Phytostabilization of trace element polluted soils using native aromatic plant species suitable for phytomanagement

Abstract

Addressing trace element pollution is one of the controversial areas of environmental research. Despite the natural existence of these elements in the environment, several anthropogenic sources have contributed to an unusually high concentration of elements, causing serious damage to health and ecosystems. In accordance with the evolution of international standards regarding care and protection of the environments, the techniques and strategies of industrial exploitation have been adapted in order to achieve sustainable processes. On this context, the conventional physical-chemical processes of soil reclamation have been gradually replaced by more efficient, economical, bio-based strategies with better public acceptance, such as phytoremediation. Accordingly, the aim of this thesis was to generate the necessary knowledge to develop a phytostabilization strategy for trace element polluted soils, based on the synergistic use of aromatic plant species and rhizobacteria that promote plant growth, in a context of economic revalorization of polluted lands. A pioneer trace element tolerant plan specie was reported as result of the bio-prospecting campaigns carried out in mining sites and in vitro studies. We could analyze the physiological response of this plants and their bacterial communities structure and diversity when was grown on contaminated soils in vitro. Tolerant and plant growth promoters rhizospheric bacteria were also isolated and identified. Through molecular and analytical microscopy tools, the sequestration of elements by these bacteria was evaluated and also the possibility of bioaugmenting them in order to improve the stabilization of the elements in the rhizosphere. Finally, the phytostabilization of a shooting range area was carried out to confirm the previous findings in vitro and in greenhouse. Also the extraction of the essential oils, as well as the element localization in the by-products obtained was analyzed. Our results indicate that Helianthus petiolaris can grow in sandy soils polluted with up to 400 mg Kg-1 of Pb, accumulate up to 40% of this element present in the soil in its roots and translocate only 10% of the accumulated element to the above ground part. In vitro studies indicated that H. petiolaris also adapts the content of photosynthetic pigments in its leaf cells to cope with the presence of these elements. In turn, essential oils obtained from plants grown in contaminated sites do not present differences in composition with those obtained from noncontaminated sites, although the essential oil yield is significantly lower when is obtained from such sites. Among the 26 isolated and identified tolerant microorganisms, 6 bacteria had the best performance in terms of plant growth promotion in vitro. Bacillus paramycoides ST9, Bacillus wiedmanni ST29, Bacillus proteolyticus ST89, Brevibacterium frigoritolerans ST30, Cellulosimicrobium cellulans ST54, and Methylobacterium sp. ST85 were selected to carry out individual and consortium bioaugmentation trials. Two strains expressed significant effects when they were inoculated individually in H. annuus: B. proteolyticus ST89, increased plant biomass by 40% and decreased element absorption by 20%. While B. paramycoides ST9, increased Pb and Cd concentrations in the aerial part of H. annuus. The tested consortia did not obtain better results of plant biomass development. However, an increase in element sequestration by 80% and 85% was visible in the rhizosphere of H. annuus plants when they were inoculated with artificial consortia C2 and C3 respectively. Lastly, we were able to localize the (sub)-cellular structures where these microorganisms accumulate trace elements: Bacillus wiedmanni ST29 and Bacillus paramycoides ST9 absorbed Pb into the cells; Bacillus paramycoides ST9 adsorbed the biggest amount of Pb on the cell wall and Cellulosimicrobium cellulans ST54 retained Pb on the extracellular matrix. In this context we can conclude that: i- the bioprospection of native species can provide new, naturally selected organisms and efficient for the application of phytoremediation. Taking advantage of the multiple ecological advantages that these plant species represent; ii- the strategically directed manipulation of the synergistic plantbacterium associations and interactions constitutes an effective and technologically viable strategy for the enhancement of this process, and iii- both the essential oil and the floral water obtained from the plant biomass can be sold with the assurance that they do not contain trace elements. In this way, the profits from the commercialization of these products could solve both the costs of implementing environmental restoration, as well as the socio-economic impact left by many extractive activities when they leave the affected areas.