Trichodesmium

Trichodesmium is a genus of cyanobacteria. They're itty bitty near-plants which float on the surface of the ocean in long, microscopic strings. Favored habitat is tropical and near-tropical ocean of either hemisphere (they can be found along the coasts of both North Carolina and Australia) and of the oligiotrophic persuasion (a mouthful meaning "low in nutrients"). A few species are responsible for toxic blooms. More importantly, however, Trichodesmium is a major source of nitrogen in the world's oceans. If plants are the base of the food chain, nitrogen is the foundation on which they stand. Trichodesmium grow in dense, tangled colonies of filaments, which are key to their nitrogen fixing capability. Unlike most other nitrogen fixing bacteria, Trichodesmium have no heterocysts, thick-walled cells which protect nitrogenase (the enzyme involved in nitrogen fixation) from oxygen. The very same O₂ we like to breathe brings nitrogen fixation to a halt. Trichodesmium, instead of going through the trouble of making heterocysts, segregates nitrogen fixation to the center of the tangle. Things are wound up so tightly that no oxygen can get in. However, turbulent seas can tease the tangles apart, so Trichodesmium is typically found in calm waters. Furthermore, Trichodesmium alternates between photosynthesis and nitrogen fixation over the course of the day. When nitrogen fixating is at its peak, at about mid-day, Photosystem II, which produces oxygen as a waste, is shut down. Photosystem II reaches peak activity once nitrogen fixation ends.

Species:
* Trichodesmium aureum
* Trichodesmium contortum
* Trichodesmium erythraeum
* Trichodesmium havanum
* Trichodesmium hildebrandtii
* Trichodesmium pelagicum
* Trichodesmium tenue
* Trichodesmium thiebautii

Sources: http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=pubmed&dopt=Abstract&list_uids=1257749
Segregation of Nitrogen Fixation and Oxygenic Photosynthesis in the Marine Cyanobacterium Trichodesmium, Science 16 November 2001