Exudates from Miscanthus x giganteus change the response of a root-associated Pseudomonas putida strain towards heavy metals.

The composition of root exudates is modulated by several environmental factors, and it remains unclear how that affects beneficial rhizosphere or inoculated microorganisms under heavy metal (HM) contamination. Therefore, we evaluated the transcriptional response of Pseudomonas putida E36 (a Miscanthus x giganteus isolate with plant growth promotion-related properties) to Cd, Pb and Zn in an in vitro study implementing root exudates from M. x giganteus. To collect root exudates and analyse their composition plants were grown in a pot experiment under HM and control conditions. Our results indicated higher exudation rate for plants challenged with HM. Further, out of 29 organic acids identified and quantified in the root exudates, 8 of them were significantly influenced by HM (e.g., salicylic and terephthalic acid). The transcriptional response of P. putida E36 was significantly affected by the HM addition to the growth medium, increasing the expression of several efflux pumps and stress response-related functional units. The additional supplementation of the growth medium with root exudates from HM-challenged plants resulted in a downregulation of 29% of the functional units upregulated in P. putida E36 as a result of HM addition to the growth medium. Surprisingly, root exudates + HM downregulated the expression of P. putida E36 functional units related to plant colonization (e.g., chemotaxis, motility, biofilm formation) but upregulated its antibiotic and biocide resistance compared to the control treatment without HM. Our findings suggest that HM-induced changes in root exudation pattern may attract beneficial bacteria that are in turn awarded with organic nutrients, helping them cope with HM stress. However, it might affect the ability of these bacteria to colonize plants growing in HM polluted areas. Those findings may offer an insight for future in vivo studies contributing to improvements in phytoremediation measures.

Post-reclamation microbial diversity and functions in hexachlorocyclohexane (HCH) contaminated soil in relation to spontaneous HCH tolerant vegetation.

The toxicity, volatility and persistence of the obsolete organochlorine pesticide hexachlorocyclohexane (HCH), makes reclamation of contaminated areas a priority for the health and welfare of neighboring human communities. Microbial diversity and functions and their relation to spontaneous vegetation in post-excavation situations, are essential indicators to consider in bioaugmentation or microbe-assisted phytoremediation strategies at field scale. Our study aimed to evaluate the effects of long-term HCH contamination on soil and plant-associated microbial communities, and whether contaminated soil has the potential to act as a bacterial inoculum in post-excavation bioremediation strategies. To scrutinize the role of vegetation, the potential nitrogen fixation of free-living and symbiotic diazotrophs of the legume Lotus tenuis was assessed as a measure of nutrient cycling functions in soil under HCH contamination. Potential nitrogen fixation was generally not affected by HCH, with the exception of lower nifH gene counts in excavated contaminated rhizospheres, most probably a short-term HCH effect on early bacterial succession in this compartment. HCH shaped microbial communities in long-term contaminated bulk soil, where we identified possible HCH tolerants such as Sphingomonas and Altererythrobacter. In L. tenuis rhizosphere, microbial community composition was additionally influenced by plant growth stage. Sphingobium and Massilia were the bacterial genera characteristic for HCH contaminated rhizospheres. Long-term HCH contamination negatively affected L. tenuis growth and development. However, root-associated bacterial community composition was driven solely by plant age, with negligible HCH effect. Results showed that L. tenuis acquired possible HCH tolerant bacteria such as the Allorhizobium-Neorhizobium-Pararhizobium-Rhizobium clade, Sphingomonas, Massilia or Pantoea which could simultaneously offer plant growth promoting (PGP) benefits for the host. Finally, we identified an inoculum with possibly HCH tolerant, PGP bacteria transferred from the contaminated bulk soil to L. tenuis roots through the rhizosphere compartment, consisting of Mesorhizobium loti, Neorhizobium galegae, Novosphingobium lindaniclasticum, Pantoea agglomerans and Lysobacter bugurensis.

Under temperate climate, the conversion of grassland to arable land affects soil nutrient stocks and bacteria in a short term.

Projected population growth and climate change will make it inevitable to convert neglected and marginal land into productive arable land. We investigate the influence of agricultural management practices on nutrient stocks and soil functions during the conversion of former extensively used grassland to arable land. Effects of grassland removal, tillage, intercropping with faba bean (Vicia faba) and its later incorporation were studied with respect to soil properties and bacterial community structure. Therefore, composite samples were collected with a core sampler from the topsoil (0-20 cm) in (a) the initial grassland, (b) the transitional phase during the vegetation period of V. faba, (c) after ploughing the legume in, and (d) untreated controls. In all samples, nitrate-N, ammonium-N, dissolved organic carbon (DOC) and total nitrogen bound (TNb) were analyzed and comparisons of the bacterial community structure after 16S-amplicon sequencing were performed to assess soil functions. Mineralization after grassland conversion followed by the biological nitrogen fixation of broad beans enhanced the nitrate-N content in bulk soil from 4 to almost 50 µg N g-1dw. Bacterial community structure on phylum level in bulk soil was dominated by Proteobacteria, Actinobacteria, Acidobacteria, Chloroflexi, and Bacteroidetes and remained almost stable. However, alpha and beta-diversity analysis revealed a change of the bacterial composition at the final state of the conversion. This change was primarily driven by increasing abundances of the genera Massilia and Lysobacter, both members of the Proteobacteria, after the decay of the leguminous plant residues. Furthermore, increasing abundances of the family Gaiellaceae and its genus Gaiella fostered this change and were related to the decreasing carbon to nitrogen ratio. In short, gentle management strategies could replace the input of mineral fertilizer with the aim to contribute to future sustainable and intensified production even on converted grassland.

HCH phytoremediation potential of native plant species from a contaminated urban site in Turda, Romania.

Current physical or chemical methods used for remediation of soils contaminated with hexachlocyclohexane (HCH), leave behind significant levels of pollutants. Given the compound's volatility and persistence in the environment, sites contaminated with HCH remain a concern for the population living in nearby areas. By making use of both the recovery capacity and the pollutant uptake ability of spontaneously growing vegetation, our study aimed to identify native plant species able to cover and moreover take up the HCH left at a former lindane production unit in Turda, Romania. The results showed that dominant species across the study site like Lotus tenuis, Artemisia vulgaris or Tanacetum vulgare, were capable of taking up HCH in their tissues, according to different patterns that combined at the scale of the plant community. Regardless of the proximity of the HCH contamination hotspots, the development of the plant cover was characteristic for vegetation succession on disturbed soils of the Central European region. Finally, we conclude that plant species which grow spontaneously at the HCH contaminated site in Turda and are capable of taking up the pollutant, represent a self-sustainable and low maintenance phytomanagement approach that would allow for the reintegration of the site in the urban or industrial circuit and nevertheless would reduce the toxicity risk to the neighboring human inhabitants.