About 65-70 per cent. of the dry matter of bacteria consists of protein.
(2) Sources of Carbon.
The biggest constituent of the dry matter of bacteria is therefore carbon. In the soil, bacteria find an abundance of organic matter from which they may derive their carbon supply. A special case, however, is furnished by the nitrifying organisms, certain sulphur oxidising bacteria, and others that derive their carbon from the CO2 of the soil atmosphere. The sources from which these special groups obtain the necessary energy to accomplish this, we have already considered.
(3) Assimilation of Nitrogen Compounds.
Of chief importance in its consequences are the means adopted by bacteria to obtain their nitrogen supply.
There is some reason to believe that soil bacteria do not take up protein and peptones as such, but must first break down these bodies into simpler compounds. When a sufficient amount of easily decomposable organic nitrogen is present in the soil, the ammonifying bacteria use such compounds as sources of energy, and in this case have a nitrogen supply exceeding their requirements.
But where there is an excess of carbohydrate or other non-nitrogenous source of energy available in the soil, the case is different. Here the organisms have a supply of energy which enables them to multiply rapidly until the organic nitrogen is insufficient for their needs. Hence they turn to the ammonia and nitrate present in the soil, and build up their proteins from this source. Doryland[17] has shown that many common soil ammonifiers assimilate ammonia and nitrate when supplied with carbohydrate. There may thus be a temporary loss of nitrate from soil when sugar, starch, straw, or such materials are added to it.
(4) Fixation of Free Nitrogen.
The bacteria that we have so far considered take up their nitrogen directly from compounds containing this element. There remain, however, a comparatively small but very important group of bacteria possessing the power of causing elemental nitrogen to combine, and of building it up into their proteins. This fixation of nitrogen by micro-organisms is a vital step in the economy of nature. Losses of nitrogen from the land are continually occurring through the washing-out of nitrates by rain, and through the evolution of gaseous nitrogen during the processes of decay. To maintain the supply of combined nitrogen which is essential to living organisms, there must therefore be a compensating process by which the supply of nitrogen compounds in the soil is kept up.
It was discovered in the middle of the nineteenth century that if soil were kept moist and exposed to the air, there was an increase in the amount of nitrogen compounds present. Berthelot, in 1893, studied the nitrogen relationships of soil, and recognised that this fixation of nitrogen in soil was the work of micro-organisms.