The Halomonas species isolated from the rhizosphere of the true mangrove Avicennia marina of Indian Sundarbans showed enhanced rice growth promotion under combined stress of salt and arsenic in pot assay. Interestingly, under abiotic stress conditions, Halomonas sp. Exo1 was observed as an efficient producer of exopolysaccharide. The study revealed that salt triggered exopolysaccharide production, which in turn, increased osmotic tolerance of the strain. Again, like salt, presence of arsenic also caused increased exopolysaccharide production that in turn sequestered arsenic showing a positive feedback mechanism. To understand the role of exopolysaccharide in salt and arsenic biosorption, purified exopolysaccharide mediated salt and arsenic sequestration were studied both under in vivo and in vitro conditions and the substrate binding properties were characterized through FT-IR and SEM-EDX analyses. Finally, observation of enhanced plant growth in pot assay in the presence of the strain and pure exopolysaccharide separately, confirmed direct role of exopolysaccharide in plant growth promotion.

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HighlightsGene clusters responsible for aromatic degradation in Sphingobium sp. PNB were sequenced.The ferredoxin from sphingomonads is structurally unique.Substrate specificities of several ring-hydroxylating oxygenases were determined.Oxygenase capable of transforming alkylaromatics was characterized.Complex regulation of degradative genes was revealed by real-time PCR analyses.

Polycyclic aromatic hydrocarbons (PAHs) comprise a group of priority organic pollutants that are toxic and/or carcinogenic. Phenanthrene, the simplest PAH among recognized priority pollutants, is commonly used as a model compound for the study of PAH biodegradation. Sphingobium sp. strain PNB, capable of degrading phenanthrene as a sole carbon and energy source, was isolated from a municipal waste-contaminated soil sample. A combination of chromatographic and spectrometric analyses, together with oxygen uptake and enzyme activity studies, suggested the presence of phenanthrene degradation pathways in this strain. Identification of metabolites suggested that initial dioxygenation of phenanthrene took place at both 3,4-and 1,2-carbon positions; meta-cleavage of resultant diols led to the formation of 1-hydroxy-2-naphthoic acid and 2-hydroxy-1-naphthoic acid, respectively. The hydroxynaphthoic acids, in turn, were metabolized by a meta-cleavage pathway(s), leading to the formation of 2,2-dicarboxychromene and 2-hydroxychromene-2-glyoxylic acid, respectively. These metabolites were subsequently transformed to catechol via salicylic acid, which further proceeds towards the tricarboxylic acid cycle leading to complete mineralization of the compound phenanthrene. The present study establishes the metabolism of hydroxynaphthoic acids by a meta-cleavage pathway in the degradation of phenanthrene, expanding our current understanding of microbial degradation of PAHs.

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