ROBERT L. STARKEY 329 



Even the existence of autotrophic iron bacteria has been questioned on theoretical 

 grounds.' The precipitation of iron irrespective of its mechanism is of considerable 

 economic importance. This is suggested by the extensive deposits of iron ore of 

 aquatic origin, deposition with rusting of iron pipes, the fouling of drinking waters, 

 and the formation of iron hardpans in soils, 



BACTERIA WHICH OXIDIZE HYDROGEN 



Hydrogen arises naturally in considerable abundance as a result of decomposition 

 processes under anaerobic conditions. This is particularly noticeable in stagnant 

 waters. It may also arise from mines and numerous non-biological sources. Bio- 

 logical oxidation may be active in preventing any great accumulation of this gas. It 

 may be oxidized under aerobic conditions by a large number of simple bacteria all 

 of which appear to be facultative autotrophs.^ Some of those isolated by Grohmann 

 have endospores. The reaction proceeds to water by the following reaction: 



XIV. H.+§0.=H.O 



AF298 = — 56,000 calories 



The quotient of H^iOj in pure-culture experiments is close to 2. No consistent ratio 

 of C02(assimilated):H2(oxidized) was observed by Ruhland, but from observations 

 on the physiology of other autotrophic bacteria one might infer that in the biological 

 transformation such a relationship exists. 



These organisms occur widely in soils and may live upon many different organic 

 compounds as well as hydrogen. 



It has been suggested that other organisms have autotrophic habits and utilize such 

 compounds as selenium,^ carbon monoxide,'' arsenites,s and methane.* As concerns the first 

 three compounds the evidence appears too limited at present to be more than suggestive. 

 Although the bacterial oxidation of methane has been quite definitely established it seems 

 undesirable to classify this process as autotrophic since methane may more logically be con- 

 sidered as an organic compound. Although classification is naturally arbitrary and its dis- 

 tinction is not as sharp in nature as seems to be suggested, it should eliminate confusion 

 and lead toward simplicity and clearness. These advantages do not seem to be gained by 

 including such organisms as methane-oxidizing bacteria among the autotrophic forms. 



' Baas-Becking and Parks, loc. cit. 



2 Kaserer, H.: CentralU.f. Bakleriol., Abt. II, 15, 573-76; 16, 681-96. 1906; Beijerinck, M. W. 

 and Van Delden, A.: ibid., 10, 33-47. 1903; Nikitinsky, J.: ibid., ig, 495-99. 1907; Nabokitch, A. 

 J., and Lebedeff, A. F.: ibid., 17, 350-55. 1907; Niklewski, B.: ibid., 20, 469-73. 1908; Jahrb.f. 

 Wiss. Botan., 48, 113-42. 1910; Lebedev, A. J.: Bcr. d. Dent. Bot. GeselL, 27, 598-602. 1910; Groh- 

 mann, G.: CentralU.f. Bakieriol., Abt. II, 61, 256-71. 1924; Ruhland, W.: Jahrb.f. Wiss. Botan., 

 63, 321-89. 1924. 



3 Lipman, J. G., and Waksman, S. A.: Science (N.S.), 57, 60. 1923. 



■* Beijerinck, M. W., and Van Delden, A.: loc. cit.; Kaserer, H.: CentralU.f. Bakteriol., Abt. II, 

 16, 681-96, 769-75. 1906; Lantzsch, K.: ibid., 57, 309. 1922. 



s Green, H. H.: Fifth and Si.xth Rpls. Vet. Res. Dept. of Agr. of the Union of So. Africa, pp. 

 595-610. 1918. 



^Sohngen, N. L.: CentralU.f .Bakteriol., Abt. II, 15, 513-17. i9o6;Munz, E.: Zur Physiologic 

 der Methanbakterien (Inaug. Diss., Halle). 63 pp. 1915; Aiyer, P. A. S.: Mem. Dept. Agr. India 

 (Pusa), Chem. Ser., 5, 173-94. 1920. 



