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''Calothrix'' is a genus of 
cyanobacteria
Cyanobacteria (), also known as Cyanophyta, are a phylum of gram-negative bacteria that obtain energy via photosynthesis. The name ''cyanobacteria'' refers to their color (), which similarly forms the basis of cyanobacteria's common name, blue ...
. They are generally found in freshwater.
References
Rivulariaceae Cyanobacteria genera {{cyanobacteria-stub