soil remediation- appearances can be deceiving

8/10/2019 Soil Remediation- Appearances Can Be Deceiving

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Reflexions, le site de vulgarisation de l'Universit de Lige

Universit de Lige - http://reflexions.ulg.ac.be/ - 22 December 2014- 3 -

Instantaneous measurement of fluorescence

Aricia Evlard subjected 41 willow clones from thecloning park of the CRA-W located at Grand-Manil (Gembloux), to a substrate contaminated with heavy metals.Instead of being satisfied with mere empirical observation (phenotypical) of their growth, she investigated the

tolerance of the young trees to these pollutants (1). For two years she used physiological methods whichconsist in measuring chlorophyll fluorescence, to measure the sugars and pigments present in the leaves and toidentify the expression of proteins (known as a "proteomic" approach). "The more the plant becomes stressed by the metals the more it emits fluorescence. This fluorescence can be measured by a fluorometer, a technique which has the advantage of being non-destructive and delivering immediate results. Pigments and sugars also constitute parameters of stress which are interesting to study. During metal stress, the concentration in sugars can increase for example. This is linked to the roles sugars play in the protection of proteins and the maintenance of cellular activities" . Finally, the identification of certain proteins can indicate the presenceof reactive oxygen species (ROS), oxygen free radicals that are more toxic than oxygen. "Under normal circumstances, there is always a minimum of ROS produced in the cells. The cell has defense mechanisms that can make them disappear before they can do any damage. These antioxidant defense mechanisms can be enzymatic (again, proteins) or non-enzymatic. However, when the plant is stressed, the ROS are formed more rapidly than they are eliminated. The result of this is that the ROS interact with the macromolecules ( DNA , lipids and proteins) to the point of deactivating the antioxidant defense mechanisms. Indirectly, the metals cause various disorders of the cellular metabolism (loss of membrane integrity, altered photosynthesis,
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accelerated senescence, etc.)" The vital functions of the plant are therefore reduced or inhibited. Finally theplant in question dies.After having subjected the 41 clones to these four examinations, it clearly emerged that one of them stands

out from the rest. Here it is not important for the purposes of the demonstration which species or sub-speciesis identified. "The main thing is that this clone was not only part of those which produced the most biomass in the presence of metals, but also stood out from the others by having the highest levels of metals in its twigs.In reality, during a supplementary study, it became clear that it was the least tolerant to metal stress from a physiological and proteomics viewpoint. This study was also able to demonstrate that the good individual "phytoextractor" or the most tolerant individual is not neccessarily that which produces the most biomass".

Accumulation and good health are not synonymous!

The main lesson learned from this research: a tree that "holds its own" against the metals, and which producesa lot of biomass, is not neccessarily in good health due to its weak tolerance to MTEs. For this reason, it isnot necessarily a good phytoextractor in the long term. This fact is even more interesting when we considerthat the duration of a short rotation coppice-the type of crop generally used for energy purposes-is at leasttwenty years.

Another observation made during this thesis: while the clones of willow tested at Gembloux seem lessinteresting from the point of view of remediating a polluted site within a reasonable timescale, they appear,on the other hand, to be more interesting to use in the context of phytostabilisation. "In fact they seem to be

tolerant to the metals but slow down the transfer of these to the upper parts of the plant and concentrate them around the area of the rhizosphere. In other words, when this type of individual is planted, the metals are better "handled" (this has yet to be studied in situ) than in the absence of planting: the metals do not travel (or at least very rarely) towards the water tables. Also, because the transfer of pollutants to the twigs and leaves is reduced, the contamination of the biomass is also reduced as well as the restitution of the metals due to leaf-fall".

A necessary precaution must be taken, however: because part of the metals remain likely to move towardsthe biomass, this woody material must be treated in controlled combustion units, that is to say, equipped with

specific filters. This is the recommendation of Aricia Evlard with regard to current legislation overseas becausehere in Belgium there are no regional regulations governing this subject.

The endophytes show themselves

Subsequently, in order to increase knowledge about the phytoremediation potential of the willow, Aricia Evlardconcentrated on the mushrooms present around the roots. What role do these play in the fixation of metals?How exactly do the fungal cells act? Does their presence slow down the flow of metals to the plant (acting as abarrier) or, on the contrary, does their presence facilitate the flow of metals to the above-ground plant surfaces?With her team, the doctoral student concentrated on the job of perfecting an in vitro protocol aimed atassessing the sensitivity of the fungal strains associated with the roots of alder and willow trees. First shemade a collection of 91 fungal strains taken from the banks of the non-navigable watercourses of Wallonia.The objective: to test the tolerance of these strains in vitro by subjecting them to various concentrations ofcadmium (1, 5, 10, 25 or 50 mg per liter of culture medium) this heavy metal was chosen due to its high levelof toxicity. The first surprise: against all expectations, the majority of the strains showed themselves to beperfectly tolerant to the metal, even for a maximal dose of 50 mg. "I was expecting lethal effects. Yet this was
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certainly not the case". Second surprise: "among the most resistant strains, I discovered endophytes, more particularly 'Dark Septate Endophytes', that is to say, mushrooms belonging to the sub-branch of ascomycetes and which live inside roots ; they are not parasites nor are they really symbiotic (2).

It seems that tolerance to these high doses of cadmium can be explained by a phenomenon of absorption ofmelanin present in the walls of the mushroom. "We can reasonably suppose that this absorption phenomenon can also be observed for other metals, but this remains to be established. We have already reproduced this procedure with Zinc: the results are in the process of being interpreted. In any event, we can, at this stage,posit the theory that the role of this microflora in the control of metals could be important in the willow and alder trees. It would be interesting to test the most highly resistant strains by inoculating them with woody species used in phytoremediation".Finally, in the light of this work, what hopes can we place in phytoextraction and phytostabilisation? "It must be remembered that these two approaches are slow and are spread out over several years. They are only useful when used as a complement to traditional physico-chemical techniques when there is persistent residual pollution. In addition, these more radical techniques very often disturb the structure of the soil and leave it very fragile. Phytoremediation projects by means of the willow tree can help to compensate for the loss of natural resources and the damage caused by traditional techniques. It is in this sense that phytoremediation can be considered as a complementary and/or compensatory measure, described in the 'soil' legislation recently passed in Wallonia.

(1) For this purpose she is the beneficiary of assistance from the Gabriel Lippmann Public ResearchCentre (Luxembourg).

(2) At the present stage of knowledge.
http://www.crpgl.lu/index.php?id=27&L=2http://www.crpgl.lu/index.php?id=27&L=2

soil remediation- appearances can be deceiving

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