New Herbicide-Degrading Microorganisms for the Biogenic Synthesis of Silver Nanoparticles

Technical Summary: New Herbicide-Degrading Microorganisms for the Biogenic Synthesis of Silver Nanoparticles

Problem Statement
The study addresses two concurrent challenges in agroecology and nanotechnology: the accumulation of herbicides (specifically pendimethalin and clethodim) in agricultural soils and the need for environmentally sustainable ("green") methods for synthesizing metallic nanoparticles. Conventional physical and chemical synthesis of silver nanoparticles (AgNPs) often involves toxic reducing agents, high energy consumption, and significant costs. Furthermore, while microorganisms capable of degrading xenobiotics are known, their dual potential to simultaneously remediate contaminated soils and synthesize stable nanomaterials remains underexplored. The research aims to isolate herbicide-degrading soil microorganisms and evaluate their capacity for the extracellular biogenic synthesis of AgNPs, which could serve as eco-friendly alternatives to chemical pesticides.

Methodology
The research utilized soil samples collected from herbicide-contaminated sites in the Tashkent region of Uzbekistan (0–10 cm and 15–30 cm depths).

• Isolation and Screening: 42 microbial isolates were obtained from anthropogenically stressed soils and the cotton rhizosphere. These were screened for resistance to pendimethalin and clethodim. Thirteen strains exhibiting high resistance were selected for further study.
• Identification: Pure cultures were identified via morphological, physiological, and biochemical characteristics, followed by molecular genetic identification using 16S rRNA gene sequencing and MALDI-TOF mass spectrometry.
• Biogenic Synthesis: Selected strains were cultivated in Meat-Peptone Broth (bacteria) or Gause's liquid medium (actinomycetes). Silver nitrate (AgNO₃) was added to the culture broth at concentrations of 50, 75, and 100 mg/L after 24 and 48 hours. Synthesis was monitored visually by color change (colorless to yellow/brown).
• Characterization: The most active strain, Acinetobacter radioresistens R27, was selected for detailed analysis.
  • UV-Vis Spectroscopy: Measured absorption spectra (190–900 nm) to confirm Surface Plasmon Resonance (SPR).
  • Scanning Electron Microscopy (SEM): Analyzed particle size, shape, and morphology at magnifications of ×15,000 to ×35,000.
  • X-Ray Diffraction (XRD): Determined crystalline structure and crystallite size using the Debye–Scherrer equation.
• Antimicrobial Activity: The synthesized AgNPs were tested against phytopathogens (Aspergillus flavus, Fusarium spp., Alternaria alternata, Clostridium sp., etc.) using the agar well diffusion method at concentrations of 50, 75, and 100 mg/L.

Key Results

• Strain Identification: From the initial 42 isolates, 13 showed high herbicide resistance. The most promising strains were identified as Acinetobacter radioresistens R27, Bacillus pumilus R25, and Streptomyces variabilis G35.
• Synthesis Efficiency: Acinetobacter radioresistens R27 demonstrated the highest biosynthetic activity, showing a rapid color transition to dark brown within 24–48 hours. Bacillus pumilus R25 and Pseudomonas putida R15 also showed significant activity. Strains Pseudomonas sp. 17 and Acinetobacter sp. 32 showed no biosynthetic activity.
• Nanoparticle Characterization:
  • UV-Vis: The synthesized AgNPs exhibited characteristic absorption peaks at λmax = 270 nm and 416 nm (for 50–75 mg/L) and 400 nm, indicating spherical, monodispersed particles. At 100 mg/L, the spectra suggested larger particles (80–190 nm) with needle-like morphologies.
  • SEM: Confirmed predominantly spherical particles. At 50 mg/L, sizes ranged from 10–75 nm; at 75 mg/L, 30–85 nm; and at 100 mg/L, irregular polydispersed particles of 60–320 nm were observed.
  • XRD: Confirmed a face-centered cubic (fcc) crystalline structure of silver. Diffraction peaks at 2θ ≈ 38.12°, 44.5°, 64.13°, and 77.48° corresponded to the (111), (200), (220), and (311) planes. Crystallite sizes calculated via the Debye–Scherrer equation ranged from 21.3 nm to 74.3 nm.
• Antimicrobial Activity: The biogenic AgNPs exhibited dose-dependent fungicidal and antibacterial activity.
  • Aspergillus flavus showed the highest sensitivity, with an inhibition zone of 35.0 ± 0.5 mm at 100 mg/L.
  • Alternaria alternata and Fusarium oxysporum showed linear dose-dependent inhibition.
  • Fusarium vasinfectum exhibited a threshold response, showing no inhibition at 50 mg/L but significant inhibition at 75 and 100 mg/L.
  • Clostridium sp. showed a non-linear response, with maximum inhibition (19.0 mm) at 75 mg/L, decreasing slightly at 100 mg/L, potentially due to nanoparticle aggregation.

Significance and Claims
The paper claims that the study successfully demonstrates the "functional multipotency" of specific soil microorganisms, particularly Acinetobacter radioresistens R27. The significance lies in the dual application of these strains:

1. Bioremediation: The strains are capable of degrading herbicides (pendimethalin and clethodim), offering a solution for cleaning anthropogenically stressed soils.
2. Green Nanotechnology: These same strains can biosynthesize stable silver nanoparticles without the need for toxic chemical reducing agents. The biogenic process creates a natural biomolecular shell on the nanoparticles, ensuring high colloidal stability and biocompatibility.

The authors conclude that integrating herbicide degradation with the production of biocontrol agents offers a promising strategy for developing multifunctional biotechnological tools. These tools could simultaneously remediate soil contamination and provide eco-friendly protection against economically significant phytopathogens, reducing reliance on conventional chemical pesticides. The study emphasizes that this approach aligns with the principles of green synthesis, offering an environmentally safe alternative for nanomaterial production in agriculture.