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 evalu- ated 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.