Identify that alga:
Web-based key to genera - 21: sheathed blue-green colonies and coenobia

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  Cyanobacterial colonies and coenobia
      Click on the thumbnail for a larger image
  planktonic, with sheath  
 
  coenobia: INFORMATION: cell number in a geometric series: 2,4,8,16,32, etc.
 

cells in a regular arrangement

 

-forming a flat plate, cells in a square 2-dimensional formation



  Merismopedia
 

-cells in a 3-dimensional formation, in groups of 4 forming cubical coenobia with a mucilage sheath

 



  Eucapsis
 

coenobia not arranged in a regular manner (other than cells in groups)

 

-coenobia of 2 or more subspherical cells enclosed in a layered mucilage sheath

 

  Chroococcus  

     
  Colony: composed of an indeterminate number of cells  
 
colony spherical or subspherical:
 
 

-cells attached to the centre of the colony by mucilage stalks;
the cells usually seen as heart-shaped

 


  Gomphosphaeria
 

-cells some distance from the outer margin of the mucilage, attached to centre of colony by distinct branched stalks, cells may be in pairs; planktonic

 


 
  Snowella
 

-somewhat elongated cells clustered in a layer at the outer edge of the colony, at the ends of branching stalks and sometimes with an individual sheath, but these may be indistinct; planktonic

 



  Woronichinia: compare carefully with Coelosphaerium
 

-closely packed spherical / subspherical cells in the outer zone of the colonial mucilage, forming a hollow ball, no stalks; planktonic.

 



  Coelosphaerium
 

-Colony of individual cells enclosed in a spherical or irregular mucilage sheath, not attached by stalks or in any definite position relative to neighbouring cells.

 

cells spherical/subspherical, arranged singly or in pairs throughout the colonial mucilage, colony spherical (irregular in terrestrial species), with a (sometimes indistinct) outer layer of mucilage free of cells


  Aphanocapsa
   

 

 


Aphanocapsa grevillei

 

cells oval or cylindrical, distributed throughout colonial mucilage, may be densely packed:


 
Aphanothece
 


Aphanothece stagnina:

     
in A. stagnina, some cells may have individual sheaths
 

colony spherical, irregular or clathrate (depending partly on species),
cells spherical or subspherical, densely packed



  Microcystis
   

Microcystis aeruginosa:


   
usually with outer margin of mucilage cell-free; mean cell diameter ~6µm.
   

 

  Microcystis flos-aquae:


 
     
without cell-free outer margin of mucilage; mean cell diameter ~3.5µm.
 
Colony a flat plate, cells not arranged in a regular pattern (cf. Merismopedia)

 



  Microcrocis
 
cells form filaments enclosed in a tough mucilage envelope; heterocysts present
(see also filamentous cyanobacteria here)

 



  Nostoc
 
Colony is a rosette of radiating tapering filaments, each with a heterocyst at the base and a sheath.
(see also filamentous cyanobacteria here)

 



  Gloeotrichia
  Genera not included: Chroococcopsis, Gloeothece, Hydrococcus, Rhabdogloea, Xenococcus
         

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John Kinross