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  • Cyanobacteria are prokaryotic organisms!
  • They lack a nucleus and organelles (chloroplast, mitochondria)

  • Circular DNA, no chromosomes, no histone protein
  • 70S ribosomes – smaller than eukaryotic
  • Pigments: Chl.a, phycobilins, carotenoids, (Chl.b)
  • Rubisco located in carboxysomes As true Bacteria, cyanobacteria contain peptidoglycan or murein in their cell walls; cell walls are gram-negative
  • They lack flagella
  • Unicellular, colonies, filaments
  • Trichomes are individual cell filaments (sharing cell walls) inside a mucilage sheath
  • False branches are outgrowths of filaments adjacent to dead or specialized cells (filaments merely take a „curve“)
  • 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
  • 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
  • Molecular Systematics: heterocystous and non-heterocystous forms in separate clusters; Prochlorophytes do not form cluster

Cyanobacteria Reproduction
  • Asexual reproduction 
    • Exospores: bud off the end of a filament
    • Endospores (baeocytes): subdivision of cell into multiple units
    • 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
    • Hormogonium: short filament from breakup of longer trichomes at specialized or dead cells (necridia)

  • Sexual reproduction: unknown
Cyanobacteria Evolution
  • Cyanobacteria are the oldest photosynthetic organisms. Oldest fossils from Australia date 3.5 billion years ago

  • 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
  • Fossil cyanobacteria appear very similar to today‘s forms and evolution seems to have been very slow in this group
  • Stromatolites date back as early as 2.7 billion years ago. Maximum 700-800 million years ago. Only 20 modern habitats

Cyanobacteria Photosynthesis
  • Pigments: Chl. a, b, carotenoids, phycobilins
  • Chl. b restricted to prochlorophytes
  • Phycobilins:
    • Phycocyanin, phycoerythrin, allophycocyanin, bound to proteins (phycobiliproteins)
    • Phycobilins close the „gap“ in the absorption spectra of chlorophyll and carotenoids and act as antenna pigments
    • Blue-green CB contain phycocyanin, redish CB contain phycoerythrin
    • Phycobiliproteins are located in disk-shaped or hemispherical phycobilisomes on the surface of thylakoids
    • Phycoerythrin exhibits yellow autofluorescence

  • Carotenoids: b-carotene, xanthophylls (e.g. zeaxanthin)
  • Chromatic adaptation: some CB can change their color and pigment composition in response to light quality
  • 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
  • Cyanobacteria are the only phototrophic organisms that perform N2 fixation
  • Nitrogenase converts N2 into NH4+; encoded by nif genes and requires 32 Fe per enzyme molecule
  • High energy required: 12-15 ATP per fixed N, H provided by NADPH
  • 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
  • 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
  • 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)