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

Abstract

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 (K_m) 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.