Nitrogen-fixing cyanobacteria in tropical oceans, with emphasis on the Western Indian Ocean

It is now widely recognised that cyanobacteria in the marine environment are represented by unicellular, filamentous as well as filamentous heterocystous taxa. The unicellular pico-cyanobacteria contribute substantially to primary production, while the larger planktonic nonheterocystous genus Trichodesmium appear to be the most potent provider of new nitrogen to oceans. Although there have been rather few studies in the Indian Ocean, recent surveys suggest a rich diversity among benthic cyanobacteria along the east African coast and many of the taxa identified are known to fix or are potential nitrogen fixers. The northeastern monsoon promotes large surface blooms of Trichodesmium, and besides some unicellular nitrogen-fixers such as Gloeocapsa sp., heterocystous cyanobacteria of the genus Richelia, living in symbiosis with certain diatoms, are particularly common in the area. Other potential cyanobacterial nitrogen-fixers are discussed as are factors known to regulate marine nitrogen fixation.