Associations of pseudomonas species and forage grasses enhance degradation of chlorinated benzoic acids in soil
Siciliano, Steven Douglas
Microorganisms and plants can be used as bioremediation agents to clean up contaminated soil sites in a cost effective and environmentally friendly manner. Furthermore, associations of plants and bacteria can act synergistically and thereby, eliminate difficulties encountered when using a single agent. The purpose of my thesis research was to develop and characterize associations of plants and bacteria that degraded chlorinated benzoic acids (CBA) in soil. The contaminants were used as model compounds because they are present in soils contaminated with polychlorinated biphenyls (PCB) or chlorinated pesticides. Sixteen forage grasses in combination with 12 bacterial inoculants were screened for their ability to promote the degradation of CBA in soil. The CBAs were added to soil as single isomers, or in tertiary mixtures. The effect of inoculants on the root associated microbial community was assessed by fatty acid methyl ester (FAME) profiles as well as carbon substrate utilization as determined by the Biolog system. In addition, inoculant stimulation of the CBA degradative activity of roots and/or rhizosphere soil was determined by in vitro and hydroponic systems. Degradative enzymes were isolated from root exudates through the use of ultrafiltration and chromatography. Five associations of plants and bacteria degraded CBA to a greater extent than plants without bacterial inoculants. Plant-bacterial associations that increased 2-chlorobenzoic acid (2CBA) degradation had little effect on di-chlorinated benzoic acid degradation. Furthermore, the effective inoculants altered the root-associated microbial community of Bromus biebersteinii and simultaneously increased the CBA degradative activity of roots. Although these inoculants had little effect on the microbial community composition of Elymus dauricus, they stimulated a plant enzyme capable of degrading 2CBA in the rhizosphere. Bacterial inoculants stimulated CBA degradation by altering the microbial community present on the root surface and thereby increasing the ability of this community to degrade CBA. Alternatively, inoculants stimulated the production of a plant enzyme(s) that degraded 2CBA. My research has demonstrated that specific interactions between plants and bacteria promote contaminant degradation in soil, and suggests that new remediation strategies can be developed based on such interactions.