Colourless dinoflagellates may establish endosymbiotic relationships with photosynthetic algae on different levels of organisation. These range from undefined numbers of endosymbionts in Noctiluca, binucleated stages with morphologically preserved endosymbionts in Peridinium foliaceum and P. balticum to a mononucleated subcellular structure. In the last case the endosymbiotic origin of the chloroplasts is revealed by a class-deviating pigmentation. A scheme involving subcellular organisation is presented to illustrate key steps in the establishment of endosymbiosis. Since chloroplast capture by horizontal evolution is a large evolutionary step, increased emphasis on photosynthetic capability when delimitating taxonomic entities should be considered.

?One group of dinoflagellates, including the toxic species Gymnodinium galatheanum, Gym. breve and Gyrodinium aff. aureolum, contains fucoxanthin and its two 19’-acyloxyderivatives as main carotenoids. A detailed spectrometric reinvestigation of Gym. galatheanum, including 1H-NMR and CD, showed the 3 fucoxanthins to possess the same chiral structures as in other algal classes. Some minor xanthophylls are structurally related to pigments first isolated as animal metabolic products in the marine food chain, including 3 new esters related to halocynthiaxanthin. A hypothesis evaluating the possibility of joint host/endosymbiont interaction in carotenoid synthesis is discussed in terms of the endosymbiont theory. The carotenoids of Gym. galatheanum and Pelagomonas calceolata are compared. Guided by the taxonomic distribution pattern of fucoxanthins and gyroxanthin diester, our earlier hypothesis on a prymnesiophycean origin of the chloroplasts is extended to also include a possible pelagophycean origin.