SUPPLEMENT 69 



water, of lakes, or of the sea (VAN DELDEN, 1903). Microspora disulphuricans 

 and a closely related form living in such places produce sulphuretted hydrogen 

 from sulphate ; in one of VAN DELDEN'S experiments the enormous amount 

 of 0-952 g. was produced from i litre of culture solution. The oxygen so 

 abstracted is found again in the carbon-dioxide which is obtained from the 

 essential organic materials in absence of free oxygen. In addition to sulphates, 

 thiosulphates are reduced by certain Bacteria, and yeasts also form sulphuretted 

 hydrogen out of thiosulphates and sulphates. 



One might imagine these reductions were the result of the action of the 

 hydrogen produced in these fermentations, or of other similar reduced bodies, 

 such as methane. When in the nascent state these substances can reduce 

 sulphates, just as they reduce the easily-reduced bodies above referred to. 

 However, the origin of H or CH 4 in the case of yeast is neither proved nor 

 probable. On the other hand, Bacteria which form hydrogen abundantly and 

 which decolourize indigo-carmine are unable to reduce sulphates. Finally, 

 many Bacteria that are able to form H 2 S from sulphates are unable to reduce 

 the more readily reducible nitrate. All these facts suggest that we have here 

 to do with quite specific reducing agents in living cells (OMELIANSKI, 1904, in 

 LAFAR'S Mykologie, III. 217). That these substances must be of the nature 

 of enzymes is supported by HAHN'S observations (comp. BUCHNER, 1903), 

 according to which dead cells can induce the same reductions as living cells. 

 His experiments were carried out with expressed yeast sap, dried yeast cells, 

 and dead Bacteria. 



Just as the combination of sulphur is aided by the reduction of sulphates 

 into a form which is of no nutritive value to the majority of plants, so also 

 by reduction nitrates are transformed into compounds which are of no value 

 as nutrients to higher plants, or at least are in many cases of less value than 

 nitrates. Nitrates may be transformed into ammonia through the inter- 

 mediate stages of nitrites, or they may be reduced to free nitrogen, or, finally, 

 NO and N 2 O may be produced (comp. JENSEN in LAFAR'S Mykologie, III. 182 ; 

 ITERSON, 1904). The last reaction is as yet little known, but ammonia forma- 

 tion is a quite common feature in the metabolism of micro-organisms. Since 

 the ammonia in the soil is easily absorbed, its formation does not result in any 

 loss of nitrogen in agriculture, but it is different when free nitrogen is formed. 

 This process, ' denitrification,'in the limited sense, appears to be very common. 

 JENSEN (1898, 1899) has studied some Bacteria which can split off nitrogen 

 only when oxygen is absent, and which are able to live as anaerobes only when 

 potassium nitrate is available. It is obvious that they take oxygen from 

 potassium nitrate, and this view is supported by MAASSEN'S (1901) observation 

 that bodies containing abundant oxygen, e. g. chlorates, inhibit the decom- 

 position of potassium nitrate. Probably these chlorates take the place of 

 nitrates, and oxygen is abstracted from them, so that they, to a certain extent, 

 protect the nitrates. It has, however, been shown by several authors that 

 denitrification is also possible in presence of oxygen, and this fact appears at 

 first to be contrary to the conception of the process as above outlined. But 

 when we bear in mind that yeast does not cease to produce alcohol when oxygen 

 is plentifully supplied, we may well assume the possibility of the existence of 

 Bacteria which can regularly split off oxygen from certain nitrogenous com- 

 pounds, even when it is also supplied to them in the free state. From MAASSEN'S 

 researches, indeed, it appears that certain Bacteria are specifically denitrifiers, 

 i. e. always induce denitrification, while others develop such a power only 

 under certain definite external conditions. The former are obviously limited 

 in number, while the power of inducing ordinary denitrification is apparently 

 widely distributed. The cause of nitrate as of sulphate reduction must 

 be attributed to enzymes. 



