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The Origin of Eukaryotic Algae
  • Endosymbiosis theory: modern eukaryotic cells evolved from simple, phagotrophic cells that ingested bacteria and cyanobacteria; the prey was not digested, and physiological processes of the endosymbionts were used by the host.

  • Lateral DNA transfer relocated endosymbiont DNA into the host nucleus
  • Polyphyletic origin: Aquisition of endo- symbionts and rise of modern algae occured several times during evolution; algae as a group do not have one common ancestor

  • Primary chloroplasts
  • The Loss of Chloroplasts:
    • Heterotrophic algae: Some relatives of algae, predominantly parasites, did loose their plastids during evolution, or their plastids became inactive
    • Toxoplasma gondii: A parasite in mammal muscular tissues; ca. 60% of domestic cats are infected by Toxoplasma
    • Human infection occurs but is of minor impact in healthy people; babies and humans with immune deficiencies (AIDS) are prone to Toxoplasmosis
    • Toxoplasmosis: transfer from mother to unborn baby can result in mental retardation, blindness, or death; AIDS patients may suffer severe brain damage leading to death

    Toxoplasma gondii: left upper shows flagellates; right upper: Toxoplasma in cardiomuscular tissue; lower: EM thin section
  • Chloroplasts by secondary endosymbiosis:
    • Origin: phagocytosis of prey with primary endosymbiont
    • Plastids possess 2 outer membranes
    • Algal groups: Euglenophytes, Cryptophytes, Chlorarachniophytes, Haptophytes, diverse Chromophytes (Ochrophytes)
    • Nucleopmorph: extremely reduced eukaryotic nucleus between outer two membranes of the plastid, thought to origin from the nucelus of the primary eukaryote


The Chlorarachniophyte Gymnochlora stellata exhibits an amoeboid growth form with secondary plastids
  • Tertiary Endosymbiosis in Dinoflagellates
    • Origin: phagotrophy of prey with secondary plastid by a heterotrophic dinoflagellate
    • Endosymbiont reduction: the endosymbiont is reduced to loose its primary and secondary nucleus; DNA is laterally transferred into the dinoflagellate nucleus (extremely large)
    • Rubisco differs in dinoflagellates in that it is only composed of large subunits (not 8/8 small/large) and only 25-30% homologous to other algae‘s Rubisco sequence; origin from proteobacteria or lateral gene transfer from mitochondria, which originated from proteo-bacteria
    • Phycoerythrin can be detected in Dinophysis

    Autofluorescence microscopy of the dinoflagellate Dinophysis norwegica (upper picture in bright field): left: blue-light excitation causes red chl.a fluorescence and yellow phycoerythrin fluorescence; right: green-light excitation produces bright red phycoerythrin fluorescence (no chl. a fluorescence under green light excitation!)

  • Algal Symbionts in Animals
    • Algal endosymbionts occur in freshwater and marine protozoa, sponges, coelenterates, flat-worms, molluscs
    • Zoochlorellae: most common symbiont in freshwater animals is green alga Chlorella

    • Zooxanthellae: dinoflagellate Symbiodonium as symbiont in corals, sponges and others

    From upper row: Nudibranch snail with zooxanthellae; cross section through nudibranch filament showing layer of endosymbiontic zooxanthellae (zoox). Middle row: soft coral with zooxanthellae; the cross section through the coral tentacles reveals that zooxanthellare in this coral species are not living within the coral tissue but in special groves in the tentacles. Lower row: left shows a coral with zooxanthellae photographed under blue light, which made the zooxanthellae's chlorophyll fluoresce red; right: isolated zooxanthellae from coral tissue.

    • Marine Systems: other endosymbionts than Symbiodinium occur as well: Chlorella sp., diatoms, cryptophytes, unicellular Red algae

  • Kleptoplastids
    • Kleptoplastids: Some heterotrophic flagellates and ciliates ingest algae and maintain them for a limited time
    • Origin: various algae, mixture of plastids
    • No symbiosis: the hosts lack essential genes expressed by the algae, and the chloroplasts lack these gene products because the prey nucleus is digested; kleptoplastids are a temporary solution
    • Oligotrich ciliates are important marine microzooplankton, which are often obligate or facultative mixotrophs
    • Sea slug (Elysia; mollusc) collects plastids from green seaweeds
    • Some dinoflagellates possess klaptoplastids instead of symbionts