Humic acids stimulate growth and activity of in vitro-tested axenic cultures of soil autotrophic nitrifying bacteria

In the present study, the effect of humic acids on activity and growth of Nitrosomonas europaea and Nitrobacter agilis was investigated in vitro under axenic conditions. Humates from compost-stabilized vegetable waste or leonardite were added to the chemolithotrophic culturing medium at concentrations of 0, 5, 50 and 100 mg l-1. It was found that both types of humic acids increased either NH4/+ or NO2 oxidation and cell growth of nitrifying bacteria in a dose-independent manner. By combining these results with data from a comparative growth evaluation of N. agilis based on possible utilization of humates or pyruvate in heterotrophic conditions, evidence was obtained that nitrifiers cannot use humic acids as an alternative carbon and energy source. Thus, the stimulating effect of this fraction of soil organic matter on chemolithotrophic ammonia and nitrite oxidizers might be attributed to an increase in microbial membrane permeability favouring a better utilization of nutrients.