Assessment of dynamic microbial community structure and rhizosphere interactions during bioaugmented phytoremediation of petroleum contaminated soil by a newly designed rhizobox system

NOVELTY STATEMENT This is the first study to characterize the rhizosphere effect of cowpea on microbial activities, population, and community structure in crude oil contaminated soil in the presence and absence of PGPB, M. luteus WN01. The rhizosphere of cowpea was found to be a degradation hotspot where microbial abundance and metabolic activities were most active. Cowpea-M. luteus association can be a good candidate that can be implemented in real field sites. To understand the plant (Vigna unguiculata) and plant-growth promoting bacteria (PGPB) (Microcococcus luteus WN01) interactions in crude oil contaminated soil, experiments were conducted based on the newly designed rhizobox system. The rhizobox was divided into three main compartments namely the rhizosphere zone, the mid-zone, and the bulk soil zone, in accordance with the distance from the plant. Plants were grown in these three-chambered pots for 30 days under natural conditions. The plant root exudates were determined by analyzing for carbohydrates, amino acids, and phenolic compounds. The degradation of alkane, polycyclic aromatic hydrocarbons (PAHs), and total petroleum hydrocarbons (TPHs) were quantified by GC-FID. Soil catalase, dehydrogenase, and invertase activities were determined. The microbial community structure was assessed using denaturing gradient gel electrophoresis (DGGE). Results showed that the inoculation of M. luteus WN01 significantly enhanced cowpea root biomass and exudates, especially the phenolic compounds. Bioaugmented phytoremediation by cowpea and M. luteus promoted rhizodegradation of TPH. Cowpea stimulated microbial growth, soil dehydrogenase, and invertase activities and enhanced bacterial community diversity in oil contaminated soil. The rhizosphere zone of cowpea inoculated with M. luteus showed the highest removal efficiency, microbial activities, microbial population, and bacterial community diversity indicating the strong synergic interactions between M. luteus and cowpea.

Automated summary

This study investigates the rhizosphere effect of cowpea on microbial activities, population, and community structure in crude oil contaminated soil, with or without the plant-growth promoting bacteria (PGPB) M. luteus WN01. Results demonstrate that the interaction between cowpea and PGPB plays a positive role in promoting rhizodegradation of total petroleum hydrocarbons (TPH) in oil contaminated soil.