The phenanthrene-acclimatised cultures were then added just before connecting each serum bottle to a respirometer cell. The respirometer system replaced the O2 consumed in the sealed headspace of the bottles and measured the O2 uptake. Any CO2 generated during respiration was absorbed by a 50% KOH solution fixed at the mouth of each bottle. The O2 taken up for the degradation of the surfactants was monitored over time using a computer data acquisition program. When the O2 uptake rate of all the serum bottles became constant, the results were compared with each other.