Photoacclimation of Phaeodactylum tricornutum (Bacillariophyceae) cultures grown outdoors in photobioreactors and open ponds

Abstract

The acclimation to high light of Phaeodactylum tricornutum cultures grown outdoors both in an open pond and in tubular photobioreactors (PBRs) was studied by means of chlorophyll fluorescence, pigment analysis and growth. Cultures grown in PBRs (5-cm tube diameter) at two biomass concentrations (0.3 and 0.6 g l−1) were compared with a culture grown in a 10-cm deep open pond (0.3 g l−1). Therefore, the performance of the cultures was compared on the basis of both the same biomass concentration and areal density. Cultures grown at 0.3 g l−1 in PBRs experienced the highest light stress, which resulted in dramatic changes in both chlorophyll fluorescence and photosynthesis parameters, and in low areal productivity. In this culture, the Fv/Fm ratio was 0.5 in the morning and dropped to about 0.1 within 1 h of exposure to bright sunlight. Similar behaviour was observed with the measurements of rETR, and the initial slope (αETR) of the photosynthesis curve, while the saturation irradiance parameter (Ik) increased about four-fold compared to the morning value. These changes were accompanied by a higher induction of the diadinoxanthin-cycle pigments, evidenced by an increasing ratio of diatoxanthin to diadinoxanthin during the day (up to 104% at 1400 h), and a higher value of non-photochemical quenching (NPQ) that rose to up to 1.2 in the high stress culture. The culture grown in the open pond at the same biomass concentration (0.3 g l−1) did not show sizeable changes either in pigments or in chlorophyll fluorescence parameters. Denser cultures in PBRs (0.6 g l−1) showed less dramatic changes in the photochemical parameters. Changes in the areal productivity of the cultures correlated well with changes in the photochemical parameters. Areal productivity was about 27% higher in the open pond than in PBRs when cultivated at the same concentration of 0.3 g l−1.