Average responses of 30 marker genes to anaerobic phases. In steady state phase of a chemostat culture (D = 0.03 h-1, pH 5) the air supply was replaced with N2 for 20, 60, 120 and 180 min. Curves show the average variation of the entire marker gene set (Table 1) in each sample point from the 0 h sample (initial steady state). In the figure for 60 min, circles show the corresponding average response for 60 min without either air or medium feed and triangles show the average response to 60 min without medium feed, but with air. The average variation in the responses between different marker genes is indicated by the bars.