Exercise 2 - Photosynthetic Prokaryotes

• Eubacteria are considered as plant allies because some of their members, e.g., the cyanobacteria and chloroxybacteria (prochlorophytes), have contributed to the chloroplast lineage of the plants (endosymbiotic theory). 
• Structurally, the prokaryotes are differentiated from eukaryotes by the absence of organelles, such as the nucleus, the chloroplast, mitochondria, dictyosomes, Iysosomes, and true vacuoles.
• Their cells are enclosed by a peptidoglycan cell wall and an outer sheath that may either be mucilaginous or rigid.  
• The cytoplasmic membrane or the plasmalemma usually shows invaginations towards the cytoplasm and these are called the mesosomes that are responsible for respiration. 
• It encloses a centrally located nuclear zone or centroplasm or nucleoid region and the peripheral zone with some granular structures and/or lamellar structures.
• Reproduction is only by asexual means either by binary fission (for unicellular to colonial forms), fragmentation (for filamentous forms) and/or by spore formations.
• Photoautotrophs, those that use light as energy source and chlorophyll as the main receptor pigment of this light energy.
• The six-kingdom system of classification of organisms as proposed by Carl Woese is followed. It consists of two prokaryotic kingdoms and for eukaryotic types. The organisms that will be studied here belong to the kingdom Eubacteria.
• They are photosynthetic prokaryotes with representatives belonging to the divisions Cyanophyta and Prochlorophyta
• The former is known as the blue-green algae or the cyanobacteria while the latter as prochlorophytes or chloroxybacteria.
• The cyanobacteria are photosynthetic mainly due to the presence of chlorophyll a.  
• Their accessory pigments are phycobilins, composed of phycocyanin (blue pigments) and phycoerythrin (red pigments) in varying proportions.  
• The cyanobacteria range from bright blue-green to golden brown due to a variety of composition of these pigments in their body. Their form varies from unicellular to colonial and filamentous types.
• These eubacteria are also photosynthetic organisms. However, their main accessory pigment is both chlorophyll a and b, as in the higher green plants but has no phycobilins. 
• Prochloron didemni, the type species, is a unicellular marine from that grows on the surface of certain colonial ascidians
• Azolla pinnata frond possesses a central ovoid cavity which houses the Anabaena azollae.
• Heterocysts - yellowish or lighter green than the vegetative cells.
• Akinetes - bigger, darker, and more granulated than the vegetative cells.
• Trichome – aggregation of cells that divide only in one direction.
• Filament – trichome and the enclosing gelatinous sheath
• Somatic cells – morphologically alike cells in the filaments. Thus, refer as a ‘homocystous’ filament. 
• Necridia – dead cells that look like biconcave separation discs. 
• Hormogonia – Fragmented section or segments of filament.
• Colonial shealth – gelatinous substance holding the colony together.
• Polar nodule (a glistening, spherical structure at the crosswall

  between the heterocyst and the adjacent cell)
• A false branch is initiated without cell division in a new plane.
• A true branch is initiated by the division of certain cells in a new plane to the axial filament.
• Cell wall - multi-layered, appears to lack an external sheath.
• Thylakoid – wide, usually peripheral band; stacked usually in pairs;
• Cytoplasm – appears to occupy two interconnected zones