Mercury, arsenic and boron resistant bacteria isolated from the phyllosphere as positive bioindicators of airborne pollution near geothermal plants

The bioindex percentage of element-resistant microorganisms is suggested to study airborne pollution near geothermal plants. A new method of determining the percentage of element-resistant bacteria is described. The index was calculated as the percentage ratio of the metal-resistant colony-forming units grown on agar plates to the whole cultivable eterotrophic colony forming units. Element resistance was regarded as the ability of bacteria to grow on media amended with high concentrations of toxic compounds: HgCl2 (10 μg/ml), NaAsO2 (750 μg/ml) and H3BO3 (1000 μg/ml). Available elements induced activity of the element-resistance in epiphytic bacteria. The mosses from which the bacteria were isolated were collected in winter from two different areas near Siena: ten sites in the geothermal area around Radicondoli and five sites in an unpolluted part of the Chianti region. Percentages of element-resistant bacteria were found to be directly correlated with concentrations of acid-leachable mercury, arsenic and boron. The percentages of mercury- and arsenic-resistant bacteria were significantly higher in the geothermal area, especially in the GEO-6 station. High concentrations of leachable boron were found in the geothermal area and in a wooded patch surrounded by vinyards in the Chianti region. Boron-resistant bacteria responded only to boron from geothermal emissions.