Root-surface colonization of black mangrove seedlings by Azospirillum halopraeferens and Azospirillum brasilense in seawater

Abstract

Inoculation of axenic black mangrove seedlings in seawater for 8 days with either the terrestrial halotolerant plant growth-promoting bacterium Azospirillum halopraeferens or with Azospirillum brasilense produced heavy colonization of the root surface. The colonization pattern was different for the two strains. A. halopraeferens yielded mainly single cells embedded in a thick sheath, whereas A. brasilense produced primarily microaggregates. A. brasilense cells were anchored to the root surfaces and to themselves by a network of fibrillar material. Both bacterial strains survived in seawater (approximately 104 colony forming units per ml) for more than 30 days, for 70 days in saline water (A. brasilense) and colonized mangrove roots at a very high population density. A. halopraeferens was a better root surface colonizer, whereas the A. brasilense population was greater in the entire root. This work is the initial stage of studies designed to assess the feasibility of using terrestrial plant growth-promoting bacteria for the inoculation of marine plants.