A STUDY ON SIMULTANEOUS PHOTOLIMITATION AND PHOTOINHIBITION IN DENSE MICROALGAL CULTURES TAKING INTO ACCOUNT INCIDENT AND AVERAGED IRRADIANCES

From chemostat cultures of the marine microalga Isochrysis galbana (CC AP 927/15) simultaneous photolimitation and photoinhibition was observ ed. The extent of each phenomenon depends on the light gradient inside the culture and therefore on the incident irradiance. Variations in b iomass concentration and average irradiance inside the culture with di lution rate at three incident irradiances, I-o, were studied (from I-o = 820 to 3270 mu E m(-2) S-1). At I-o above 1630 mu E m(-2) s(-1) a p hotoinhibition effect was observed, although the specific growth rate remained a hyperbolic function of average irradiance regardless of inc ident irradiance. To calculate average irradiance, a three-dimensional irradiance distribution model for cylindrical geometry is proposed, i mproving the estimation of the irradiance field inside culture with re gard to other methods used up to now since the variations in illuminat ion along the vertical axis are considered. Lastly, a new approach to model simultaneous photolimitation and photoinhibition is proposed by considering that specific growth rate is related to average irradiance and that parameters representing the cell adaptability to light are a function of the maximum irradiation at which cells are exposed, that is, the incident irradiance.