Enhanced removal of polychlorinated biphenyls from alfalfa rhizosphere soil in a field study: the impact of a rhizobial inoculum

Phytoremediation of soil contaminated with cadmium, copper and polychlorinated biphenyls

A pot experiment and afield trial were conducted to study the remediation of an aged field soil contaminated with cadmium, copper and polychlorinated biphenyls (PCBs) (7.67 +/- 0.51 mg kg(-1) Cd, 369 +/- 1 mg kg(-1) Cu in pot experiment; 8.46 +/- 0.31 mg kg(-1) Cd, 468 +/- 7 mg kg(-1) Cu, 323 +/- 12 microg kg(-1) PCBs for field experiment) under different cropping patterns. In the pot experiment Sedum plumbizincicola showed pronounced Cd phytoextraction. After two periods (14 months) of cropping the Cd removal rates in these two treatments were 52.2 +/- 12.0 and 56.1 +/- 9.1%, respectively. Total soil PCBs in unplanted control pots decreased from 323 +/- 11 to 49.3 +/- 6.6 microg kg(-1), but with no significant difference between treatments. The field microcosm experiment intercropping of three plant species reduced the yield of S. plumbizincicola, with a consequent decrease in soil Cd removal. S. plumbizincicola intercropped with E. splendens had the highest shoot Cd uptake (18.5 +/- 1.8 mg pot(-1)) after 6 months planting followed by intercropping with M. sativa (15.9 +/- 1.9 mg pot(-1)). Liming with S. plumbizincicola intercropped with M. sativa significantly promoted soil PCB degradation by 25.2%. Thus, adjustment of soil pH to 5.56 combined with intercropping with S. plumbizincicola and M. sativagave high removal rates of Cd, Cu, and PCBs.

Poplar clones of different sizes, grown on a heavy metal polluted site, are associated with microbial populations of varying composition

We performed a field trial to evaluate the response of different poplar clones to heavy metals. We found that poplar plants of the same clone, propagated by cuttings, had a marked variability of survival and growth in different zones of the field that were characterized by very similar physical-chemical prosperities. Since metal uptake and its accumulation by plants can be affected by soil microorganisms, we investigated soil microbial populations that were collected in proximity to the roots of large and small poplar plants. We used microbiological and molecular tools to ascertain whether bacterial strains or species were associated with large, or small poplars, and whether these were different from those present in the bulk (without plants) soil. We found that the culturable fraction of the bacteria differed in the three cases (bulk soil, small or large poplars). While some taxa were always present, two species (Chryseobacterium soldanellicola and Variovorax paradoxus) were only found in the soil where poplars (large or small) were growing, independently from the plant size. Bacterial strains of the genus Flavobacterium were prevalent in the soil with large poplar plants. The existence of different microbial populations in the bulk and in the poplar grown soils was confirmed by the DGGE profiles of the bacterial culturable fractions. Cluster analysis of the DGGE profiles highlighted the clear separation of the culturable fraction from the whole microbial community. The isolation and identification of poplar-associated bacterial strains from the culturable fraction of the microbial community provided the basis for further studies aimed at the combined use of plants and soil microorganisms in the remediation of heavy metal polluted soils.

Characterization of the bacterial archaeal diversity in hydrocarbon-contaminated soil

A polyphasic approach combining culture-based methods with molecular methods is useful to expand knowledge on microbial diversity in contaminated soil. Microbial diversity was examined in soil samples from a former industrial site in the European Alps (mainly used for aluminum production and heavily contaminated with petroleum hydrocarbons) by culture-dependent and culture-independent methods. The physiologically active eubacterial community, as revealed by fluorescence-in-situ-hybridization (FISH), accounted for 6.7% of the total (DAPI-stained) bacterial community. 4.4% and 2.0% of the DAPI-stained cells could be attributed to culturable, heterotrophic bacteria able to grow at 20°C and 10°C, respectively. The majority of culturable bacterial isolates (34/48) belonged to the Proteobacteria (with a predominance of Alphaproteobacteria and Gammaproteobacteria), while the remaining isolates were affiliated with the Actinobacteria, Cytophaga-Flavobacterium-Bacteroides and Firmicutes. A high fraction of the culturable, heterotrophic bacterial population was able to utilize hydrocarbons. Actinobacteria were the most versatile and efficient degraders of diesel oil, n-alkanes, phenol and PAHs. The bacterial 16S rRNA gene clone library contained 390 clones that grouped into 68 phylotypes related to the Proteobacteria, Bacteroidetes, Actinobacteria and Spirochaetes. The archaeal 16S rRNA gene library contained 202 clones and 15 phylotypes belonging to the phylum Euryarchaeota; sequences were closely related to those of methanogenic archaea of the orders Methanomicrobiales, Methanosarcinales, Methanobacteriales and Thermoplasmatales. A number of bacterial and archaeal phylotypes in the clone libraries shared high similarities with strains previously described to be involved in hydrocarbon biodegradation. Knowledge of the bacterial and archaeal diversity in the studied soil is important in order to get a better insight into the microbial structure of contaminated environments and to better exploit the bioremediation potential by identifying potential hydrocarbon degraders and consequently developing appropriate bioremediation strategies.

Uptake, translocation and metabolism of polybrominated diphenyl ethers (PBDEs) and polychlorinated biphenyls (PCBs) in maize (Zea mays L.)

A hydroponic experiment was conducted in the present study to investigate and compare plant uptake, translocation and metabolism of polybrominated diphenyl ethers (PBDEs) of BDE-15, BDE-28 and BDE-47 and polychlorinated biphenyls (PCBs) of PCB-15, PCB-28 and PCB-47 in maize. Root concentrations of BDE-15, BDE-28 and BDE-47 were consistently higher than PCB-15, PCB-28 and PCB-47, respectively. A significantly positive correlation was found between logRCF (root concentration factor) and logKow of these PBDEs and PCBs, suggesting a control role of their partitioning in plant uptake. The translocation factors (TFs, Cstem/Croot) of PBDEs were generally lower than those of PCBs of the same halogen-substitutions, demonstrating easier transport of PCBs than PBDEs. Metabolites mono-, di- and tri-BDEs and PCBs were detected, suggesting the existence of in vivo metabolism of PBDEs and PCBs in maize. Dehalogenation and rearrangement of halogen atoms were identified, and some similarities but also significant differences existed between the PBDEs and PCBs. PBDEs in maize were, in general, more susceptible to metabolism compared with PCBs of the same halogen-substitutions. This is the first comparative report on the uptake, translocation and metabolism of PBDEs and PCBs in plants.

Endogeic earthworms shape bacterial functional communities and affect organic matter mineralization in a tropical soil

Priming effect (PE) is defined as a stimulation of the mineralization of soil organic matter (SOM) following a supply of fresh organic matter. This process can have important consequences on the fate of SOM and on the management of residues in agricultural soils, especially in tropical regions where soil fertility is essentially based on the management of organic matter. Earthworms are ecosystem engineers known to affect the dynamics of SOM. Endogeic earthworms ingest large amounts of soil and assimilate a part of organic matter it contains. During gut transit, microorganisms are transported to new substrates and their activity is stimulated by (i) the production of readily assimilable organic matter (mucus) and (ii) the possible presence of fresh organic residues in the ingested soil. The objective of our study was to see (i) whether earthworms impact the PE intensity when a fresh residue is added to a tropical soil and (ii) whether this impact is linked to a stimulation/inhibition of bacterial taxa, and which taxa are affected. A tropical soil from Madagascar was incubated in the laboratory, with a (13)C wheat straw residue, in the presence or absence of a peregrine endogeic tropical earthworm, Pontoscolex corethrurus. Emissions of (12)CO(2) and (13)CO(2) were followed during 16 days. The coupling between DNA-SIP (stable isotope probing) and pyrosequencing showed that stimulation of both the mineralization of wheat residues and the PE can be linked to the stimulation of several groups especially belonging to the Bacteroidetes phylum.

Behavior of decabromodiphenyl ether (BDE-209) in soil: effects of rhizosphere and mycorrhizal colonization of ryegrass roots

A rhizobox experiment was conducted to investigate degradation of decabromodiphenyl ether (BDE-209) in the rhizosphere of ryegrass and the influence of root colonization with an arbuscular mycorrhizal (AM) fungus. BDE-209 dissipation in soil varied with its proximity to the roots and was enhanced by AM inoculation. A negative correlation (P < 0.001, R(2) = 0.66) was found between the residual BDE-209 concentration in soil and soil microbial biomass estimated as the total phospholipid fatty acids, suggesting a contribution of microbial degradation to BDE-209 dissipation. Twelve and twenty-four lower brominated PBDEs were detected in soil and plant samples, respectively, with a higher proportion of di- through hepta-BDE congeners in the plant tissues than in the soils, indicating the occurrence of BDE-209 debromination in the soil-plant system. AM inoculation increased the levels of lower brominated PBDEs in ryegrass. These results provide important information about the behavior of BDE-209 in the soil-plant system.

Influence of Rhizobium meliloti on phytoremediation of polycyclic aromatic hydrocarbons by alfalfa in an aged contaminated soil

Microbe-assisted phytoremediation is emerging as one of the most effective means by which plants and their associated rhizosphere microbes degrade organic contaminants in soils. A pot study was conducted to examine the effects of inoculation with Rhizobium meliloti on phytoremediation by alfalfa grown for 90 days in an agricultural soil contaminated with weathered polycyclic aromatic hydrocarbons (PAHs). Planting with uninoculated alfalfa (P) and alfalfa inoculated with R. meliloti (PR) significantly lowered the initial soil PAH concentrations by 37.2 and 51.4% respectively compared with unplanted control soil. Inoculation with R. meliloti significantly increased the counts of culturable PAH-degrading bacteria, soil microbial activity and the carbon utilization ability of the soil microbial community. The results suggest that the symbiotic association between alfalfa and Rhizobium can stimulate the rhizosphere microflora to degrade PAHs and its application may be a promising bioremediation strategy for aged PAH-contaminated soils.

Potential for biodegradation of polychlorinated biphenyls (PCBs) by Sinorhizobium meliloti

Resting cell assay and soil microcosms were set up to investigate the biodegradation capability and metabolic intermediate of polychlorinated biphenyls (PCBs) by a rhizobial strain Sinorhizobium meliloti. Biodegradation was observed immediately after 2,4,4'-TCB was supplied as a sole source of carbon and energy in liquid cultures. After 6 days, the percent biodegradation of 2,4,4'-TCB was 77.4% compared with the control. The main intermediate was identified as 2-hydroxy-6-oxo-6-phenylhex-2,4-dienoic acid (HOPDA) for 2,4,4'-TCB as determined by gas chromatography-mass spectrometry (GC-MS). Inoculation with S. meliloti greatly enhanced the degradation of target PCB mixtures in the soil. Moreover, soil culturable bacteria, fungi and biphenyl degrading bacteria counts showed significant increase after inoculation of S. meliloti. This study suggests that S. meliloti is promising in PCB bioremediation.