Effect of Plant Root Exudate Constituents on the Degradation of Phenanthrene by the Rhizobacterium Mycolicibacterium gilvum (Mycobacteriaceae, Actinobacteria)

—The influence of the major components of plant root exudates, namely, carboxylic acids (succinic acid as an example) and secondary plant phenolic metabolites, i.e., flavonoids (rutin as an example), on the microbial degradation of the three-ring polycyclic aromatic hydrocarbon (PAH) phenanthrene by the rhizobacterium Mycolicibacterium gilvum was studied. The degradative activity of this microorganism relative to PAH was studied by cultivating it in a liquid mineral medium containing phenanthrene (0.2 g/L), succinic acid (5 g/L), and rutin (0, 0.05, 0.1, or 0.2 mmol/L) at 30°C under aeration on a shaker (130 rpm) for 14 days. The stimulating effect of succinic acid and rutin on the microbial degradation of phenanthrene was revealed. It was found that carboxylate was utilized as the main growth substrate for the microorganism, while flavonol and PAH had little effect on bacterial growth. Rutin had no antimicrobial effect on the microorganisms studied; on the contrary, in combination with succinic acid, it significantly increased the biomass growth. At high concentrations (0.1 and 0.2 mmol/L), rutin inhibited the degradation of phenanthrene by 22 and 56%, respectively. However, at a concentration of 0.05 mmol/L, rutin increased phenanthrene degradation by 10%. Thus, the results obtained showed the dependence of the PAH microbial degradation efficiency on the presence, combination, and concentration of the constituents of the plant root exudates.