Effects of nitrate availability and the presence of Glyceria maxima the composition and activity of the dissimilatory nitrate-reducing bacterial community

Abstract

The effects of nitrate availability and the presence of Glyceria maxima on the composition and activity of the dissimilatory nitrate-reducing bacterial community were studied in the laboratory. Four different concentrations of NO₃⁻, 0, 533, 1434, and 2,905 µg of NO₃⁻-N g of dry sediment⁻¹, were added to pots containing freshwater sediment, and the pots were then incubated for a period of 69 days. Upon harvest, NH₄⁺ was not detectable in sediment that received 0 or 533 µg of NO₃⁻-N g of dry sediment⁻¹. Nitrate concentrations in these pots ranged from 0 to 8 µg of NO₃⁻-N g of dry sediment⁻¹ at harvest. In pots that received 1,434 or 2,905 µg of NO₃⁻-N g of dry sediment⁻¹, final concentrations varied between 10 and 48 µg of NH₄⁺-N g of dry sediment⁻¹ and between 200 and 1,600 µg of NO₃⁻-N g of dry sediment⁻¹, respectively. Higher input levels of NO₃⁻ resulted in increased numbers of potential nitrate-reducing bacteria and higher potential nitratereducing activity in the rhizosphere. In sediment samples from the rhizosphere, the contribution of denitrification to the potential nitrate-reducing capacity varied from 8% under NO₃⁻-limiting conditions to 58% when NO₃⁻ was in ample supply. In bulk sediment with excess NO₃⁻, this percentage was 44%. The nitrate-reducing community consisted almost entirely of NO₂⁻-accumulating or NH₄⁺-producing gram-positive species when NO₃⁻ was not added to the sediment. The addition of NO₃⁻ resulted in an increase of denitrifying Pseudomonas and Moraxella strains. The factor controlling the composition of the nitrate-reducing community when NO₃⁻ is limited is the presence of G. maxima. In sediment with excess NO₃⁻, nitrate availability determines the composition of the nitrate-reducing community.