Use of Endophytic and Rhizosphere Bacteria To Improve Phytoremediation of Arsenic-Contaminated Industrial Soils by Autochthonous Betula celtiberica

The aim of the study was to investigate the potential of indigenous arsenic-tolerant bacteria to enhance arsenic phytoremediation by autochthonous pseudometallophyte Betula celtiberica . The first goal was to perform an initial analysis of the entire rhizosphere and endophytic bacterial communities of the above-named accumulator plant, including the cultivable bacterial species. B. celtiberica 9s microbiome was dominated by taxa related to Flavobacteriales, Burkholderiales, and Pseudomonadales, specially the Pseudomonas and Flavobacterium genera. A total of 54 cultivable rhizobacteria and 41 root endophytes, mainly affiliated to the phyla Proteobacteria, Bacteroidetes, Firmicutes, and Actinobacteria, were isolated and characterized with respect to several potentially useful features for metal plant accumulation, such as the ability to promote plant growth, metal chelation, and/or mitigation of heavy metal stress. Seven bacterial isolates were further selected and tested for in vitro arsenic plant-accumulation; four of them were finally assayed in field-scale bioaugmentation experiments. The exposure to arsenic in vitro caused increased total non-protein thiol compounds content in roots, suggesting a detoxification mechanism through phytochelatins complexation. In the contaminated field, the siderophore and IAA producers of the endophytic bacterial consortium enhanced As-accumulation in the leaves and roots of Betula celtiberica , whereas the rhizosphere isolate Ensifer adhaerens strain 91R mainly promoted plant growth. Field experimentation showed that additional factors, such as soil arsenic content and pH, influenced arsenic uptake in the plant, attesting to the relevance of field conditions in the success of phytoextraction strategies. IMPORTANCE Microorganisms and plants have developed several ways of dealing with arsenic, allowing them to resist and metabolize this metalloid. These properties form the basis of phytoremediation treatments and understanding the interactions of plants with soil bacteria is crucial for the optimization of As-uptake. To address this in our work, we initially performed a microbiome analysis of the autochthonous Betula celtiberica plants growing in As-contaminated soils, including endophytic and rhizosphere bacterial communities. We then proceeded to isolate and characterize the cultivable bacteria, potentially better suited to enhance phytoextraction efficiency. Eventually, we went to the field application stage. Our results corroborated that recovery of pseudometallophytes-associated bacteria adapted to a large historically contaminated site and their use in bioaugmentation technologies are affordable experimental approaches and potentially very useful for implementing effective phytoremediation strategies with plants and their indigenous bacteria.