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 and the red sea, where they were first described by Captain Cook). Trichodesmium is a diazotroph; that is, it fixes atmospheric nitrogen into ammonium, usable also for other organisms.
Trichodesmium forms blooms, which can be extensive on surface waters and led to its widespread recognition as "sea sawdust/straw"; in fact, the Red Sea gets most of its eponymous colouration from the corresponding pigment in Trichodesmium eryhtraeum. Colonies of Trichodesmium provide a pseudobenthic substrate for many small oceanic organisms including bacteria, diatoms, dinoflagellates, protozoa, and copepods (which are its primary predator); in this way, the genus can support complex microenvironments.
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. Trichodesmium is thought to fix nitrogen on such a scale that it accounts for almost half of the nitrogen-fixation within marine systems on a global scale.
Unlike other nitrogen fixing bacteria, Trichodesmium does not have heterocysts, nor any other specialised cells for this task. Furthermore, nitrogen fixation peaks at midday, 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.
- Bergman, B.; Sandh, G.; Lin, S.; Larsson, H.; and Carpenter, E.J. (2012). "Trichodesmium – a widespread marine cyanobacterium with unusual nitrogen fixation properties.". FEMS Microbiology Reviews: 1–17.
- Kana, T.M. (1993) Rapid oxygen cycling in Trichodesmium thiebautii. Limnology and Oceanography 38: 18–24.
- Berman-Frank, I., Lundgren, P., Chen, Y.-B., Küpper, H., Kolber, Z., Bergman, B., and Falkowski, P. (2001) Segregation of nitrogen fixation and oxygenic photosynthesis in the marine cyanobacterium Trichodesmium. Science 294: 1534–1537.
- Küpper, H., Ferimazova, N., Šetlík, I., and Berman-Frank, I. (2004) Traffic lights in Trichodesmium: regulation of photosynthesis for nitrogen fixation studied by chlorophyll fluorescence kinetic microscopy. Plant Physiology 135: 2120–2133.
- Capone, D.G., Zehr, J., Paerl, H., Bergman, B., and Carpenter, E.J. (1997) Trichodesmium: A globally significant marine cyanobacterium. Science 276: 1221–1229.
- Publications on Trichodesmium from a Marine Biogeochemistry laboratory at the University of Southern California
- Charles Darwin's description of sailing through a Trichodesmium bloom
- Trichodesmium in Florida — 2004, Florida Fish and Wildlife Conservation Commission Fish and Wildlife Research Institute
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