Trichodesmium

Trichodesmium, also called sea sawdust, is a genus of filamentous cyanobacteria. They are 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 are among the most important of the marine varieties, and are being extensively studied for their role in nutrient cycling in the ocean. Unlike other nitrogen fixing bacteria, Trichodesmium does not have heterocysts, nor any other specialised cells for this task. Furthermore, nitrogen fixation peaks at mid-day, i.e. occurs during the same time as photosynthesis. Inhibitor studies even revealed that photosystem II activity is essential for nitrogen fixation in this organism. All this may seem contradictory at first glance, because the enzyme responsible for nitrogen fixation, nitrogenase, is irreversibly inhibited by oxygen. However, Trichodesmium utilises photosynthesis for nitrogen fixation by carrying out the Mehler reaction, during which the oxygen produced by PSII is reduced again after PSI. This regulation of photosynthesis for nitrogen fixation involves rapidly reversible coupling of their light-harvesting antenna, the phycobilisomes, with PSI and PSII. Trichodesmium forms blooms and provides substrate for many small oceanic organisms (bacteria, diatoms, dinoflagellates, protozoa, and copepods).