Influence of humic acids on the growth of the microorganisms utilizing toxic compounds (comparison between yeast and bacteria)

Humic acids, a heterogenic group of natural organic macromolecules with a complex polymeric structure, form a significant part of the soil and water environment. They have a high surface activity, thus they are able to interact with other components in the environment (mineral and microbial cell surfaces, organic and inorganic compounds). This work is focused on humic acid effects on the growth of microorganisms utilizing phenolic compounds as the sole carbon and energy source. For this purpose a bacterial population Rhodococcus erythropolis and a yeast strain Candida maltosa were used. Both microorganisms had been adapted to the degradation of phenol and phenolic compounds for many years. However, a toxic effect of these compounds still occurs. The addition of humic acids into the cultivation medium extended toleration of the bacteria to higher concentrations of phenolic compounds and increased the growth of this microbial population as well. In the case of the yeast, humic acids inhibited the growth. Humic acids easily form an additive layer on the surface of the investigated microorganisms. The generated humic acid layer can probably serve as a transport barrier and thus influence the growth of microbial populations.