Time course of the enzyme activity of hydrolases of the biofilm in the percolator. (a) Values of xylanase, cellulase, and amylase activities. (b) Values of aminopeptidase and protease activity. Depth 1 (D1) corresponds to a sampling of about 100 cm and depth 2 (D2) to approximately 20 cm below the surface. The data points are arithmetic means of two independent experiments, while the error bars denote the standard deviation.