Bacterial resources for assisted phytostabilization of acid mine drainage-affected mountain stream bank

Several areas of the Italian Alps have an arsenic content that exceeds the Italian law limit (20 mg kg-1, D.Lgs 152/2006), due to mineralogy of bedrock and to mining activities. Rio Rosso mountain stream (Anzasca Valley, Piedmont), is affected by arsenic (1015 mg kg-1) and iron (22 g kg-1) contaminated sediments that leach from an abandoned gold mine. In view of a bacterial-assisted phytostabilization action, rhizospheric bacteria and root endophytes of liverwort, fern and willow inhabiting stream bank were isolated and characterized. A total of 240 colonies were isolated and screened for arsenic-related features and plant grow-promoting traits. Most of rhizobacteria and endophytes were resistant up to 7500 mg L-1 arsenate (21 and 20 respectively) and up to 750 mg L-1 arsenite (17 and 24 respectively), regardless plant species. More than 80% of the isolates were capable to reduce 75 mg L-1 of arsenate, while arsenite oxidation rarely occurred (<5% of the isolates), reflecting the prevalence of As(V) in the sediments. The endophytes of fern and willow mostly exhibited indole acetic acid and exopolysaccharide production, while rhizobacteria primarily possessed P solubilizing activity. This evidenced that the two different plant compartments exerted an effect on plant grow-promoting traits. These were homogenously distributed among rhizobacteria and endophytes of liverwort. A super imposed selective pressure was exerted by arsenic on the plants of the mountain stream bank, thus resulting in the distribution of arsenic resistance in plant rhizobiomes. The bacterial resources characterized in the present study will be exploited for the development of a bacterial-assisted phytostabilization of the arsenic contaminated stream.