RECHAR-M2C: MICROBIAL ISOLATION FROM LEGACY-CONTAMINATED SOILS

Abstract

RECHAR-M2C: MICROBIAL ISOLATION FROM LEGACY-CONTAMINATED SOILS

The present study aimed to isolate Zn-solubilising and metal-tolerant microbial strains from legacy-contaminated soils affected by potentially toxic elements (PTEs), persistent organic pollutants (POPs), or their combination. A total of 242 microbial strains were isolated. Subsequent sequential screening for Zn-solubilisation potential identified 13 isolates with solubilisation indices exceeding 1.5. Of these, four isolates (S21, S56, TA5, and TE32) exhibited strong tolerance to 0.03–0.06% Zn, with relative growth intensities (RGIs) ranging from 86.6% to 104%, while S56 and TE32 maintained sufficient tolerance at a Zn concentration of 0.12%. Regarding Ni tolerance, only TE32 exhibited sufficient resistance at a Ni concentration of 0.03%, with an RGI of 47.2%. For Cu tolerance, isolates S21, S56, and TE32 demonstrated strong resistance at a Cu concentration of 0.03%, with RGIs of 92.8%, 82.4%, and 79.6%, respectively. Furthermore, TE32 maintained sufficient tolerance at 0.06% Cu, with an RGI of 63.0%. Based on these results, three isolates (S21, S56, and TE32) were selected for comprehensive characterisation of their enzymatic (amylolytic, cellulolytic, lipolytic, and proteolytic) and plant growth-promoting traits, including siderophore, extracellular polymeric saccharide (EPS), and indole-3-acetic acid (IAA) production. Strain S21 exhibited moderate proteolytic activity, siderophore production, and superior IAA synthesis, reaching a peak concentration of 38.7 ± 3.29 µg mL⁻¹. Strain S56 demonstrated moderate proteolytic activity, strong lipolytic and proteolytic activities, siderophore production, and moderate IAA synthesis, with a maximum concentration of 17.5 ± 1.48 µg mL⁻¹. In contrast, strain TE32 exhibited strong amylolytic activity, proteolytic activity, EPS production, and the lowest IAA-producing capacity, reaching a peak concentration of 12.8 ± 0.88 µg mL⁻¹. Future research will focus on the development of microbe–biochar complexes through the immobilisation of microbial cells onto biochar surfaces, followed by validation of the developed complexes in marginal and/or contaminated soils to enhance plant growth.

Keywords: legacy soil contamination; potentially toxic elements; Zn-solubilising microorganisms; tolerance to heavy metals; plant growth-promoting microorganisms.