Siderophore-mediated iron removal from chrysotile: Implications for asbestos toxicity reduction and bioremediation

TL;DR

At environmentally relevant concentrations (1–10 µM), fungal and bacterial siderophores removed structural iron from chrysotile and reduced asbestos-induced ROS in murine peritoneal macrophages, whereas organic acid ligands at 100 µM did not. The authors conclude siderophore-based bioremediation is feasible for lowering asbestos fiber toxicity, though not necessarily eliminating risk.

Key findings

• Both fungal (FC) and bacterial (DFO) siderophores released similar amounts of iron from chrysotile: 17±2 µM Fe per g fiber per µM siderophore. DFO at 1 µM reached dissolution equilibrium within 24 h, while 10 µM DFO delayed equilibrium to at least 120 h.
• Organic acids at 100 µM did not directly release iron; with oxalic acid, iron was below the analytical detection limit (1 µg/L), and pre-exposure to oxalic, malonic, or citric acid did not enhance DFO-mediated iron release, except citric acid increased release with 1 µM DFO.
• Removal of plant-available/exchangeable iron by DTPA did not diminish siderophore-mediated iron removal, indicating that siderophores can remove iron from chrysotile’s crystal structure.
• Siderophore treatment lowered ROS generation by murine peritoneal macrophages; DTPA treatment alone significantly lowered ROS (p<0.05), and for DTPA-treated fibers, fungal FC but not bacterial DFO significantly lowered ROS compared with control.

Methods

• Chrysotile from Glove, Arizona, was ground, purified, and suspended at 250–300 mg/L in 1 mM MOPS and 10 mM NaCl at pH 7; iron content was measured as 2.3% by weight using EDS.
• Dissolution experiments exposed fibers to DFO or FC at 1 or 10 µM, with or without organic acids (100 µM), and after DTPA pretreatment; dissolved iron was measured by ICP-OES over up to 16 days, with duplicate batches.
• Toxicity was assessed by exposing elicited murine peritoneal macrophages to fibers at 20 µg/cm² for 6 h and measuring ROS with CellROX Green fluorescence; differences were tested with two-way ANOVA followed by Tukey post-hoc analysis.

Limitations

• The paper states that future studies should use chrysotile fibers isolated from the environmental matrix and examine organic acid effects, and notes that siderophores may lower but not eliminate risk because fiber toxicity also depends on physical properties such as aspect ratio and rigidity.
• Evident from the text: toxicity was measured only in vitro in elicited murine peritoneal macrophages at 6 h, not in vivo or during chronic exposure.
• Evident from the text: experiments used a single ground, purified chrysotile source at pH 7, so relevance to environmental asbestos mixtures, coatings, and long-term soil communities remains uncertain.