One of the most striking results of bacterial activity is this phenomenon. The particular color which results may be almost any one throughout the range of the spectrum, though shades of yellow and of red are of more frequent occurrence.
In the red sulphur bacteria the “bacteriopurpurin” which they contain appears to serve as a true respiratory pigment in a manner similar to the chlorophyl in green plants, except that these bacteria oxidize H2S in the light as a source of energy instead of splitting up CO2. The red pigment produced by certain bacteria has been shown to have a capacity for combining with O resembling that of hemoglobin, and some investigators have believed that such bacteria do store O in this way for use when the supply is diminished. With these few exceptions the pigments seem to be merely by-products of cell activity which are colored and have no known function.
The red sulphur bacteria above mentioned and one or two other kinds retain the pigments formed within the cell. Such bacteria are called chromophoric as distinguished from the chromoparic bacteria whose pigment lies outside the cell.
The chemical composition of no bacterial pigment has been determined up to the present. Some are soluble in water, as shown by the discoloration of the substances on which they grow. Others are not soluble in water but are in alcohol, or in some of the fat solvents as ether, chloroform, benzol, etc. These latter are probably closely related to the lipochromes or “fat colors” of higher plants and animals. Attempts have been made to render the production of pigments a still more reliable means of identification of species of bacteria through a careful examination of the spectra of their solutions, but such study has not as yet led to any valuable practical results.
The production of pigment depends on the same general factors which determine the growth of the organism but does not necessarily run parallel with these. It is especially influenced by the oxygen supply (only a very few organisms are known which produce pigment anaërobically—Spirillum rubrum is one); by the presence of certain food substances (starch, as in potato, for many bacteria producing yellow and red colors; certain mineral salts, as phosphates and sulphates, for others); by the temperature (many bacteria cease to produce color at all if grown at body temperature, 37°—Erythrobacillus prodigiosus—or if grown for a longer time at temperatures a few degrees higher).
REDUCING ACTIONS.
Reduction of nitrates to nitrites or to ammonia or even to free nitrogen is brought about by a great many different kinds of bacteria. In many instances this phenomenon is due to a lack of free oxygen, which is obtained by the bacteria from these easily reducible salts. In other cases a portion of the nitrogen is removed to be used as food material in the building up of new protein in the bacterial cell. This latter use of the nitrogen of nitrates by bacteria might theoretically result in considerable loss of “available nitrogen” in the soil as has actually been shown in a few experiments. The reduction of nitrates as above mentioned would also diminish this supply, but probably neither of these results has any very great practical effect on soil fertility. The building up of protein from these mineral salts by bacteria in the intestines of herbivorous animals has been suggested by Armsby as a considerable source of nitrogenous food, and this suggestion appears possible.
The liberation of nitrogen from nitrates or nitrites, either as free nitrogen or as ammonia, is spoken of as “dentrification,” though this term was formerly applied to such liberations, from compounds of nitrogen generally even from proteins.
Certain bacteria may also reduce sulphates and other sulphur compounds to H2S, a phenomenon frequently observed in sewage and likewise of importance in the soil. It is possible that phosphates may be similarly reduced.[14] Further and more careful study of the reducing actions of bacteria is needed.