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''Calothrix'' is a genus of 
cyanobacteria
Cyanobacteria ( ) are a group of autotrophic gram-negative bacteria that can obtain biological energy via oxygenic photosynthesis. The name "cyanobacteria" () refers to their bluish green (cyan) color, which forms the basis of cyanobacteri ...
. They are generally found in freshwater.
References
Rivulariaceae Cyanobacteria genera {{cyanobacteria-stub