In this study we compared the biomass productivity and the chemical composition of the diatom Phaeodactylum tricornutum grown outdoors at different biomass concentrations, in open ponds and photobioreactors (PBRs). Optimal biomass concentration of 0.6 gL(-1) and 1.0 g L-1 was found in open ponds and in PBRs, respectively. During summer the mean net areal biomass yield was 11.7 g m(-2) day(-1) and 13.1 g m(-2) day(-1) in open ponds and in PBRs, respectively. Night biomass loss was comparable (17.4% and 21.4% of the daylight productivity, in open ponds, and PBRs, respectively). Lipid content ranged between 25% and 27.5% of dry weight, and increased up to 34.7% of biomass grown in dense cultures (>0.6 g L-1 in ponds; >1.0 g L-1 in PBRs). In the evening they ranged between 21% and 31%, while a reduced amount was found in the morning between 14.5% and 24%. An induction of the diadino diatoxanthin cycle was observed in the cultures when they were grown at lower biomass concentrations, particularly in cultures grown in photobioreactors, indicating down-regulation of the photosynthetic apparatus due to high irradiance, which was confirmed by a sizeable reduction in the F-v/F-m ratio in the middle of day. It was found that the productivity of cultures was higher in photobioreactors compared to that in open ponds most likely as a result of a better light dark regime experienced by the cells in short light-path tubular PBRs, which may have allowed a more efficient use of light. (C) 2013 Elsevier Ltd. All rights reserved.