Plant root exudates impact the hydrocarbon degradation potential of a weathered-hydrocarbon contaminated soil

Abstract

a b s t r a c tPhytoremediation is a promising low cost technology for the cleanup of contaminated sites. However, specific plants may promote degradation under one set of conditions but not under another, and knowledge limitations surrounding the mechanisms of phytoremediation hamper attempts at optimization. We addressed this issue by examining how exudates released by Elymus angustus (wildrye) and Medicago sativa (alfalfa), grown under hydrocarbon-stressed or non-stressed conditions, impacted the degradation potential of microbial communities in a weathered hydrocarbon-contaminated soil. Degradation potential was assessed using mineralization assays with 14 C-labeled hydrocarbons (hexadecane, naphthalene, phenanthrene) followed by DGGE of microbial communities and quantitative-PCR of genes associated with hydrocarbon degradation. All root exudates repressed hydrocarbon mineralization in soil microcosms, with exudates from hydrocarbon-stressed wildrye having the least repressive impact. Changes in degradation potential were not associated with changes in the dominant microbial community structure or with significant shifts in general microbial abundance. Degradation was, however, associated with functional changes in microbial communities. Mineralization of polyaromatic hydrocarbons (PAH) was highly correlated with copy numbers of catechol 2,3 dioxygenase and naphthalene dioxygenase, two genes involved in PAH degradation. Both gene copy numbers and mineralization parameters were significantly impacted by exudate composition, with specific compounds associated with either increased (acetate, alanine) or decreased (malonate) degradative capacity. The success of a given phytoremediation treatment is likely influenced by the relative amount of these and similar compounds within root exudates.

Importance of soil physical characteristics for petroleum hydrocarbons phytoremediation: A review

“…Abiotic factors can greatly influence the rate of crude oil biodegradation (Sonawdekar, 2012). As reported by Phillips et al (2012) the biodegradation of petroleum hydrocarbons in loamy and sandy soil under proper SOM content and favorable degradation conditions of microorganism is, inversely proportional to the concentration of the contaminants. The hydrocarbons of crude oil were totally or partially biodegraded: low molecular weight n-alkanes were completely degraded by oil degraders while the high molecular weight n-alkanes are less degradable (Moubasher et al, 2015).…”

“…It is basically the soil conditions which manage plantmicrobe interactions (Alrumman et al, 2015), therefore an ideal soil for phytoremediation should have proper characteristics in order to, let the plant grow well and produce biomass as high as possible (Khan et al, 2013;Phillips et al, 2012). Soil physical properties such as texture, structure, aeration and water status are among the factors affecting root-organism activity and performance.…”

Root Exudation: The Ecological Driver of Hydrocarbon Rhizoremediation

“…Phillips et al [228] studied two catabolic genes involved in hydrocarbon degradation commonly carried by conjugative plasmids: C2,3O (catechol 2,3 dioxygenase) and nahAc (naphthalene dioxygenase (NDO) iron sulfur protein). On one hand, C2,3O is an enzyme encoding an archetypal pathway for the catabolism of monocyclic aromatics to aliphatic metabolites [291].…”

“…Root exudation is now considered to be the most important factor in the mediation of hydrocarbon biodegradation in the rhizosphere [54, 67,104,141,[227][228][229][230][231][232]. Root exudates serve as a carbon source and energy for microorganisms, and also improve the hydrocarbon degradation in the rhizosphere by stimulating hydrocarbon-degrader populations [67].…”

Mineralization of pyrene induced by interaction between Ochrobactrum sp. PW and ryegrass in spiked soil

“…In the presence of PW, the growth of bacteria, actinobacteria and ryegrass possess a synergistic effect. The roots are known to release root exudates, such as amino acids, sugars, enzymes and complex carbohydrates, providing carbon source and energy for the growth of rhizosphere microorganisms (Kaimi et al, 2007;Phillips et al, 2012;Sun et al, 2010). Parrish et al (2005) reported that the PAH-degrading microbial population in vegetated treatments were much higher than those in unvegetated controls after 12 months of plant growth, the same tendency was also found in our research, the number of bacteria and actinobacteria increased by the rhizosphere effect, meanwhile treatment inoculate with PW also possess a higher level of microbe in some extent.…”