The Acquisition of Plastids/Phototrophy in Heterotrophic Dinoflagellates

Several dinoflagellates are known to practice acquired phototrophy by either hosting intact algal endosymbionts or retaining plastids. The acquisition of phototrophy in dinoflagellates appears to occur independently over a variety of orders, rather than being restricted to any specific order(s). While dinoflagellates with intact algal cells host endosymbionts of cyanobacteria, pelagophyte, prasinophyte or dictyochophyte, most organelle-retaining dinoflagellates acquire plastids from cryptophytes. In dinoflagellates with acquired phototrophy, the mechanism by which symbionts or plastids are obtained has not been well studied at sub-cellular or ultrastructural level, and thus little is known regarding their mechanism to sequester and maintain photosynthetic structures, except for three cases, Amphidinium poecilochroum, Gymnodinium aeruginosum, and Dinophysis caudata with peduncle feeding. Dinoflagellates with acquired phototrophy display different degrees of reduction of the retained endosymbiont and organelles, ranging from those which contain intact whole algal cells (e.g. green Noctiluca scintillans), to those which have retained almost a full complement of organelles (e.g., Amphidinium poecilochroum and Podolampas bipes), to those in which only the plastids remain (e.g., Amphidinium wigrense and Dinophysis spp.). A series of events leading to acquisition and subsequent degeneration of a whole-cell endosymbiont have been widely recognized as evolutionary pathway of the acquisition of plastids. However, recent work on D. caudata suggests that acquisition of phototrophy by predation (i.e. kleptoplastidy) may be a mechanism and evolutionary pathway through which plastids originated in dinoflagellates with ‘foreign’ plastids other than the ‘typical’ peridinin-type plastids. Most organelle-retaining dinoflagellates are facultative mixotrophs, with Dinophysis species and an undescribed Antarctic dinoflagellate being the only obligate mixotrophs known so far. The establishment of dinoflagellates with acquired phototrophy in cultures and careful research using the cultures would help improve our knowledge of the evolution of the dinoflagellate plastids and their ecophysiology