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Zooplankton synthesise astaxanthin, a carotenoid pigment believed to protect against high-energy irradiance, from precursors in their diet. Different patterns of astaxanthin vertical distribution would be expected from the benefits of photoprotection, the costs of visual predation and the availability of food. Despite a highly resolved sampling approach (4 m depth intervals), no clear pattern of vertical distribution or correlation with chlorophyll a concentrations was found for Calanus helgolandicus astaxanthin concentrations in the German Bight. This may be attributable to photochromatic adaptation in phytoplankton, diurnal rhythms of copepod grazing and, particularly, vertical migration. A 2 × 2 factorial (light × food) experiment showed that total astaxanthin concentrations in C. helgolandicus were entirely determined by the dynamics of esterified astaxanthin. As expected, the concentrations of astaxanthin esters decreased in starved individuals in the dark, while the combined presence of food and light resulted in an increase of esterified astaxanthin. Similar levels of esterified astaxanthin were maintained in both starving individuals exposed to light and feeding individuals in the dark. The latter implies that astaxanthin esterification is per se a process independent of light, although light exposure enhances it. It is proposed that the function of astaxanthin esters is to generally improve the antioxidant protection of storage lipids, also in situations where photoprotection is not required.

Astaxanthin in the calanoid copepod Calanus helgolandicus: dynamics of esterification and vertical distribution in the German Bight, North Sea