ASSIMILATION OF CARBON 49 



to burn readily; i.e., to liberate heat. On ibis account these oxidizable in- 

 organic substances can supply energy for these bacteria. Thus, nitrifying 

 bacteria utilize ammonia, and sulphur bacteria make use of hydrogen sulphide. 



To obtain a solid substratum for cultures where organic substances must be 

 avoided, silicic acid 1 may be used instead of gelatine or agar-agar. 



Vinogradskii 2 also proved that bacteria living in sulphur springs, as Beggia- 

 toa and some other species, use hydrogen sulphide as a source of energy. This 

 is first oxidized only to sulphur and water; H 2 S + О = H 2 + S. The sul- 

 phur thus formed accumulates within the cells, to be further oxidized, in the 

 presence of carbonates (e.g., calcium carbonate), to form calcium sulphate and 

 carbonic acid. The sulphur bacteria play a very important role in the 

 economy of nature; without them the circulation of sulphur might be impossible. 



In order to obtain sulphur bacteria, freshly cut pieces of roots of Butomus 

 umbcllatus, with the mud clinging to them, are placed in a deep vessel, in from 

 3 to 5 1. of water; some calcium sulphate is added and the vessel is left uncovered 

 at room temperature. After several days the formation of hydrogen sulphide 

 is evident, consequent upon the decomposition of calcium sulphate by various 

 bacteria contained in the mud. Some time after the appearance of hydrogen 

 sulphide the development of sulphur bacteria begins. They usually collect at 

 some distance from the free surface of the liquid and, as they move upwards ■ 

 and downwards, they sometimes absorb hydrogen sulphide and sometimes 

 oxygen. 



When grown upon a microscope slide, in a liquid containing hydrogen sul- 

 phide, the sulphur bacteria assemble to form a ring, about a millimeter from the 

 edge of the cover glass. If the drop of liquid is not covered they do not develop 

 at all. There is therefore a definite optimum of oxygen supply for these bac- 

 teria. According to the researches of Yegunov, 3 this point is well brought out 

 by growing them in deep vessels. A bacterial membrane is formed at a cer- 

 tain distance from the surface of the liquid and short, tassel-like outgrowths 

 project downwards from this membrane. A part of such a membrane with its 

 projections is shown, enlarged, in Fig. 30. If these outgrowths are examined 

 with a horizontal microscope it becomes evident that they consist of bacterial 

 cells that are moving up and down with a boiling motion, like water in a spring. 



The occurrence of hydrogen sulphide is not confined to bogs and sulphur 

 springs, for this substance is also found in the sea. The water of the Black Sea 

 below a depth of about 200 m. becomes richer in hydrogen sulphide as the depth 

 increases. One hundred liters of water, collected at the depths given, contained 

 the following amounts of hydrogen sulphide. 



1 Omeliansky, V., Sur la culture des microbes mitrificateurs du sol. Arch. sei. biol. St.-Petersbourg 

 7: 291-302. i8oo- 



= Winogradsky, Sergius, Ueber Schwefelbacterien. Bot. Zeitg. 45: 480-507, 513-523. 520-530, 

 545-559. 569—576, 585-594. 606-610. 1887. Nathansohn, Alexander, Ueber eine neue Gruppe von Schwe- 

 felbacterien und ihren Stoffwechsel. Mittheil. Zool. Sta. Neapel 15: 655-680. 1902. Beijerinck, 

 M. W., Ueber die Baketerien welche sich im Dunkeln mit Kohlensaure als Kohlenstoffquelle ernähren 

 können. Centralbl. Bakt. //, 11: 403-599. 1904. Omelianski, W., Ueber eine neue Art farbloser 

 Thiospirillen. Ibid. II, 14: 769-772. 1905. 



8 Yegounow, M., Sur les sulfobacteries des limans d'Odessa. Arch. sei. biol. St.-Petersbourg 3: 381- 

 397- I895- Idem, Die Mechanik und Typen der Teilung der Bakterienscharen. Centralbl. Bakt. //, 

 4:97-109. 1898. 



