Nitrification, denitrification and growth in artificial Thiosphaera pantotropha biofilms as measured with a combined microsensor for oxygen and nitrous oxide

Abstract

Cells of the aerobic denitrifier and heterotrophic nitrifier Thiosphaera pantotropha and of the traditional denitrifier Paracoccus denitrificans were immobilized in a 1.5 mm thick agar layer (biofilm) and submersed in liquid medium. A combined microsensor for O 2 and N 2O was used to record microprofiles of these two species in biofilms where the reduction of N 2O was inhibited by acetylene. Nitrification in T. pantotropha was not affected by the addition of acetylene and by using a diffusion-reaction model to simulate the N 2O profiles it was possible to calculate depth profiles of both nitrification and denitrification. The validity of the calculations when both nitrification and denitrification were operating in concert was confirmed by performing identical calculations on data obtained for a P. denitrificans biofilm. At high NO 3 − concentrations, part of the NO 3 − reduced by T. pantotropha biofilms was reduced only to NO 2 − and N 2O production thus did not reflect total NO 3 − reduction. When NO 2 − and no NO 3 − was present in the water above the biofilm N 2O production was recorded in the anoxic zone directly below the oxic zone. Nitrous oxide production was never detected in the oxic zone of the biofilms, although aerobic denitrification was described for the original isolate of this bacterium. The growth rate of T. pantotropha in the oxic region of the biofilms was estimated to be 0.42 h −1 which is slightly higher than rates previously obtained in liquid culture. In the T. pantotropha biofilms nitrification was calculated to account for more than 50% of the O 2 consumption whereas this process only consumed about 10% of the O 2 in liquid culture.