Humic substances (HSs) are important for the functioning and stability of soil ecosystems; their physical and chemical properties, as well as their influence on plants and animals are well studied. However, the role of HSs in the physiology of microorganisms, in particular, in their survival under unfavorable conditions, is understood insufficiently. To evaluate the role of various chemical groups and physicochemical properties of HSs in the survival of soil bacteria, experiments were performed using eight HS preparations with different structure and properties. Addition of HSs to cultures of hydrocarbonoxidizing soil bacteria Rhodococcus erythropolis and Pseudomonas extremaustralis increased the titers of viable cells after 30 days of storage up to 10 times. This biological effect of HSs is due to their ability to stimulate the formation of stress-resistant persister cells. Correlation analysis established a relationship between the biological activity of HS and the atomic content of hydrogen and carbon, hydrophobicity, and the average molecular weight of HS. The atomic content of oxygen, as well as the content of CHn groups and various oxygen-containing groups (CH3O, CH2O, CHO, C=O, Alk-O) correlated with a negative effect of HS on bacterial viability. The content of aromatic groups and antioxidant capacity correlated with both activating and inhibiting biological activity of HSs. The stabilizing effect of HSs on bacterial cells is explained by their antioxidant activity,as well as by their ability to serve as a carbon source and to act as a terminal electron acceptor in the absence of oxygen. The obtained results elucidate an important function of HSs: to ensure long-term survival of bacteria in soil ecosystems in the periods between their active development.
Chemical Structure and Biological Activity of Humic Substances Define Their Role as Plant Growth Promoters
