The light intensity at the backside of the culture was detected with a PAR photon flux density sensor that was connected to an ADAM-5000 data acquisition card. The light transmitted through the culture (and consequently the turbidity) was evaluated every minute and compared to the set point, i.e. the amount of light transmitted 24 h after the start of light stress (see above). This set point was controlled via a Lab View virtual instrument running on a PC (Lab View 7.1, National Instruments), by automatically switching on or off the feed medium pump. The culture volume was maintained constant by continuous removal of culture broth via a horizontal out- let tube connected to a long vertical tube. This vertical tube had one end (bended upwards) in the culture broth and the top end was situated high above the surface level. This way a constant level of the aqueous phase could be realised even with an organic phase on top.