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

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 mu g 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 mu g of NO3--N g of dry sediment(-1). Nitrate concentrations in these pots ranged from 0 to 8 mu g of NO3--N g of dry sediment(-1) at harvest. In pots that received 1,434 or 2,905 mu g of NO3--N g of dry sediment(-1), final concentrations varied between 10 and 48 mu g of NH4+-N g of dry sediment(-1) and between 200 and 1,600 mu g 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 nitrate-reducing 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.