Isolation and Bioassay of N-fixing Bacteria from Mercury-Contaminated Tailing and Soil of Artisanal and Small-Scale Gold Mining

Abstract

This study aims to isolate Nitrogen-fixing bacteria from tailing and heavy metal (HM) polluted soils and examine their effectiveness as a plant growth-promoting rhizobacteria (PGPR) to increase foxtail millet plant growth. We analyzed artisanal and small-scale gold mining (ASGM) sites in Pongkor Village, West Java, Indonesia, and identified nine areas with significant contamination of Mercury (Hg), Lead (Pb), Cadmium (Cd), Chromium (Cr), and Arsenic (As). We isolated sixteen strains of nitrogen-fixing bacteria different carbon sources. We identified three strains (3.2.h, 4.2.b, and 9.2.a) that the most robust nitrogenase activity and indole acetic acid (IAA) production. They were identified based on their molecular characteristics (16S rRNA) as Ralstonia pickettii strain LMQOTU12, Azotobacter chroococcum subsp. isscasi strain P205, and Azotobacter chroococcum subsp. isscasi strain P205. The three isolates were inoculated to foxtail millet plant grown under Andosol slightly acidic (pH 5.8), moderate nitrogen content (0.26%). The three strains isolates demonstrated significant enhancement to foxtail millet plants’ growth. Improvements (compared to control) in shoot dry weight: 8% (3.2.h), 32% (4.2.b), 88% (9.2.a); longest root length: 88.71% (3.2.h), 108.90% (4.2.b), 140.16%; root dry weight: 350% (3.2.h), 450% (4.2.b), 250% (9.2.a); and root-shoot ratio 312% (3.2.h), 312% (4.2.b), 75% (9.2.a). This study demonstrates that the three strains are potential PGPRs for increasing plant growth of foxtail millet.