Biological N_2-fixation on Mangrove Pneumatophores: Preliminary Observations and Perspectives

摘要

Mangroves are highly productive ecosystems in tropical marine environment (1, 2). It is generally accepted that mangrove swamps supply both organic and inorganic material to adjacent coastal waters and related biotopes (3-5). Despite their high production rates, mangrove trees grow and flourish in highly oligotrophic tropical areas. Cyanobacteria, common in mangrove swamps may play an important role as atmospheric nitrogen fixers in these habitats (6, 7). Other diazotrophic but heterotrophic eubacteria are also common in marine sediments and are suggested to contribute substantially to fixed nitrogen inputs (8, 9). In addition, some nitrogen-fixing bacteria inhabiting barks of mangrove trees has been isolated and characterized (10). A previous survey in mangrove areas around Zanzibar revealed a high diversity among free-living cyano-bacteria ranging from unicellular to non-heterocystous filamentous and heterocystous species (11). A heterocystous cy anobacterium of the genus Rivularia was particularly common on mangrove pneumatophores. Other potential diazotrophic cyanobacteria described from mangrove areas include the genera Aphanocapsa, Lyngbya, Nodularia, and Scytonema (11). The current study was aimed to assess nitrogenase activity associated with cyanobacteria and other diazotrophs on mangrove pneumatophores and in adjacent sediments.