Two new nitrogen‐fixing bacteria from the rhizosphere of mangrove trees: Their isolation, identification and in vitro interaction with rhizosphere Staphylococcus sp.

Two new diazotrophic bacteria, Listonella anguillarum and Vibrio campbellii , and one non‐nitrogen‐fixing bacterium, Staphylococcus sp., were isolated from the rhizosphere of mangrove trees. Strains of these newly‐defined diazotrophs are known as pathogenic bacteria in fish and shellfish. During the purification of diazotrophic species from the entire rhizosphere population, N 2 ‐fixation of the bacterial mixtures decreased. When grown in vitro in mixed cultures, the non‐fixing bacterium Staphylococcus sp. increased the nitrogen‐fixing capacity of L. anguillarum by 17% over the pure culture; the nitrogen‐fixing capacity per bacterial cell increased 22%. This interaction was not due to a change in O 2 concentration. Staphylococcus sp. decreased the nitrogen‐fixing capacity of V. campbellii by 15%. These findings indicate that (i) other species of rhizosphere bacteria, apart from the common diazotrophic species, should be evaluated for their contribution to the nitrogen‐fixation process in mangrove communities; and (ii) the nitrogen‐fixing activity detected in the rhizosphere of mangrove plants is probably not the result of individual nitrogen‐fixing strains, but the sum of interactions between members of the rhizosphere community.