Pigment transfer from phytoplankton to zooplankton with emphasis on astaxanthin production in the Baltic Sea food web

The carotenoid astaxanthin is a powerful antioxidant and a compound of vital importance for many marine organisms such as crustaceans and salmonids. Although astaxanthin deficiency may have serious consequences for ecosystem functioning, its origin and transfer in the food web have been little studied. Astaxanthin is produced by planktonic crustaceans, but these cannot synthesise carotenoid pigments de novo; they rely on the availability of astaxanthin precursors through the consumption of phytoplankton algae. We performed 4 laboratory experiments to test how the production of astaxanthin in wild pelagic copepod communities (mainly consisting of Acartia) is affected when different phytoplankton communities (unfertilised, fertilised with N and P, or fertilised with N, P and Si) are supplied as prey. We show that phytoplankton community composition and biomass have profound effects on the production of astaxanthin in calanoid copepods. When they were grazing on a diverse phytoplankton community with high biomass dominated by chlorophytes, dinoflagellates and diatoms with thin silica frustules, astaxanthin production in the copepods was highest. It was lower when the copepods were fed with low phytoplankton biomass or high biomass dominated by large heavily silicified diatoms; these diatoms were not consumed and grazing was mainly on prymnesiophytes. When the astaxanthin production was high, maximum astaxanthin content per copepod individual of about twice the initial level was reached during sunrise. These results suggest increased feeding activity and astaxanthin production during the night and utilisation of astaxanthin for photoprotection and other antioxidant activities during the day.