Effect of 2-hydroxybenzoate on the rate of naphthalene mineralization in soil

Abstract

The effect of 2-hydroxybenzoate (2-OHB, salicylate) on the mineralization rate of [¹⁴C]naphthalene, the population density of naphthalene-degrading bacteria, and the concentration of genes encoding for naphthalene dioxygenase in a soil bacterial community was investigated. Six different concentrations of 2-OHB (10, 20, 50, 100, 150 and 200 μg g⁻¹ soil) were tested in 100-g portions of soil. The addition of 10, 20 or 50 μg 2-OHB g⁻¹ soil produced a general increase in total soil bacterial population density, whereas the addition of 100 μg or 200 μg 2-OHB g⁻¹ soil specifically increased the proportion of naphthalene degraders relative to the total population. The addition of 50 μg 2-OHB g⁻¹ soil produced a fourfold increase (the maximum observed) in the rate of naphthalene mineralization relative to the rate in unamended soil. The concentration of 2-OHB (≤ 100 μg/g) added to soil correlated with the population density of naphthalene degraders (r=0.961). Addition of up to 200 μg 2-OHB g⁻¹ correlated with the abundance of DNA sequences homologous to known naphthalene dioxygenase genes (nahAB) (r=0.958). However, mineralization of [¹⁴C]naphthalene was stimulated significantly only by the addition of 50 μg 2-OHB g⁻¹ soil. Results of the mineralization experiments were supported by the detection of nahAB mRNA extracted directly from soil. The specificity of the effect of 2-OHB on naphthalene biodegradation was confirmed in a control experiment using equivalent concentrations of 4-OHB which repressed naphthalene mineralization by about 50%. Addition of ammonium nitrate to the soil also increased the rate of naphthalene mineralization. Ammonium nitrate added together with 2-OHB reduced the mineralization enhancement effect of either compound alone. The study confirmed that specific induction of biodegradative genes can enhance chemical pollutant removal in situ.