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 in situ at Balandra lagoon, Baja California Sur, Mexico, for 18 consecutive months. Year-round vertical zonation of cyanobacterial colonization was determined along the pneumatophores. The bottom part close to the sediment was colonized mainly by nonheterocystous, filamentous cyanobacteria 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 colonies resembling Aphanothece sp. mixed with undefined filamentous cyanobacteria. Two of the cyanobacteria (Microcoleus sp. and Anabaena sp.) isolated from the pneumatophore were diazotrophs. Massive biofilm production along the pneumatophores was evident throughout the observation period. The surrounding sediment was seasonally dominated by heterocystous Anabaena sp. Glass and dead-wood surfaces incubated for 18 months in the pneumatophore vicinity showed no zonation in the colonization pattern, although they were heavily colonized. In situ N2fixation showed seasonal and diurnal fluctuations. N2fixation was low during winter, increased in early summer, and reached its peak in midsummer. N2fixation in the summer showed diurnal peaks: one in the morning until midday and the second in the late afternoon. N2fixation was at its lowest levels near midnight. Light and water temperature are probably primary environmental factors governing N2fixation on the pneumatophores.Key words: Avicennia germinans, black mangrove, diazotrophic cyanobacteria, nitrogen fixation, pneumatophore.