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Cyanobacteria
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Cyanobacteria
are prokaryotic organisms!
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They
lack
a nucleus and organelles (chloroplast, mitochondria)
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Circular
DNA, no chromosomes, no histone protein
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70S ribosomes
– smaller than eukaryotic
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Pigments:
Chl.a, phycobilins, carotenoids, (Chl.b)
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Rubisco
located in carboxysomes As true Bacteria, cyanobacteria contain peptidoglycan
or murein in their cell walls; cell walls are gram-negative
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They
lack flagella
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Unicellular,
colonies, filaments
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Trichomes
are individual cell filaments (sharing cell walls) inside a mucilage sheath
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False
branches are outgrowths of filaments adjacent to dead or specialized cells
(filaments merely take a „curve“)
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True
branches outgrow from cells that change their division axis 90° to
the trichome axis
Left: false branching in Scytenoema;
right: true branching in Stigonema
Cyanobacteria Systematics
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Classical
approach: growth form and presence/abscence
of morphological features: 5 groups (orders)
1. Unicelluar (rods or cocci) - Synechococcus
2. Pleurocapsalean - unicells which produce endospores - Dermocarpa
3. Non-heterocystous filaments - Spirulina, Oscillatoria, Trichodesmium.
4. Heterocystous unbranched filaments - Anabaena, Nostoc
5. Heterocystous branched filaments - Scytonema, Fisherella.
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Molecular
Systematics: heterocystous and non-heterocystous
forms in separate clusters; Prochlorophytes do not form cluster
Cyanobacteria Reproduction
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Asexual
reproduction
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Exospores:
bud off the end of a filament
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Endospores
(baeocytes): subdivision of cell into multiple units
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Akinetes:
large, thick-walled cells
Two Anabaena species with
akinetes (long, dark cells) and heterocysts (dark round cells)
Anabaena sp., young filaments
germinating from akinetes
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Hormogonium:
short filament from breakup of longer trichomes at specialized or dead
cells (necridia)
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Sexual
reproduction: unknown
Cyanobacteria Evolution
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Cyanobacteria
are the oldest photosynthetic organisms. Oldest fossils from Australia
date 3.5 billion years ago
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Cyanobacteria
changed our atmosphere into oxygen-rich air; they provided the basis for
today‘s lifeforms but caused mass mortality in old, anaerobic bacteria
forms
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Fossil
cyanobacteria appear very similar to today‘s forms and evolution seems
to have been very slow in this group
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Stromatolites
date back as early as 2.7 billion years ago. Maximum 700-800 million years
ago. Only 20 modern habitats
Cyanobacteria Photosynthesis
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Pigments:
Chl. a, b, carotenoids, phycobilins
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Chl.
b restricted to prochlorophytes
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Phycobilins:
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Phycocyanin,
phycoerythrin, allophycocyanin, bound to proteins
(phycobiliproteins)
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Phycobilins
close the „gap“ in the absorption spectra
of chlorophyll and carotenoids and act as antenna pigments
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Blue-green
CB contain phycocyanin, redish CB contain phycoerythrin
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Phycobiliproteins
are located in disk-shaped or hemispherical phycobilisomes
on the surface of thylakoids
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Phycoerythrin
exhibits yellow autofluorescence
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Carotenoids:
b-carotene,
xanthophylls (e.g. zeaxanthin)
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Chromatic
adaptation: some CB can change their color
and pigment composition in response to light quality
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Storage
products: cyanophycean
starch (does not react with iodine), cyanophycin
particles (amino acids), volatin
(polyphosphate particles)
Cyanophycin particles in the cyanobacterium Oscillatoria
(dark
blue to violet); centroplasm light blue; staining by Loeffler's methylene
blue, 1000x bright field
Cyanobacteria Nitrogen
Fixation
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Cyanobacteria
are the only phototrophic organisms that perform N2 fixation
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Nitrogenase
converts N2 into NH4+; encoded by nif
genes and requires 32 Fe per enzyme molecule
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High
energy required: 12-15 ATP per fixed N, H
provided by NADPH
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Heterocysts
are special cells for N2 fixation: thick cell wall, low oxygen
concentration, photosystem II (light reaction) inactive but photosystem
I active to provide ATP; connected to vegetative cells by cell wall pores
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Nitrogenase
activity: during the day in heterocystous
species, at night in non-heterocystous species; induced by low NH4+
concentrations in the environment
Cyanobacteria in Extreme
Habitats
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Anoxygenic
photosynthesis: cyanobacteria lacking photosystem
II perform photosynthesis without O2 production and use hydrogen sulfide
(H2S)
as electron donator: 2H2S + CO2 ---> CH2O
+ 2S + H2O
Thermophilic
forms: live in extremely high temperature of up to 70°C (Yellowstone
Park)
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