Maintaining the cultivation of vegetables with low Pb accumulation while remediating the soil of an allotment garden (Nantes, France) by phytoextraction.

Lead (Pb) is commonly found in urban soils and can transfer to vegetables. This entails a health risk for consumers of garden crops. The increasing demand of gardening on urban soil linked to the population increase and concentration in urban areas induces an increase in the risk, as people could be forced to cultivate contaminated soils. The aim of this study was to evaluate the performance of a cropping system that allows simultaneously (i) growing eatable vegetables that accumulate few Pb and (ii) cleaning up the soil with other plants by phytoextraction. The tests were carried out in an allotment garden (Nantes, France) where soils are moderately enriched by Pb from geogenic origin (178 mg.kg-1 of dry soil on average). Four vegetables known to accumulate slightly Pb (Solanum lycopersicum, Brassica oleracea cv. "Capitata," Solanum tuberosum, and Phaseolus vulgaris) were grown. The in situ ability of Brassica juncea L. to progressively absorb the phytoavailable Pb of the soil was assessed during four seasons. Analyses of the edible parts of the four vegetables confirmed that they can all be safely cultivated. The accumulation of Pb in B. juncea shoots was too low (ca. 1 mg.kg-1 of dry matter at best) for phytoextraction purposes. Our results confirm that it is possible to grow very low Pb-accumulating vegetables on soils moderately contaminated with Pb, although it was not possible to reduce phytoavailable Pb rapidly enough with B. juncea. This study identifies possible avenues of research to improve this cropping system by using appropriate vegetables that will allow food production to continue on moderately contaminated soil while cleaning it up.

Drivers to improve metal(loid) phytoextraction with a focus on microbial degradation of dissolved organic matter in soils.

Bioaugmentation of soils can increase the mobilization of metal(loid)s from the soil-bearing phases. However, once desorbed, these metal(loid)s are mostly complexed to the dissolved organic matter (DOM) in the soil solution, which can restrict their availability to plants (roots mainly taking up the free forms) and then the phytoextraction performances. Firstly the main drivers influencing phytoextraction are reminded, then the review focuses on the DOM role. After having reminding the origin, the chemical structure and the lability of DOM, the pool of stable DOM (the most abundant in the soil) most involved in the complexation of metal(loid)s is addressed in particular by focusing on carboxylic and/or phenolic groups and factors controlling metal(loid) complexation with DOM. Finally, this review addresses the ability of microorganisms to degrade metal(loid)-DOM complexes as an additional lever for increasing the pool of free metal(loid) ions, and then phytoextraction performances, and details the origin of microorganisms and how they are selected. The development of innovative processes including the use of these DOM-degrading microorganisms is proposed in perspectives.

This review focuses on the available drivers to increase the pool of free (i.e. phytoavailable) metal(loid)s in the soil solution, with a specific focus on the ability of microorganisms to degrade dissolved organic matter for enriching this pool, and then to substantially improve phytoextraction performance.

Isolation and screening of indigenous bacteria from phosphogypsum-contaminated soils for their potential in promoting plant growth and trace elements mobilization.

Bacteria isolated from soils in the vicinity of phosphogypsum (PG) stockpiles were studied for their potential use in bioaugmentation-assisted phytoextraction. Quick, miniaturized biochemical tests were performed in the presence of metal trace elements (MTE), including rare earth elements (Cd, Sr, Ce, La, Nd and Y), corresponding to their bioavailable concentrations in PG. The intention herein was to assess the capacity of bacteria to: i) grow in PG; ii) produce indole acetic acid and ACC deaminase to promote plant growth and reduce stress; and iii) produce siderophores, including pyoverdine, to mobilize MTE. Results showed that even at maximum PG concentration (10 g/L and pH 3.40), 7 out of 32 isolates were able to grow. The biochemical tests showed differences in the presence or absence of MTE. The presence of MTE seems to promote the production of IAA by a factor of 3.25. On the contrary, it inhibits ACC deaminase and siderophore production, including pyoverdine. According to a scoring method applied, the two most efficient isolates exhibiting maximum metabolite production were identified as Bacillus sp.

Fate and transport of metal trace elements from phosphogypsum piles in Tunisia and their impact on soil bacteria and wild plants.

The phosphate industry in Tunisia generates large amounts of phosphogypsum (PG) with more than 107 t per year. Environmental impact of this solid waste was studied. Cd, Ce, La, Nd, Sr and Y were analyzed from soils near PG stockpiles (Sfax and M'dhilla) and sediments from marine discharge (Gabes). Their impacts on the bacterial community structure and wild plants were investigated. Metal trace elements (MTE) concentrations (in mg Kg-1 DM) were much higher in contaminated soil than in the control (at 12 km from PG stockpiles). Highest concentrations were recorded in top soil and decreased with depth. A low bacterial diversity was shown (impacted by plants more than by MTE). The MTE concentrations in aerial parts (AP) and roots varied according to the plant species and were higher in contaminated sites. Sr, La and Cd in the AP ranged 33.10-657.56, 2.22-11.05 and 0.21-14.20 mg Kg-1 DM respectively. Plants exhibiting the maximal metal concentrations in AP (in mg Kg-1 DM) were the following: Zygophylum album for Sr (657.56) >Zygophylum album for Cd (14.20) >Zygophylum album (11.05) for La >Conyza canadensis (1.11) for Ce >Conyza canadensis (0.75) for Nd >Arthrocemum inducum (0.72) for Y. Kochia indica showed the highest bioconcentration factor (1.60) for Cd, while Zygophylum album exhibited the highest translocation factor (6.12) for La. Zygophylum album would be the most suitable candidate for MTE phytoextraction. CAPSULE: Phosphogypsum contaminates soils near stockpiles with metal trace elements including rare earth element and selects wild plants able to be used for phytostabilization and phytomining.

A defined terminal region of the E. coli chromosome shows late segregation and high FtsK activity.

BACKGROUND: The FtsK DNA-translocase controls the last steps of chromosome segregation in E. coli. It translocates sister chromosomes using the KOPS DNA motifs to orient its activity, and controls the resolution of dimeric forms of sister chromosomes by XerCD-mediated recombination at the dif site and their decatenation by TopoIV. METHODOLOGY: We have used XerCD/dif recombination as a genetic trap to probe the interaction of FtsK with loci located in different regions of the chromosome. This assay revealed that the activity of FtsK is restricted to a ∼400 kb terminal region of the chromosome around the natural position of the dif site. Preferential interaction with this region required the tethering of FtsK to the division septum via its N-terminal domain as well as its translocation activity. However, the KOPS-recognition activity of FtsK was not required. Displacement of replication termination outside the FtsK high activity region had no effect on FtsK activity and deletion of a part of this region was not compensated by its extension to neighbouring regions. By observing the fate of fluorescent-tagged loci of the ter region, we found that segregation of the FtsK high activity region is delayed compared to that of its adjacent regions. SIGNIFICANCE: Our results show that a restricted terminal region of the chromosome is specifically dedicated to the last steps of chromosome segregation and to their coupling with cell division by FtsK.

A dual role for the FtsK protein in Escherichia coli chromosome segregation.

FtsK is a multifunctional protein that acts in Escherichia coli cell division and chromosome segregation. Its C-terminal domain is required for XerCD-mediated recombination between dif sites that resolve chromosome dimers formed by recombination between sister chromosomes. We report the construction and analysis of a set of strains carrying different Xer recombination sites in place of dif, some of which recombine in an FtsK-independent manner. The results show that FtsK-independent Xer recombination does not support chromosome dimer resolution. Furthermore, resolution of dimers by the Cre/loxP system also requires FtsK. These findings reveal a second role for FtsK during chromosome dimer resolution in addition to XerCD activation. We propose that FtsK acts to position the dif regions, thus allowing a productive synapse between dif sites.