The Aquatic Budding Bacterium Blastobacter denitrificans Is a Nitrogen-Fixing Symbiont of Aeschynomene indica

Abstract

ABSTRACT Blastobacter spp. are freshwater bacteria that form rosette structures by cellular attachment to a common base. Comparative analyses of ribosomal 16S rRNA gene and internally transcribed spacer region sequences indicated that B. denitrificans is a member of the α-subdivision of Proteobacteria . Among the α- Proteobacteria , B. denitrificans was related to a cluster of genera, including Rhodopseudomonas palustris , Afipia felis , Nitrobacter hamburgensis , and Bradyrhizobium spp. Although the precise phylogenetic relationships among these genera could not be established with a high degree of confidence, the sequences of B. denitrificans and several bradyrhizobial isolates from nodules of Aeschynomene indica were almost identical. Bradyrhizobia are bacteria that form nitrogen-fixing symbioses with legumes, including soybeans ( Glycine max ) and members of the genus Aeschynomene . From symbiotic infectiveness tests we demonstrated that the type strain for B. denitrificans , IFAM 1005, was capable of forming an effective nitrogen-fixing symbiosis with A. indica . Not only do these results reveal a previously unknown ecological adaptation of a relatively obscure aquatic bacterium, but they also demonstrate how evidence gathered from molecular systematic analyses can sometimes provide clues for predicting ecological behavior.