The objective of this Study was to evaluate the effect of the photoperiod oil the biomass production and carbondioxide fixation rates using a photosynthetic Culture of the cyanobacterium Aphanothece microscopica Nageli in bubble column photobioreactors. The cultures were carried Out at temperatures of 35 degrees C, air enriched with carbon dioxide at concentrations of 15% and photon flux density of 150 mu mol m(-2) s(-1). The light cycles evaluated were 0:24, 2:22, 4:20. 6:18, 8:16, 10:14, 12:12, 14:10, 16:8, 18:6, 20:4, 22:2 and 24:0 (night:day), respectively. The results obtained indicated that the duration of the light periods was a determinant factor in the performance of the photobioreactors. A linear reduction in biomass production and carbon dioxide fixation with reductions in the duration of the light period was evident, with the exception of the 12:12 (night:day) cycles. Reductions of up to 99.69% in the carbon-fixation rates as compared with cultures under continuous illumination were obtained. (c) 2008 Elsevier B.V. All rights reserved.