Carotenoid composition in Rhodophyta: insights into xanthophyll regulation inCorallina elongata

Macroalgae possess several photoprotection mechanisms, including xanthophyll cycles. Among these cycles, the VAZ (violaxanthin–anteraxanthin–zeaxanthin) cycle controls the interconversion of epoxidated xanthophyll (V) to de-epoxidated forms anteraxanthin and zeaxanthin. This conversion modulates the amount of excess energy that it is dissipated as heat. Presence of functional xanthophyll cycles in Rhodophyta is controversial. In this work we investigated the presence of xanthophyll cycles in red algae. Carotenoid composition of 13 red macroalgae collected in northern Spain was studied by HPLC. This screening showed that carotenoid composition is a conservative trade in rhodophytes, with very few exceptions to the general pattern formed by β-carotene and one to three xanthophylls: anteraxanthin, zeaxanthin or lutein. Corallina elongata and Jania rubens were the only algae containing anteraxanthin as the main xanthophyll. The first species was selected to study whether any truncated xanthophyll cycle between anteraxanthin and zeaxanthin could operate under photoinhibitory conditions. Upon illumination, xanthophyll composition remained stable, without any short-term light-induced de-epoxidation of anteraxanthin to zeaxanthin. However, an inverse relation between both xanthophylls was found. In addition, a seasonal trend of changes in anteraxanthin and zeaxanthin was observed when pigment composition was studied in field samples over the course of a year. We concluded that these variations were more likely due to differential rates of synthesis and degradation of xanthophylls than to the operation of a xanthophyll cycle. The great amount of anteraxanthin in C. elongata may play an structural stability role on light-harvesting complexes.