Denitrification and oxygen respiration in biofilms studied with a microsensor for nitrous oxide and oxygen

Depth distributions of O2 respiration and denitrification activity were studied in 1- to 2-mm thick biofilms from nutrient-rich Danish streams. Acetylene was added to block the reduction of N2O, and micro-profiles of O2 and N2O in the biofilm were measured simultaneously with a polarographic microsensor. The specific activities of the two respiratory processes were calculated from the microprofiles using a one-dimensional diffusion-reaction model. Denitrification only occurred in layers where O2 was absent or present at low concentrations (of a fewμM). Introduction of O2 into deeper layers inhibited denitrification, but the process started immediately after anoxic conditions were reestablished. Denitrification activity was present at greater depth in the biofilm when the NO3 (-) concentration in the overlying water was elevated, and the deepest occurrence of denitrification was apparently determined by the depth penetration of NO3 (-). The denitrification rate within each specific layer was not affected by an increase in NO3 (-) concentration, and the half-saturation concentration (Km) for NO3 (-) therefore considered to be low (<25μM). Addition of 0.2% yeast extract stimulated denitrification only in the uppermost 0.2 mm of the denitrification zone indicating a very efficient utilization of the dissolved organic matter within the upper layers of the biofilm.