290 THE NITRIFYING BACTERIA 



+ 2 = 0^ 

 \ N = 



As is apparent from this equation, their powers differ from that of the 

 nitroso-baeteria, inasmuch as the latter convert the pentavalent nitrogen of 

 ammonia into the trivalent nitrogen of nitrous acid, whilst the nitro-bacteria 

 re-convert the element into the pentad condition. 



It is evident that these oxidation processes can be effected only in the presence 

 of bases which take up the acids with which the ammonia was initially combined, 

 and also neutralise the resulting nitrous or nitric acid thus protecting the 

 bacteria from injury from this source. This task is excellently performed in the 

 soil by calcium carbonate. The favourable influence exercised on the course of 

 nitrification by the presence of this salt is therefore readily explainable without 

 dragging in any hypothesis about the condensation of oxygen. Free alkali is 

 unsuitable here for the fixation of the acids, since the presence of this reagent 

 in quantity would be injurious to the bacteria. In artificial cultures calcium 

 carbonate can be replaced by magnesium carbonate, a practice adopted by 

 Winogradsky. 



205. Nitrosomonas and Nitrosococcus. 



Two main types of nitroso-baeteria can be differentiated in consequence of 

 the results of existing investigations. One of them (in several species) is found 

 in all the soils of the Old World (Europe, Asia, Africa) hitherto examined, and 

 is known as Nitrosomonas. The second is peculiar to the soil of the two remaining 

 continents, and has received the name of Nitrosococcus. The individual organisms 

 of the first-named type are each provided with a single cilium, and exhibit powers 

 of locomotion which are manifested at an early stage in the cultures, and cause 

 these to become decidedly opalescent. Subsequently the cells become quiescent 

 and collect as zooglcea, which rest in the form of greyish gelatinous clouds on the 

 carbonate at the bottom of the liquid. We will describe this (Nitrosomonas) 

 genus first. 



Only a single species of nitroso-bacterium has been discovered in European 

 soils, viz., Nitrosomonas europtva. At the opalescent stage of the culture this 

 organism appears as briskly motile cells (fitted with a short flagellum) in the 

 shape of short rods 1.2-1.8 p. long and 0.9-1.0 /z broad. The cells of Nitroso- 

 tnoucLS javanica, cultivated from the .soil of the Botanical Garden at Buitenzorg, 

 near Batavia, are globular, and only attain a diameter of 0.5-0.6 /*, but their 

 flagellum is very long as much as 30 /u. The Nitrosomonas japonica, found in 

 soil from Tokio, is like the Nitrosomonas qfricana, isolated from samples of 

 soil from Tunis and from La Reghai'a, in Algeria very similar to the European 

 species, only somewhat smaller. 



Differing from these species are those of the genus Nitrosococcus, found in 

 South American and Australian soils. They do not form zooglcea, neither are 

 they ciliated. That obtained from Quito (Ecuador) is a coccus 1.5-1.7 /* in 

 diameter. A similar species, except in point of size, is the Xitrosococcii^s brasili- 

 ensis, obtained as a pure culture from the soil of Campinas (Brazil), and attaining 

 a diameter of 2 p ; and the species grown from Melbourne soil is undistinguish- 

 able from this latter. The nitroso-bacteria are, as observed by WINOGKADSKY (IX.), 

 very susceptible to desiccation, and consequently tlm amount of such organisms 

 in the soil decreases as drying progresses. They arc almost entirely lacking in 

 the air. 



