Abstract
In most rhodophytes, the quenchers zeaxanthin or lutein seem to be used for carotenoid photoprotection next to β-carotene. However, our study shows that the tropical intertidal alga Gracilaria birdiae possesses high concentrations of antheraxanthin, as well as lower concentrations of both zeaxanthin and violaxanthin. In higher plants and some algal groups, these three oxygenated carotenoids are known to protect photosystem II in a xanthophyll cycle. We were able to manipulate the carotenoid concentrations in G. birdiae by varying irradiance and temperature in laboratory experiments. Higher concentrations of violaxanthin were found in dark incubations and higher concentrations of zeaxanthin in light incubations, which is an indication that a xanthophyll cycle is active in G. birdiae. Our results also suggest a biosynthesis pathway from lycopene to β,β-carotene and β-cryptoxanthin in the light and synthesis of new lycopene in darkness. This would imply that the production of functional carotenoids is regulated by their need in photoprotection. Despite cultivation in the laboratory under the same conditions for at least 6 years, there were differences in photosynthetic properties and carotenoid dynamics between algae obtained from a population close to the Equator and another population close to the Tropic of Capricorn. These differences most probably have their origin in genetic differentiation mediated by ambient irradiance and temperature regimes. Four phycoerythrin-deficient mutant strains of G. birdiae were studied as well. Photosynthetic properties and carotenoid dynamics were basically the same in the mutants and the wild strains, but two green strains had remarkably low dark respiratory rates. This is an indication of low production of metabolites, which may be a competitive disadvantage.