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Additional file 1. Transmission electron microscope image of the silver nanoparticles used in the study. Nanoparticles were obtained in solid form from US Research Nanomaterials, Inc. (Houston, TX, USA), and were applied to soil in this form.

Air pollution, containing fossil fuel-related volatile organic compounds (VOCs), constitutes a major problem in most urban areas. Worldwide, more people die from exposure to air pollution than from malaria and HIV infection together. Many years of research show that plant–microbe interactions can be exploited to significantly enhance phytoremediation of contaminated environments. Microorganisms living on plant leaves constitute promising candidates to detoxify harmful VOCs and promote plant growth. Hedera helix, an evergreen plant known for its hardiness and climbing ability, proves to be an excellent host species in the context of phytoremediation of air pollution. We employed 16S rRNA gene amplicon-based metagenomics to characterize the taxonomic structure of phyllospheric microbial communities associated with H. helix. Further, we investigated 33 bacterial isolates from 8 different genera using culture-dependent approaches and whole genome sequencing. Our results indicate that the relative composition of the H. helix phyllobiome is similar to that of the model plant Arabidopsis thaliana. The presence of pathways related to detoxification of fossil fuel-related VOCs could be confirmed by genome-level analyses of bacterial isolates showing a high in vitro capacity to metabolize selected VOCs and promote plant growth. We provide an integrated description of the H. helix phyllobiome, including the identification of interesting bacteria with significant potential as inoculants in phytoremediation approaches. Het Fonds voor Wetenschappelijk Onderzoek – Vlaanderen (FWO)

Air pollution, containing fossil fuel-related volatile organic compounds (VOCs), constitutes a major problem in most urban areas. According to recent data, more people die from exposure to air pollution than from malaria and HIV infection together. Many years of research show that plant–microbe interactions can be exploited to significantly enhance phytoremediation of contaminated environments. Microorganisms living on plant leaves constitute promising candidates to detoxify harmful VOCs and promote plant growth. Hedera helix, an evergreen plant known for its hardiness and climbing ability, proves to be an excellent host species in the context of phytoremediation of air pollution. We employed 16S rRNA gene amplicon-based metagenomics and real-time PCR to characterize the taxonomic structure of phyllospheric microbial communities associated with H. helix, and compared the communities present at polluted high-traffic locations with non-polluted sites. Further, we investigated bacterial isolates using culture-dependent approaches and sequenced the genomes of promising bacteria with regard to phytoremediation applications. Our results indicate that Actinobacteria, Firmicutes and Betaproteobacteria are more abundantly present on H. helix leaves in a polluted environment. Moreover, the relative composition of the H. helix phyllobiome appears to be similar to that of the model plant Arabidopsis thaliana. We further show that air pollution increases the prevalence of bacterial functional traits affiliated to plant growth and detoxification of fossil fuel-related VOCs. The presence of relevant pathways could be confirmed by genome-level analyses of bacterial isolates showing a high in vitro capacity to metabolize selected VOCs and promote plant growth. In conclusion, an integrated description of the H. helix phyllobiome is presented, including the identification of interesting bacteria with significant potential as inoculants in phytoremediation approaches. This work is supported by the Hasselt University Methusalem project 08M03VGRJ

Phytoextraction has been reported as an economically and ecologically sound alternative for the remediation of metal-contaminated soils. Willow is a metal phytoextractor of interest because it allows to combine a gradual contaminant removal with production of biomass that can be valorized in different ways. In this work two willow clones growing on a metal-contaminated site were selected: ‘Belgisch Rood’ (BR) with a moderate metal extraction capacity and ‘Tora’ (TO) with a twice as high metal accumulation. All cultivable bacteria associated with both willow clones were isolated and identified using 16SrDNA ARDRA analysis followed by 16SrDNA sequencing. Further all isolated bacteria were investigated for characteristics that might promote plant growth (production of siderophores, organic acids and indol acetic acid) and for their metal resistance. The genotypic and phenotypic characterization of the isolated bacteria showed that the TO endophytic bacterial population is more diverse and contains a higher percentage of metal-resistant plant growth promoting bacteria than the endophytic population associated with BR. We hypothesize that the difference in the metal accumulation capacity between BR and TO clones might be at least partly related to differences in characteristics of their associated bacterial population. Funding Information This research was funded by the Institute for the Promotion of Innovation through Science and Technology in Flanders (IWT-Vlaanderen), PhD grant for J.J., and by the Fund for Scientific Research Flanders (FWO-Vlaanderen), PhD grant for S.C. and post doc grant for N. We. and N. Wi. This work has been financially supported by the UHasselt Methusalem project 08M03VGRJ and by the European Commission under the Seventh Framwork Programme for Research (FP7-KBBE-266124, GREEN-LAND).

Gentle remediation options (GROs) are risk management strategies or technologies involving plant (phyto-), fungi (myco-), and/or bacteria-based methods that result in a net gain (or at least no gross reduction) in soil function as well as effective risk management. GRO strategies can be customised along contaminant linkages, and can generate a range of wider economic, environmental and societal benefits in contaminated land management (and in brownfields management more widely). The application of GROs as practical on-site remedial solutions is still limited however, particularly in Europe and at trace element (typically metal and metalloid) contaminated sites. This paper discusses challenges to the practical adoption of GROs in contaminated land management, and outlines the decision support tools and best practice guidance developed in the European Commission FP7-funded GREENLAND project aimed at overcoming these challenges. The GREENLAND guidance promotes a refocus from phytoremediation to wider GROs- or phyto-management based approaches which place realisation of wider benefits at the core of site design, and where gentle remediation technologies can be applied as part of integrated, mixed, site risk management solutions or as part of “holding strategies” for vacant sites. The combination of GROs with renewables, both in terms of biomass generation but also with green technologies such as wind and solar power, can provide a range of economic and other benefits and can potentially support the return of low-level contaminated sites to productive usage, while combining GROs with urban design and landscape architecture, and integrating GRO strategies with sustainable urban drainage systems and community gardens/parkland (particularly for health and leisure benefits), has large potential for triggering GRO application and in realising wider benefits in urban and suburban systems. Quantifying these wider benefits and value (above standard economic returns) will be important in leveraging funding for GRO application and soft site end-use more widely at vacant or underutilized sites. Further details of the GREENLAND project, and downloadable versions of the GREENLAND decision support tool and GRO best practice guidance, can be found at http://www.greenland-project.eu. The authors are grateful for financial support from the European Commission under the Seventh Framework Programme for Research (FP7-KBBE-266124), and thank the wider GREENLAND consortium and project advisory board for their support and input during the development of the DST and GRO best practice guidance.

Additional file 1. Soil characteristics analyzed in 2012 and 2013. a organic matter, b electrical conductivity of a saturated soil paste extract (ECe), c Total soil N by Kjeldahl method.

Phytoremediation is a promising technology to clean-up contaminated soils based on the synergistic actions of plants and microorganisms. However, to become a widely accepted, and predictable remediation alternative, a deeper understanding of the plant-microbe interactions is needed. A number of studies link the success of phytoremediation to the plant-associated microbiome functioning, though whether the microbiome can exist in alternative, functional states for soil remediation, is incompletely understood. Moreover, current approaches that target the plant host, and environment separately to improve phytoremediation, potentially overlook microbial functions and properties that are part of the multiscale complexity of the plant-environment wherein biodegradation takes place. In contrast, in situ studies of phytoremediation research at the metaorganism level (host and microbiome together) are lacking. Here, we discuss a competition-driven model, based on recent evidence from the metagenomics level, and hypotheses generated by microbial community ecology, to explain the establishment of a catabolic rhizosphere microbiome in a contaminated soil. There is evidence to ground that if the host provides the right level and mix of resources (exudates) over which the microbes can compete, then a competitive catabolic and plant-growth promoting (PGP) microbiome can be selected for as long as it provides a competitive superiority in the niche. The competition-driven model indicates four strategies to interfere with the microbiome. Specifically, the rhizosphere microbiome community can be shifted using treatments that alter the host, resources, environment, and that take advantage of prioritization in inoculation. Our model and suggestions, considering the metaorganism in its natural context, would allow to gain further knowledge on the plant-microbial functions, and facilitate translation to more effective, and predictable phytotechnologies. This work was supported by a PhD grant to ST and a post doc grant for NW from the Fund for Scientific Research Flanders, Belgium (FWO-Vlaanderen) and the Methusalem project 08M03VGRJ.

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Many endophytes have beneficial effects on plants and can be exploited in biotechnological applications. Studies hypothesize that only 0.001–1% of all plantassociated bacteria are cultivable. Moreover, even after successful isolations, many endophytic bacteria often show reduced regrowth capacity. This research aimed to optimize isolation processes and culturing these bacteria afterwards. We compared several minimal and complex media in a screening. Beside the media themselves, two gelling agents and adding plant extract to media were investigated to enhance the number and diversity of endophytes as well as the growth capacity when regrown after isolation. In this work, 869 medium delivered the highest numbers of cultivable bacteria, as well as the highest diversity. When comparing gelling agents, no differences were observed in the numbers of bacteria. Adding plant extract to the media lead to a slight increase in diversity. However, when adding plant extract to improve the regrowth capacity, sharp increases of viable bacteria occurred in both rich and minimal media. The financial support of the agency for Innovation by Science and Technology (IWT) for project 121243 is greatly appreciated.

Gentle remediation options (GRO) are risk management strategies/technologies that result in a net gain (or at least no gross reduction) in soil function as well as risk management. They encompass a number of technologies, including the use of plant (phyto-), fungi (myco-), and/or bacteria-based methods, with or without chemical soil additives or amendments, for reducing contaminant transfer to local receptors by in situ stabilization, or extraction, transformation, or degradation of contaminants. Despite offering strong benefits in terms of risk management, deployment costs, and sustainability for a range of site problems, the application of GRO as practical on-site remedial solutions is still in its relative infancy, particularly for metal(loid)-contaminated sites. A key barrier to wider adoption of GRO relates to general uncertainties and lack of stakeholder confidence in (and indeed knowledge of) the feasibility or reliability of GRO as practical risk management solutions. The GREENLAND project has therefore developed a simple and transparent decision support framework for promoting the appropriate use of gentle remediation options and encouraging participation of stakeholders, supplemented by a set of specific design aids for use when GRO appear to be a viable option. The framework is presented as a three phased model or Decision Support Tool (DST), in the form of a Microsoft Excel-based workbook, designed to inform decision-making and options appraisal during the selection of remedial approaches for contaminated sites. The DST acts as a simple decision support and stakeholder engagement tool for the application of GRO, providing a context for GRO application (particularly where soft end-use of remediated land is envisaged), quick reference tables (including an economic cost calculator), and supporting information and technical guidance drawing on practical examples of effective GRO application at trace metal(loid) contaminated sites across Europe. This article introduces the decision support framework. The authors are grateful for financial support from the European Commission under the Seventh Framework Programme for Research (FP7-KBBE-266124).