Comparative Ecophysiology of the Xanthophyll Cycle in Six Marine Phytoplanktonic Species

The ecophysiology of the photoprotective xanthophyll cycle (XC) was compared in six chlorophyll c-containing pico- and nano-phytoplankton species. Different accessory pigment combinations and ecological properties characterize the six studied species, Bolidomonas mediterranea, Pelagomonas calceolata, Phaeocystis cordata, Phaeocystis sp. (strain RCC186), Mesopedinella arctica and Ochromonas sp. The first experimental set consisted in the study of the activity of the xanthophyll cycle and non-photochemical quenching (NPQ) during two gradual light increases, with photon flux density (PFD) ranging from 40 to 400 μmol photons m−2 s−1. Pigments, absorption spectra, flow cytometry measurements for cell counts and chlorophyll a autofluorescence and Electron Transport Rate (ETR) vs. light curves were determined at different times during the experiment. The second set of experiments consisted in using two inhibitors: carotenogenesis inhibitor (norflurazon) and de-epoxidation step (occurring in the xanthophyll cycle) inhibitor (dithiotreitol) during high-light shift, to compare the functioning of the xanthophyll cycle among the different species. Results highlighted a large diversity in the xanthophyll cycle functioning, possibly related to the ecological traits of the species. In view of the results, three groups of species have been identified as (i) high light-adapted, (ii) low light-adapted and (iii) variable light-adapted species.