Phenotypic Plasticity in Groundwater Denitrifiers

Abstract

Bioassays were made in a gradient of NO 3 - concentrations to examine how phenotypic plasticity in denitrification rates was associated with adaptation in groundwater bacteria to aquifers with different NO 3 - concentrations. Denitrifying activity of single strains were quantified using progress curves of N 2 O production in sediment-groundwater microcosms in which the NO 3 concentrations ranged between 0.5 and 48 mg NO 3 - -N l -1 . Denitrifiers from a fertiliser contaminated aquifer with 24.1 mg NO 3 - N 1 -1 had a shorter lag phase and more rapid growth compared to strains from an uncontaminated aquifer with 2.8 mg NO 3 - -N 1 -1 . regardless of the NO 3 concentrations they were maintained in. Strains from the uncontaminated site showed a harrow plastic response with a denitrification optimum at low NO 3 - concentration (2.8 6 mg NO 3 - -N l -1 ) suggesting that denitrifiers in the uncontaminated aquifer have undergone stabilising selection to the in situ NO 3 - concentration. Strains from the contaminated site had a broader plasticity with virtually the same denitrification rate from low to high NO 3 concentrations (2.8-24 mg NO 3 - -N l -1 ). They were superior denitrifiers also at the in situ concentration of the uncontaminated aquifer. which we suggest is due to the expression of more than one copy of the reductase genes. The average denitrification rates for strains from the contaminated site were 2-3 times higher compared to rates for strains from the uncontaminated site, but the variability was larger, possibly because two of the anaerobic denitrifiers from the contaminated site also were capable of aerobic denitrification. The results presented here can be useful when in situ bioremediation of NO 3 - contaminated aquifers is considered, since denitrifiers adapted to the conditions in the contaminated aquifer maintain high rates of N 2 O production down to a NO 3 - concentration of about 3 mg NO 3 -N l -1 .