Structural and metabolic diversity of rhizosphere microbial communities of Phragmites karka in a tropical coastal lagoon

The structural and metabolic diversity of rhizosphere microbial communities of Phragmites karka, a perennial reed, was examined through high-throughput sequencing of 16S rRNA genes, taxonomic-to-metabolic mapping, and the BIOLOG® community level physiological profiling. Rhizosphere sediments were collected from P. karka stands along the shoreline of Chilika Lagoon during the year 2014-2015. At the genus level, bacterial communities were largely composed of Thiobacillus, Methylotenera, Bacillus, Steroidobacter and Escherichia/Shigella whereas Methanomassiliicoccus was one of the most abundant genus in the archaeal communities. In general, rhizosphere communities of samples from the same locations were more similar in their composition than from other sites. Salinity played an important role in driving the community composition along with 'rhizosphere effect'. Carbon substrate utilization profiles of rhizosphere communities were highly diverse and the highest utilization was recorded in wells containing tween 40 (polymer), D-mannitol (carbohydrates), L-asparagine (amino acids), 4-hydroxybenzoic acid (phenols), putrescine (amines), and D-galacturonic acid (carboxylic acids). Of these, L-asparagine and D-mannitol were preferentially utilized by all rhizosphere communities. Metabolic mapping suggested that rhizosphere microbial communities could play a critical role in maintaining the ecological health of coastal wetlands through cycling of carbon, nitrogen, and sulfur, and degradation of hydrocarbon and xenobiotic compounds. Overall, this study demonstrated a high variability in the community structure and carbon metabolic profiles of rhizosphere communities of P. karka and suggested that nutrient cycling related ecosystem services that are mediated by the rhizosphere microbial communities of reeds must be included in the management and conservation plan of wetlands.