Environmental pollution by metals represents a serious risk to human health and to the environment in both urban, industrial and neighboring areas (Ali et al. 2013). Among natural metalliferous soils, serpentine soils have nutrient scarcity and high concentrations of metals (bioavailable Ni 7-100 mg kg-1; total Ni 500-8000 mg kg-1) (Roccotiello et al., 2015; Turgay et al., 2012). These habitats are inhospitable for the most of the plant species, but highly or exclusively preferred hyperaccumulating plants: these species can live and reproduce on these metalliferous soils without showing toxicity symptoms (Rascio and Navari-Izzo, 2011), being able to accumulate metals such as nickel (Ni) at high concentrations in the aboveground biomass (Krämer, 2010). In some taxa of hyperaccumulators, the concentration of metals and metalloids in the shoot biomass is four orders of magnitude greater than in other non-hyperaccumulating species (Kramer 2010). In these taxa, the rhizosphere, defined as the soil-root interface, plays a key role because it represents the first area of exchange and potential uptake of contaminants, where the roots have free access to the elements of the soil (Alford et al. 2010). Improving the effectiveness of the absorption of metals by the root system of native hyperaccumulators, using the rhizospheric microbiota as a natural chelator of metals, allows increasing phytoextraction with consequent increased potential for environmental restoration and significant economic benefits (Rosatto et al. 2019). This study aims to develop an integrated phytoremediation protocol, employing the facultative Ni hyperaccumulator Alyssoides utriculata (L.) Medik., that was of interest for its environmental plasticity (Roccotiello et al. 2017). This species was inoculated with microbial strains isolated from its rhizosphere: the bacterial strain of Pseudomonas fluorescens SERP1, the fungal strain of Penicillium ochrochloron SERP03S and a mixed inoculum consisting of both strains. These strains, investigated in previous studies developed in collaborations with the Mycology laboratory and the Microbiology laboratory of DISTAV, have been proved to be able to grow in synergy, maintaining the morphological traits typical of each species and reaching maturity, without mutual inhibition (Rosatto et al. 2019). The thesis consists into six thematic chapters: Chapter 1 examines the natural and anthropogenic sources of heavy metals and the risks they pose to human health. In particular, the Nickel element is investigated, being naturally present in considerable amounts in serpentinite soils. The metallophytes and their mechanisms of tolerance and accumulation with respect to metals and the role of Nickel (Ni) in plant physiology are then examined, in regard to hyperaccumulating plants and the physiological mechanisms involved in the hyperaccumulation process. Finally, the processes of phytoremediation and the species used for this purpose are described and the integrated phytoremediation technique, a recently developed technique that involves the use of bacterial and fungal strains as root growth promoters to relieve stress from metals, is explored. The objectives of the current study are subsequently described. Chapter 2 illustrates the soil sampling in the field, the experimental design, the monitoring plan in the different months of the experimentation (t0, t6, t12, t18, respectively) and the parameters measured to evaluate the response of the plants under different treatments (control, single bacterial and fungal inoculum, co-inocula mix of bacterial and fungal strains). The accumulation of leaf Ni is qualitatively assessed with dimethylglyoxime (DMG test). The methodology for the isolation and culture of bacterial and fungal strains is described and the measured biometric and ecophysiological plant parameters are described: fresh and dry biomass, photosynthetic efficiency, and performance index of the plants in the experiment. Chapter 3 illustrates the data obtained and the main results. The data analysis revealed that plants inoculated with a single inoculum (bacterial or fungal), compared to control plants without inoculum, have a greater development of belowground and aboveground biomass but do not show and increased accumulation of Ni. The Student T-test for unpaired samples does not highlight any significant differences for both the photosynthetic efficiency (Fv/Fm) and the Performance Index (PI) between the 'control' group and single inoculated ('Bacteria' and 'Fungi') or co-inoculated plants ('Mix'). Furthermore, the data show that there is no further increase in biomass and physiological response in co-inoculated plants compared to those with single inoculum. Chapter 4 illustrates and discusses the main results obtained, highlighting the potential applicability of the proposed methodological approach in the field. In the end, the conclusions (chapter 5) summarize the main evidence and outline future scenarios. Further studies will validate the effectiveness of selected co-inocula in the field in polymetalliferous soils to understand their potential applicability on a larger scale. The study of the possible use of single inocula and co-inocula in heavy metal hyperaccumulators is currently rising and can significantly contribute to set up more sustainable and effective phytoremediation techniques.
|Titolo della tesi:||Phytoremediation integrata di contaminanti inorganici|
|Data di discussione:||22-apr-2022|
|Appare nelle tipologie:||Tesi di dottorato|