HETEROTROPHIC DENITRIFICATION POTENTIAL AS AN ADAPTIVE RESPONSE IN GROUNDWATER BACTERIA

The denitrification rate of bacteria indigenous to aquifers with different nitrate concentrations was investigated. The objective was to test how high nitrate loads affect community composition and select for enhanced nitrate reduction rates among denitrifying strains. Groundwater bacteria were isolated from two aquifers with high in situ concentrations of nitrate (24.1 and 35.2 mg NO3--Nl(-1)) and one aquifer with a lower in situ concentration (6.3 mg NO3--Nl(-1)). The isolates were characterized with respect to 15 colony and cell morphological and 8 physiological features. The frequency of heterotrophic denitrifiers of all isolated strains was 45-50% in the nitrate contaminated aquifers compared to slightly more than 20% in the uncontaminated aquifer. Kinetics of nitrate reduction were measured by using progress curves of nitrous oxide production. Bacterial respiration was measured to determine whether proposed differences in denitrification rates were due to differences in carbon utilization. Strains from the aquifers with enhanced nitrate concentrations showed shorter lag periods and higher growth rates than strains from the aquifer with low nitrate concentration. Denitrification rates for unamended incubations varied between 0.10 and 0.42 ml cells(-1) day(-1) and between 0.42 and 2.10 ml cells(-1) day(-1) for the glucose amended incubations. The average denitrification rate for the strains from the contaminated sites was 2-fold higher than for the strains from the uncontaminated site, and glucose amendment increased the difference to 2.5-fold. The differences in rates were not explained by differences in carbon utilization. The strains from the contaminated sites reduced more nitrate per unit of carbon dioxide produced than the strains from the uncontaminated aquifer. This suggests that strains in soils which have been heavily N-fertilized may have developed denitrification reductases with optimum activities at high in situ concentrations of nitrate.