The interaction between rhizobacteria and the hyperaccumulator fern Pteris vittata in arsenic transformation

Arsenic is an ubiquitous element which occurs naturally in the earth's crust. Arsenic is toxic to both plants and animals and inorganic arsenicals are proven carcinogens in humans (Ng, 2005). Arsenic toxicity to human health ranges from skin lesions to brain, liver, kidney, and stomach cancer (Smith et al., 1992). Arsenic contamination of groundwater used for domestic supplies has been reported in over 70 countries, affecting health of an estimated 150 million people (Ravenscroft et al., 2009). Because of its wide distribution and marked toxic effects, arsenic contamination is a problem of global concern. Inorganic arsenic is indeed regarded as the number one toxin in the USEPA list of priority pollutants (Ng et al., 2003). In this respect, an interesting case study is that discussed in this PhD thesis focusing on the Scarlino area, a Site of Regional Interest (SRI) which requires prominent attention from the environmental point of view. The contamination in this context is the result of dumping of ash from arsenopyrite roasting for more than 30 years as a consequence of sulfuric acid production by the Nuova Solmine Company operating with industrial facilities in this site. Disposal of such ash in addition to the natural arsenic background has thus provoked either a serious soil contamination or a diffuse pollution of aquifers within the whole industrial district, where As can be detected in an average concentration of 140 mg/kg. An approach that might alleviate this problem is represented by the biotechnological option defined as microbe-enhanced phytoremediation. Phytoremediation is a low-cost and eco-friendly technology that uses vascular plants for in situ environmental restoration and reclamation of contaminated soils, sludges and sediments. Particularly, in the case of As contamination, phytoextraction – which is the removal of toxic metals/metalloids from soil or whatever other environmental matrix and their concentration into the harvestable plant portion – appears quite useful (Khan, 2005). Microorganisms are known to play a very important role in this process. Actually, bacteria can enhance the mobility of arsenic in the soil matrix (eliciting the metalloid uptake by plants) while plant growth promoting rhizobacteria (PGPR) can improve plant biomass production (Abou- Shanab et al., 2003a; Glick et al., 1995; Glick, 2003). on the basis of obtained data it can be stated that:The high contamination due to arsenic and heavy metals present in the Nuova Solmine industrial area has determined a selection of the soil autochthonous bacterial cenoses towards a more tolerant and well adapted community, with wide biodiversity, resistance and As-transforming potential; • Most of the strains identified in the area with the highest pollutant concentrations (M, the arsenopyrite ash dumping pile) belonged to Gram positive Firmicutes and Actinobacteria, including strains possessing interesting genotypic traits involved in As transformation; • The isolates belonging to Delftia lacustris, and Pseudomonas putida specie reduced As(V) in vitro at high efficiency also showing the ability to solubilize arsenic in soils; • Among the members of the above mentioned bacterial community isolated in pure culture, strains belonging mainly to Gamma-proteobacteria were carrying particular PGP traits useful in a phytoremediation perspective based on bioaugmentation;Ochrobactrum cytisi, Pseudomonas putida and Achromobacter marplatensis were selected and tested for a microbe-enhanced phytoextraction experiment in association with the hyperaccumulator fern P. vittata. They exerted positive effects on both plant biomass production and total phytoextraction efficiency when compared with the not inoculated plants. However, all the plants – with or without bacterial inoculants – were able to extract an appreciable amount of As; however, where the inocula were present, ferns lowered As concentration in soil more effectively. The results of a second phytoextraction trial with another selection of As-resistant PGP strain (P. putida, Delftia lacustris, B. thuringiensis, Variovorax paradoxus, Pseudoxanthomonas mexicana) are coming. When completed, these preliminary evidences will allow either to evaluate the phytoextraction potential of the bacterial strains tested so far at lab scale or to schedule a future pilot trial at the Scarlino contaminated site. In conclusion an integrated system of plants and bacteria would be perfected as a reliable remediation tool to be applied in the Scarlino industrial area.