Effect of diverse nitrogen-fixing bacterial community inhabiting the rhizoplane of acid sulfate soil-tolerant plants on nitrogen circulation around the rhizophere.

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In this study, we cultured rot washings in nitrogen-free, soft gel medium as a primary screening of functional rhizoplane and/or rhizosphere microbes, and we efficiently and selectively obtained α- and β-proteobacteria to exclude saprophytic γ-proteobacteria. Those oligotrophic and/or diazotrophic bacteria were visualized their behaviors in the soft gel medium, including their biocomplex formation and tripartite partnerships. The bacterial flora propagated in the N-free soft gel medium of gellan gum base survived relatively longer in the medium at room temperature due to a biofilm-like colonization and quorum-sensing like dormancy. Thus, we were able to trail new area of functional rhizo-biocomplex connected to the study of behabior of rhizoplane microfloral communities. As a demonstration, we have isolated and sub-cultured two rhizoplane bacteria, Sphingomonas rosa and Burkholderia cepacia, from rhizoplane of Melastoma malabathricum, a representative acid sulfate soil-tolerant plant, and identified them with 16S rRNA gene sequencing and data base search. The S. rosa showed. abilities to alter rhizospherous conditions and also to enhance nitrogen supply from the rhizo-biocomplex in a sotrongly acidic conditions. Like S. rosa, many other Sphingomonas spp. often isolated from the rhizoplane of acid-tolerant plants in Kalimantan had characteristics of 1) contributing to reliving divert soil in the rhizosphere, 2) fixing nitrogen in the rhizoplane or at least adjusting conditions for nitrogen fixation, and 3) being regulated the microfloral population via Mg^<2+> in the rhizosphere. In addition, Sphingomonas spp. had tight partnerships and tripartite-like co-works in the rhizoplane and/or inner tissues of host plants. Sphingomonas spp. are thus expected to be a motive force for nitrogen circulation and high genetic diversity in the rhizo-biocomplex.