Laboratory experiments were performed to study the capacity of CO2 sequestration and carbon fixation into biomass during the cultivation of the cyanobacteria Aphanothece microscopica Nageli in refinery wastewater. The influence of the photoperiod (day/night) on the rates of CO2 sequestration and O-2 release was also determined. Rates of CO2 sequestration were measured both in the liquid and gaseous phases. The results showed that the capacity of CO2 sequestration and O-2 release during the day/night experiment was about one-fourth less than that achieved in the continuously illuminated experiment. Equivalence was found between rates of CO2 sequestration measured in the two phases. Despite large amounts of CO2 that were sequestered during the cultivation, it is demonstrated that only a small fraction (about 3%) was effectively fixed as microalgae biomass, indicating the existence of other routes of CO2 conversion in the photobioreactor. (C) 2009 Elsevier Ltd. All rights reserved.