Organic compounds that reach subsoil may threaten groundwater quality; effect of benzotriazole on degradation kinetics and microbial community composition

Toxic compounds in soils threaten groundwater quality in two ways: as potential contaminants themselves, and by retarding the microbial degradation of other organic compounds, thus enhancing their deep penetration. Benzotriazole (BTA) is a chemical with versatile industrial applications, used in large quantities worldwide, and represents a potential threat to the environment due to its apparent toxicity and recalcitrance. When used as an additive in aircraft deicing/antiicing fluid on airports, substantial spills of these mixtures and jet fuel will inevitably reach the soil. We have investigated the subsoil (1-2 m depth) microbial degradation and growth on four relevant organic substrates found in airport run-off (acetate, formate, glycol and toluene) in the presence of concentrations of BTA which can be found in airport run-off. Monitoring CO₂ evolution showed growth-dependent degradation rates for all substrates (sigmoid CO₂ accumulation curves), which were significantly affected by BTA. The mineralization of acetate was only moderately retarded and only by the highest BTA concentration used (400 mg l⁻¹ in soil solution); formate and glycol mineralization was substantially retarded at 200 mg l⁻¹, and toluene mineralization already at 10 mg l⁻¹ BTA. Mass balances (fraction of added C recovered as CO₂) suggested that the microbial growth yield (g biomass-C formed per g substrate C) was severely reduced with increasing concentrations of BTA. The analysis of phospholipid fatty acids (PLFA) demonstrated that Gram-negative bacteria were dominating among the organisms growing on all four substrates. The total amount of PLFA increased with approximately 1000 pmol PLFA g⁻¹ soil in response to a dose of 0.93 μmol glycol-C g⁻¹ soil, but this increase was gradually reduced with increasing BTA concentrations. This was in agreement with C mass balances based on CO₂ measurements, verifying that BTA severely reduced the growth yields. The response of individual PLFA's to BTA and substrates demonstrated that non-growing organisms were largely unaffected (i.e. the PLFA's of which the absolute amounts did not increase in response to substrates were not affected by BTA), whereas those which were growing on the added substrates were uniformly reduced by BTA (all the PLFA's which increased in response to the substrates were negatively affected by BTA). The results suggest that BTA functions as an uncoupler, i.e. a substance that reduces the yield of ATP per mole of substrate used, or that the defence mechanisms represent a large energy burden to all microbial cells.