Tropical marine ecosystems are in general highly oligotrophic habitats. Some free-living and symbiotic prokaryotic organisms, such as cyanobacteria, often common in these habitats can utilize atmospheric nitrogen to meet their nitrogen requirements. Being photosynthetic, cyanobacteria also play an important role in oceanic production and sequestration of atmospheric carbon dioxide to the ocean interior. Consequently, attempts are now being made to fully assess the source (diversity) and rates of N₂ and CO₂ fixation in the world’s oceans and to determine their regulation at molecular, cellular, ecosystem and global levels. The present investigation was aimed at determining cyanobacterial diversity in coastal areas of the western Indian Ocean around the Island of Zanzibar, and to assess their ecological significance with regard to their role in delivering carbon and ‘new’ nitrogen to this ecosystem.

A total of 50 cyanobacterial species from within 21 genera were encountered and described morphologically. Most of these are new records from the area. Typical habitats were plankton, as free living and symbiotic, and benthic, as epiphytes and microbial mats. The filamentous non-heterocystous forms were most common comprising 64% of the species described. Heterocystous forms constituted 24% and unicellular cyanobacteria 12%. Cyanobacterial spatial and temporal distribution regulated by environmental parameters such as water temperature and nutrient concentration was apparent and discussed.

Eleven potentially diazotrophic cyanobacteria were found to be capable of N₂-fixation, some of which were often abundant in their habitat. The non-heterocystous cyanobacterium Lyngbya majuscula, common in the area, was shown to fix nitrogen only during the night as nitrogenase, present in all cells, was degraded during the day. The most common planktonic cyanobacterium, Trichodesmium was estimated to fix up to 42.7 mmol N m^-3y^-1 while contributing between 0.03% and 20% of total CO2 fixation in the surface waters during the dry and rainy seasons, respectively. Unicellular, picocyanobacteria were estimated to contribute on average 16% of the total plankton primary production and between 37 to 74% of the carbon needed by heterotrophic nanoflagellates during the rainy season. Photosynthetic activity in submerged cyanobacteria dominated biofilms showed light adaptation at different depths and quickly responded to changes in irradiance. Preliminary observations at the study site suggested CO₂ fixation rates of 0.14 kg C m^-2 y^-1.

The data presented here suggest higher cyanobacterial diversity in the area. Many cyanobacteria that are widespread and abundant are also capable of a diazotrophic life. It is proposed that cyanobacteria play a key role in cycling of C and N in coastal areas of Tanzania.

ISBN 91-72-65-447-3