Mapping the Centimeter-Scale Spatial Variability of PAHs and Microbial Populations in the Rhizosphere of Two Plants

Abstract

Rhizoremediation uses root development and exudation to favor microbial activity. Thus it can enhance polycyclic aromatic hydrocarbon (PAH) biodegradation in contaminated soils. Spatial heterogeneity of rhizosphere processes, mainly linked to the root development stage and to the plant species, could explain the contrasted rhizoremediation efficiency levels reported in the literature. Aim of the present study was to test if spatial variability in the whole plant rhizosphere, explored at the centimetre-scale, would influence the abundance of microorganisms (bacteria and fungi), and the abundance and activity of PAH-degrading bacteria, leading to spatial variability in PAH concentrations. Two contrasted rhizospheres were compared after 37 days of alfalfa or ryegrass growth in independent rhizotron devices. Almost all spiked PAHs were degraded, and the density of the PAH-degrading bacterial populations increased in both rhizospheres during the incubation period. Mapping of multiparametric data through geostatistical estimation (kriging) revealed that although root biomass was spatially structured, PAH distribution was not. However a greater variability of the PAH content was observed in the rhizosphere of alfalfa. Yet, in the ryegrass-planted rhizotron, the Gram-positive PAH-degraders followed a reverse depth gradient to root biomass, but were positively correlated to the soil pH and carbohydrate concentrations. The two rhizospheres structured the microbial community differently: a fungus-to-bacterium depth gradient similar to the root biomass gradient only formed in the alfalfa rhizotron.

pH is the primary determinant of the bacterial community structure in agricultural soils impacted by polycyclic aromatic hydrocarbon pollution

“…However, this does not mean the enrichment of GP degraders by PAH pollution, since they could be affected by soil characteristics or the presence of plant. It has been previously observed that abundance of PAH-RHDα gene was often changed by root exudates23, organic matter24 and pH25. Controlled experiments with more comprehensive and sensitive tests, for example the transcription activities analysis2627 are therefore required to reveal the effects of PAH pollution on degrader community.…”

Assessment of derelict soil quality: Abiotic, biotic and functional approaches

“…Anthropic soils can contain PAH concentrations beyond 1,000 mg.kg -1 (Biache et al, 2008;Ouvrard et al, 2013). But PAH concentrations can largely vary in soils from coking plant sites owing to contamination heterogeneity, age, natural attenuation with time, and to remediation measures (Bourceret et al, 2015).…”

Rhizosphere effect is stronger than PAH concentration on shaping spatial bacterial assemblages along centimetre-scale depth gradients

“…The present study focuses on the bacterial community structure along two plant root systems (alfalfa and ryegrass) in a PAH-contaminated soil in order to point out the principal environmental biotic and abiotic drivers at centimeter scale using rhizobox experimental device. Spatial variability of bacterial community has been rarely studied in the past, at small scale, and never in contaminated soil presenting spatial patchy distribution of pollutants and depth rhizosphere gradients as found in Bourceret et al (2015). Based on these previous findings, we hypothesized that bacterial community could be influenced by two main forcing factors identified previously: i) rhizosphere processes linked to plant species and root depth gradients of root biomass and carbon from exudates, and/or ii) pollutant content that shows a random spatial distribution with cold and hot spots of PAHs.…”

“…Europe) and ryegrass (Rye; Lolium multiflorum var. Podium) at 22.3 ± 2.0 seeds per cm 2 , plants were grown in a regulated growth chamber (22°C /18°C day/night, 80% relative humidity, ca 250 µmol photons m -2 s -1 , 16-hour photoperiod) (Bourceret et al 2015). Rhizoboxes were opened after 37 days (T37) when few roots reached the bottom of the rhizoboxes and root density gradients could easily be visualized for both plants.…”

“…At the field scale, spatial patchy distribution of pollution can be found, influencing the relative proportion of some bacterial phyla (Mukherjee et al 2014), as well as microbial diversity, evenness and PAH catabolic genes (Bengtsson et al 2013). At local centimeter-scale, organic matter and pollutants are heterogeneously distributed and can form hotspots of concentration, conducting to a nugget effect (Bourceret et al 2015, Hybolt et al 2011. Unfortunately, the link between PAH hotspots and variation of community assemblage was never explored at this scale and needs to be specified.…”