The growth yield of the PUFA-producing marine microalga Isochrysis galbana grown in Eight-limited continuous operation was measured under a wide variety of conditions of incident irradiance (I-o) and dilution rates (D). The experiments were conducted under laboratory conditions at 20 degrees C with continuous light. D ranged from 0.0024-0.0410 h(-1) at three values of I-o (820; 1,620; and 3,270 mu E m(-2) s(-1)) close to those found in outdoor cultures. A maximum efficiency of psi(max) = 0.616 g E-1 was obtained at I-o = 820 mu E m(-2) s(-1) and D = 0.030 h(-1), and the maximum capacity of the biomass to metabolize light was found to be 13.1 mu E g(-1) s(-1). Above this value, a significant drop in system efficiency was observed. A new approach based on averaged irradiance is used to assess the photon flux absorbed by the biomass. The biomass productivity to be expected in an outdoor tubular photobioreactor growing I. galbana was predicted from efficiency data obtained in laboratory cultures and was found to be in agreement with the actual value.