To test the feasibility of CO2 remediation by microalgal photosynthesis, a modified type of flat-plate photobioreactor [Hu et al. (1996) Biotechnol Bioeng 51:51-60] has been designed for cultivation of a high-CO2-tolerant unicellular green alga Chlorococcum littorale. The modified reactor has a narrow light path in which intensive turbulent flow is provided by streaming compressed air through perforated tubing into the culture suspension. The length of the reactor light path was optimized for the productivity of biomass. The interrelationship between cell density and productivity, as affected by incident light intensity, was quantitatively assessed. Cellular ultrastructural and biochemical changes in response to ultrahigh cell density were investigated. The potential of biomass production under extremely high CO2 concentrations was also evaluated. By growing C. littorale cells in this reactor, a CO2 fixation rate of 16.7 g CO2 l(-1) 24 h(-1) (or 200.4 g CO2 m(-2) 24 h(-1)) could readily be sustained at a light intensity of 2000 mu mol m(-2) s(-1) at 25 degrees C, and an ultrahigh cell density of well over 80 g l(-1) could be maintained by daily replacing the culture medium.