Spirulina: The thing that's sold as Spirulina is an entirely different plant
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Specifically (lol), Spirulina is a genus, and the stuff sold as Spirulina has long since been reclassified as belonging to a different genus, although plenty of other plants still belong to it
From the wiki article on Arthrospira:
"Although the introduction of two separate genera [Arthrospira and Spirulina] is now generally accepted, there has been much dispute in the past and the resulting taxonomical confusion is tremendous."