THE PHYSIOLOGICAL NATURE OF HUMIC ACID STIMULATION OF AZOTOBACTER GROWTH

Abstract

An analysis of various physiological aspects of stimulation of growth of Azotobacter by humic acid and other Fe compounds is presented. Stimulation involves an induction period of several hours, during which time no stimulation takes place. Organic acid Fe compounds such as citrate and tartrate require longer periods than humic acids; and in-organic Fe compounds, still longer. The induction period is sharply defined, and when stimulation commences, a new velocity constant of growth is assumed within about 1 hr. Stimulation attains a maximum at about 0.5 p.p.m. Fe, when the supply is maintained, but as the Fe is consumed the initial concentration giving maximum final growth may be increased to 5-15 p.p.m., corresponding to 500-1500 p.p.m. of natural humic acid. At constant temp., all factors causing increased growth in absence of humic acid show both relative and absolute decreased stimulation in its presence; but when growth is increased as a function of sub-optimal temp., the converse relation obtains, i.e., stimulation is increased. Humic acids exert no significant influence upon the oxidation-reduction character of either cultures of Azotobacter or of the culture medium in the absence of organisms, both with respect to rates of potential change and to ultimate potentials assumed by the culture in absence of humic acid. The oxidation-reduction character of Azotobacter cultures in absence of humic acid has been described; under aerobic and anaerobic conditions, potentials of about + 0.20 and[long dash]0.15 volts, respectively, are assumed.