Exercise 2: Photosynthetic Prokaryotes

• Eubacteria


  Plant allies, some members like cyanobacteria and chloroxybacteria contributed to the chloroplast lineage of plants (endosymbiotic theory)
• Prokaryotes


  • Lack organelles like nucleus, chloroplast, mitochondria, dictyosomes, lysosomes, and true vacuoles
  • Mostly unicellular, may aggregate into colonial and filamentous forms
  • Cells enclosed by peptidoglycan cell wall and outer sheath
  • Cytoplasmic membrane shows invaginations called mesosomes responsible for respiration
  • Centrally located nuclear zone or nucleoid region and peripheral zone with granular and/or lamellar structures
  • Reproduction only by asexual means like binary fission, fragmentation, and/or spore formation
• Photosynthetic prokaryotes studied belong to the kingdom Eubacteria, divisions Cyanophyta and Prochlorophyta
• Cyanobacteria (Cyanophytes)


  • Photosynthetic mainly due to chlorophyll a, accessory pigments are phycobilins (phycocyanin and phycoerythrin)
  • Colors range from bright blue-green to golden brown due to varying composition of pigments
  • Forms vary from unicellular to colonial and filamentous
• Chloroxybacteria (Prochlorophytes)


  • Photosynthetic, main accessory pigment is chlorophyll a and b like higher green plants, no phycobilins
  • Include unicellular, filamentous forms; free-living or associated types
• Anabaena azollae


  • Chain-like organization of cells
  • Heterocysts - yellowish or lighter green than vegetative cells
  • Akinetes - bigger, darker, and more granulated than vegetative cells, resting spores and reproductive agents
  • Vegetative cells
• Unicellular and colonial cyanobacteria

  1. Distinct parts like in Anabaena azollae

  2. Genus identification
• Filamentous cyanobacteria


  • Trichome - aggregation of cells that divide only in one direction
  • Filament - trichome and enclosing gelatinous sheath
  • Somatic cells - morphologically alike cells in filaments (homocystous)
  • Necridia - dead cells that look like biconcave separation discs
  • Hormogonia - fragmented, motile filament segments
  • Colonial sheath - gelatinous substance holding colony together
  • Polar nodule - glistening, spherical structure at crosswall between heterocyst and adjacent cell
• Branching types in filamentous cyanobacteria

  • False branch - initiated without cell division in new plane, trichomes/hormogonia develop faster than enclosing sheath
  • True branch - initiated by division of certain cells in new plane to axial filament
• Prochloron didemni (Chloroxybacteria)


  • Cell wall - multi-layered, appears to lack external sheath
  • Thylakoid - wide, usually peripheral band; stacked usually in pairs; stacked lamellae form discontinuous bands
  • Cytoplasm - appears to occupy two interconnected zones, narrow band and matrix for thylakoids
• Photosynthetic pigments


  Used to create cladogram showing relationships between cyanobacteria, chloroxybacteria, and plants
• Cell wall


  • Multi-layered, appears to lack an external sheath
• Thylakoid


  • Wide, usually peripheral band; stacked usually in pairs; stacked lamellae form discontinuous bands
• Cytoplasm


  • Appears to occupy two interconnected zones; the first one forms a narrow band and the second forms a matrix for the thylakoids
• Morpho-cytological differences between various types of cells in Anabaena azollae, a heterocystous filamentous cyanobacteria

  • Somatic cell
  • Heterocyst
  • Akinete
• Somatic cell (sc)


  • Shape
  • Color
  • Presence of granulation (+/-)
  • Presence of polar nodules or number of nodules per cell
  • Relative thickness of cell wall (relative to sc)
• Heterocyst


  • Shape
  • Color
  • Presence of granulation (+/-)
  • Presence of polar nodules or number of nodules per cell
  • Relative thickness of cell wall (relative to sc)
• Akinete


  • Shape
  • Color
  • Presence of granulation (+/-)
  • Presence of polar nodules or number of nodules per cell
  • Relative thickness of cell wall (relative to sc)