Trichodesmium

Trichodesmium, or "sea sawdust", is a genus of filamentous cyanobacteria. They are found in nutrient-poor tropical and subtropical ocean waters.
They are most common around Australia and in the Red Sea, where they were first described by Captain Cook. 
"Trichodesmium" is a diazotroph; that is, it fixes atmospheric nitrogen into ammonium, a nutrient used by other organisms. "Trichodesmium" is thought to fix nitrogen on such a scale that it accounts for almost half of the nitrogen fixation in marine systems globally.