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The Origin
of Eukaryotic Algae
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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.
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Lateral
DNA transfer relocated endosymbiont DNA into
the host nucleus
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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
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Primary
chloroplasts:
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The Loss
of Chloroplasts:
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Heterotrophic
algae: Some relatives of algae, predominantly
parasites, did loose their plastids during evolution, or their plastids
became inactive
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Toxoplasma
gondii: A parasite in mammal muscular
tissues; ca. 60% of domestic cats are infected by Toxoplasma
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Human
infection occurs but is of minor impact in
healthy people; babies and humans with immune deficiencies (AIDS) are prone
to Toxoplasmosis
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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
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Chloroplasts
by secondary endosymbiosis:
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Origin:
phagocytosis of prey with primary endosymbiont
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Plastids
possess 2 outer membranes
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Algal
groups: Euglenophytes, Cryptophytes, Chlorarachniophytes,
Haptophytes, diverse Chromophytes (Ochrophytes)
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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
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Tertiary
Endosymbiosis in Dinoflagellates
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Origin:
phagotrophy of prey with secondary plastid by a heterotrophic dinoflagellate
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Endosymbiont
reduction: the endosymbiont is reduced to
loose its primary and secondary nucleus; DNA is laterally transferred into
the dinoflagellate nucleus (extremely large)
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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
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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!)
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Algal
Symbionts in Animals
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Algal
endosymbionts occur in freshwater and marine
protozoa, sponges, coelenterates, flat-worms, molluscs
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Zoochlorellae:
most common symbiont in freshwater animals is green alga Chlorella
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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.
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Marine
Systems: other endosymbionts than Symbiodinium
occur as well: Chlorella sp., diatoms, cryptophytes, unicellular
Red algae
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Kleptoplastids
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Kleptoplastids:
Some heterotrophic flagellates and ciliates ingest algae and maintain them
for a limited time
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Origin:
various algae, mixture of plastids
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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
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Oligotrich
ciliates are important marine microzooplankton,
which are often obligate or facultative mixotrophs
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Sea slug
(Elysia; mollusc) collects plastids from green seaweeds
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Some
dinoflagellates possess klaptoplastids instead
of symbionts
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