They can, however, only carry on their work extensively under anaerobic conditions, as in waterlogged soils or in those which are badly tilled, so that there is but little loss of nitrates through their agency.
The bacterium, Clostridium pasteurianum, common in most soils, is able to utilize free nitrogen under anaerobic conditions, and an organism known as Azotobacter chroococcum and some others closely allied to it, have similar powers which they can exercise under aerobic conditions.
As the former are only active in the air while the latter are anaerobic, the activity of either agent is conditioned by variation in the water level of the bog.
Anaerobic species usually require little oxygen, but aerobic species need a free supply.
The work of van Tieghem, van Senus, Fribes, Omeliansky and others has now shown that while certain anaerobic bacteria decompose the substance of the middle lamella - chiefly pectin compounds - and thus bring about the isolation of the cellulose fibres when, for instance, flax is steeped or " retted," they are unable to attack the cellulose itself.
There exist in the mud of marshes, rivers and cloacae, &c., however, other anaerobic bacteria which decompose cellulose, probably hydrolysing it first and then splitting the products into carbon dioxide and marsh gas.
Beyerinck and Jegunow have shown that some partially anaerobic sulphur bacteria can only exist in strata at a certain depth below the level of quiet waters where SH 2 is being set free below by the bacterial decompositions of vegetable mud and rises to meet the atmospheric oxygen coming down from above, and that this zone of physiological activity rises and falls with the variations of partial pressure of the gases due to the rate of evolution of the SH 2.
Beyerinck has shown that Spirillum desulphuricans, a definite anaerobic form, attacks and reduces sulphates, thus undoing the work of the sulphur bacteria as certain de-nitrifying bacteria reverse the operations of nitro-bacteria.
No sharp line can be drawn between pathogenic and nonpathogenic Schizomycetes, and some of the most marked steps in the progress of our modern knowledge of these pasteurianum, which is anaerobic, and can fix nitrogen only if protected from oxygen by aerobic species.
It is very probable that numerous symbiotic fermentations in the soil are due to this co-operation of oxygen-protecting species with anaerobic ones, e.g.
Some, however - the strictly anaerobic bacteria - grow only in the absence of oxygen; hence means must be adopted for excluding this gas.
It is found that if the inoculation be made deep down in a solid medium, growth of an anaerobic organism will take place, especially if the medium contains some reducing agent such as glucose.
To obtain growth of an anaerobic organism on the surface of a medium, in using the plate method, and also for cultures in fluids, the air is displaced by an indifferent gas, usually hydrogen.