Trichodesmium, also called sea sawdust,'s a genus of filamentous cyanobacteria. They're found in nutrient poor tropical and subtropical ocean waters (particularly around Australia, where they were first described by Captain Cook). Trichodesmium fixes atmospheric nitrogen into ammonium, usable also for other organisms. While far from the only nitrogen fixing bacteria, they're among the most important of the marine varieties, and are being extensively studied for their role in nutrient cycling in… (More on Trichodesmium)