CYANOBACTERIA AND BLACK MANGROVES IN NORTHWESTERN MEXICO - COLONIZATION, AND DIURNAL AND SEASONAL NITROGEN-FIXATION ON AERIAL ROOTS

Nitrogen fixation and colonization by associative cyanobacteria in the aerial roots (pneumatophores) of black mangrove trees was evaluated i n situ at Balandra lagoon, Baja California Sur, Mexico, for 18 consecu tive months. Year-round vertical zonation of cyanobacterial colonizati on was determined along the pneumatophores. The bottom part close to t he sediment was colonized mainly by nonheterocystous, filamentous cyan obacteria resembling Lyngbya sp. and Oscillatoria sp. The central zone was colonized mainly by filaments resembling Microcoleus sp. and the upper part was colonized by coccoidal cyanobacteria within defined col onies resembling Aphanothece sp. mixed with undefined filamentous cyan obacteria. Two of the cyanobacteria (Microcoleus sp. and Anabaena sp.) isolated from the pneumatophore were diazotrophs. Massive biofilm pro duction along the pneumatophores was evident throughout the observatio n period. The surrounding sediment was seasonally dominated by heteroc ystous Anabaena sp. Glass and dead-wood surfaces incubated for 18 mont hs in the pneumatophore vicinity showed no zonation in the colonizatio n pattern, although they were heavily colonized. In situ N-2 fixation showed seasonal and diurnal fluctuations. N-2 fixation was low during winter, increased in early summer, and reached its peak in midsummer. N-2 fixation in the summer showed diurnal peaks: one in the morning un til midday and the second in the late afternoon. N-2 fixation was at i ts lowest levels near midnight. Light and water temperature are probab ly primary environmental factors governing N-2 fixation on the pneumat ophores.