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 <it>Azospirillum halopraeferens</it> or with <it>Azospirillum brasilense</it> produced heavy colonization of the root surface. The colonization pattern was different for the two strains. <it>A. halopraeferens</it> yielded mainly single cells embedded in a thick sheath, whereas <it>A. brasilense</it> produced primarily microaggregates. <it>A. brasilense</it> 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 (<it>A. brasilense</it>) and colonized mangrove roots at a very high population density. <it>A. halopraeferens</it> was a better root surface colonizer, whereas the <it>A. brasilense</it> 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.