NITRATE AND NITRITE BACTERIA 105 



To these must be added from 20 to 50 mgr. of sulphate of ammonia that 

 should be renewed in larger doses (i gr.) at intervals of about a week. 

 The only source of nitrogen is the sulphate of ammonia ; the carbon is 

 obtained from the CO 2 of the atmosphere, not from the sodium or mag- 

 nesium carbonate, which are added only to neutralize the nitric and nitrous 

 acid that arise from the activity of the bacteria. The sodium chloride 

 benefits them in some way at present unknown. 



The nitrogen of the ammonia is not converted directly into nitric acid, 

 as was formerly supposed, but oxidized first of all to nitrous acid and then 

 to nitric. There are two distinct reactions going on, the formation of 

 nitrites from ammonia, and of nitrates from nitrites. Each is performed by 

 specific bacteria, which are always present together, so that the compounds 

 formed by the one kind are immediately taken up by the other and the 

 end-products alone appear in the soil. 



Only in pure cultures is it possible to follow each process separately. 

 Both are very slow; in a sixteen days' old culture, 60 mgr. of (NH 4 ) 2 SO 4 

 were converted into nitrous acid daily, and in a six weeks' old culture 64 mgr. 

 potassium nitrite daily into nitric acid. Probably, under natural conditions, 

 the process is more rapid. 



To isolate the nitrite from the nitrate 

 bacteria, Winogradsky used the customary \ 

 methods of plate culture, substituting colloid m\ 

 silica for the gelatine *. Very thoroughly 

 washed agar may also be used. For the 

 nitrite organism (NH 4 ) 2 SO 4 is added to the 

 medium, for the nitrate bacteria KNO 2 . FIG. . Nitrifying bacteria (from wino- 



TII *i_ , i_ i* i i i TTT- sradskv). a, Nitrosotnonas europea, a 



1 he nitrite bacteria are divided by W mo- nitrite bacterium from Zurich. 6, ttitroso- 



11 , ! 1 1 nr-j monas javanensis. a similar organism from 



gradsky intO tWO biological genera, Nltl'OSO- l*m..J Nitrobacter, a nitrate bacterium 

 ,717-.. -NT-., from Quito. Magn. 1000. 



coccus and rJitrosomonas. Nitrosococcus is 

 a non-motile spherical cell (3/tx. in diameter, 



and less) found in soil from South America and Australia. Nitrosomonas 

 is a very short ellipsoidal, motile form, of which two species in particular 

 may be mentioned. Nitrosomonas europea (Fig. 23, a), ubiquitous in the 

 soil in Europe, Africa, and Japan, is from o-9/ix to i \JL wide, and i-2/x to 

 i'8/z long, with a short cilium. N. javanensis from Buitenzorg (Fig. 23, b) 

 is almost spherical (05-06 ju.), with a cilium 30^ long. This is the longest 

 cilium known among bacteria. Spores have not been seen. The nitrite 

 bacteria render the culture turbid as long as they are motile. They also 

 form zoogloeae, and if crystals of Mg C O 3 be added the bacteria collect 

 on them in slimy masses and eat into the mineral just as lichens eat into 

 a stone. 



* For details respecting the composition of the media, see works cited in Notes (pp. 169-184). 



