Growth and photosynthetic pigments of Gracilariopsis tenuifrons (Rhodophyta, Gracilariaceae) under high light in vitro culture

High levels of irradiance may affect the growth and development of photosynthetic organisms, changing concentrations of carotenoids and chlorophylls. These changes may indicate different photoprotection strategies. In this study, gametophytic apical portions of Gracilariopsis tenuifrons were cultivated under controlled laboratory conditions for 1 week, at different light irradiances: 60 (control), 600, and 1,000 μmol photons m⁻²s⁻¹. Growth rate, amount, and composition of pigments were analyzed daily. Color of seaweeds exposed to 600 and 1000 μmol photons m⁻²s⁻¹varied along the days, from red to yellowish, suggesting a decrease in vital processes as photosynthesis and growth. However, no decrease in biomass was observed. Actually, there was an increase at growth rates for the algae kept under higher light intensities. The main registered pigments were chlorophyll a, β-carotene, and zeaxanthin. β-carotene and chlorophyll a levels were lower in algae exposed to high light intensity. In treatment exposed to 600 μmol photons m⁻²s⁻¹, this reduction was 42 and 35 %, respectively, while in those exposed to 1000 μmol photons m⁻²s⁻¹the values were 55 and 50 % lower than the control. The lower levels of these pigments may be associated with the reduction in energy harvesting by the photosynthetic complexes-antennae, in an effort to dissipate the high excitation impinged over the photosynthesis system as a whole. For zeaxanthin levels, a 20 % increase was observed in the beginning of the experiment, which was followed by a drop to the initial levels, suggesting the role of this pigment in this alga’s photoprotection process.