enzyme at once decomposes, or is only able to act under the conditions 

 existing in the living cell. Indeed, Miquel 6 isolated a urea ferment (urase) 

 only after many futile attempts, and this ferment acts in precisely the same 

 manner as the bacteria which decompose urea. It is possible also that an 



1 Cf. Fliigge, 1. c., p. 170, and the literature here given ; Beyerinck, Centralbl. f. Bact., 1895, I, 

 Bd. I, p. i. 



2 Giltay et Aberson, Archiv. Neerland., 1891, T. xxv, p. 341 ; Burri u. Stutzer, Centralbl. f. 

 Bact., 1895, Bd. I, p. 257 ; 1896, Bd. n, p. 473. 



1 The supposition that aerobic respiration could be maintained by N 2 O has been disproved by 

 Detmer (Landw. Jahrb., 1882, Bd. xi, p. 213; Moller, Ber. d. Bot. Ges., 1884, p. 25). Cf. also 

 Correns, Flora, 1892, p. 150. 



4 Cf. Loew, Biol. Centralbl., 1890, Bd. x, p. 589. 



s Godlewski, Centralbl. f. Bact., 1896, Abth. ii, Bd. n, p. 458. Cf. Sect. 63. 



6 Cf. literature by Fliigge, Mikroorganismen, 1896, 3. Aufl., Bd. i, p. 211, and Herfeldt, 

 Centralbl. f. Bact, 1895, Abth. ii, Bd. i, p. 114. 



