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Trichodesmium A genus of filamentous cyanobacteria which are closely related to Oscillatoria and perhaps should be included in that genus. The filaments occur in masses embedded in a flocculent mucilage. Species are unusual among cyanobacteria in that they are planktonic, often forming a major component of the plankton in tropical seas. They are capable of nitrogen fixation. Heterocysts are not formed. Gas vacuoles allow the colonies to float, and when seas are calm they form conspicuous surface blooms (sea sawdust) measuring up to several kilometres across.