In vitro colonization and increase in nitrogen fixation of seedling roots of black mangrove inoculated by a filamentous cyanobacteria

Abstract

An isolate of the filamentous cyanobacterium Microcoleus sp. was obtained from black mangrove aerial root (pneumatophore) and inoculated onto young mangrove seedlings to evaluate N2-fixation and root-colonization capacities of the bacterium under in vitro conditions in closed-system experiments. N2 fixation (acetylene reduction) gradually increased with time and reached its peak 5 days after inoculation. Later, it decreased sharply. The level of N2 fixation in the presence of the plant was significantly higher than the amount of nitrogen fixed by a similar quantity of cyanobacteria on a N-free growth medium. The main feature of this root colonization was the gradual production of a biofilm in which the cyanobacterial filaments were embedded. Visible biofilm production increased with time until it completely covered the entire root system of the plant. The in-and-out movement of cyanobacterial filaments from the biofilm probably allows colonization of uncolonized portions of the root several days after the initial inoculation. This is, to the best of our knowledge, the first report of the artificial inoculation of cyanobacteria on marine mangroves.Key words: Avicennia germinans, beneficial bacteria, biofilm, black mangrove, diazotrophic cyanobacteria, Microcoleus sp., nitrogen fixation, plant growth-promoting bacteria, root colonization.