Iron removal from raw asbestos by siderophores-producing Pseudomonas

Our plain-English summary of Iron removal from raw asbestos by siderophores-producing Pseudomonas, by S.R. David, D. Ihiawakrim, R. Regis, et al (Journal of Hazardous Materials, 2020).

The study

• Authors: S.R. David, D. Ihiawakrim, R. Regis, V.A. Geoffroy
• Journal: Journal of Hazardous Materials
• Published: 2020;385:121563
• Cited by: 11 articles, as counted by NIH iCite
• Read it: PubMed · Publisher

Abstract

Asbestos, mineral present in soil, are highly toxic due to the presence of iron. Microbes-mineral interactions occur naturally through various processes leading to their alteration. We examined the effect of siderophore-producing Pseudomonas with a particular focus on the role of pyoverdine and pyochelin on raw asbestos fibers such as amosite, crocidolite and chrysotile. We compared the efficiency of pyoverdine to the iron chelating agent EDTA in the release of iron from raw asbestos fibers. Pyoverdine was able to extract iron from all the tested raw asbestos with the higher efficiency observed for chrysotile and crocidolite. When asbestos were grinded, the iron removal was more important for all types. We monitored the effect of bacterial growth and siderophores containing bacterial supernatant on raw asbestos dissolution by solution chemistry analysis and transmission electron microscopy. The siderophore-containing supernatant allowed a higher iron solubilisation than the one obtained after bacterial growth. Moreover, the iron dissolution was faster with pyoverdine-containing supernatant than pyochelin-containing supernatant, with approximately the same iron level for the maximum extraction with a delay of 48 h. Our study clearly showed the involvement of bacterial siderophores, pyoverdine and pyochelin on chrysotile, crocidolite and amosite fibers weathering.

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In plain English

In this laboratory experiment, the authors exposed samples of three raw asbestos fiber types (chrysotile, crocidolite and amosite), hand-ground or not, to iron-binding substances and measured how much iron dissolved out of the fibers. They compared pyoverdine, an iron-binding molecule purified from the soil bacterium Pseudomonas aeruginosa, with the chemical EDTA. They also tested live bacteria and the liquid the bacteria had grown in. Over three incubation cycles, pyoverdine released more iron than EDTA from chrysotile (0.38 to 1.04 mg/L versus 0.08 to 0.19 mg/L) and from crocidolite (0.59 to 3.84 versus 0.12 to 1.92 mg/L), but about the same amount from amosite. Grinding increased iron release. The bacterial growth liquid released more iron than the bacterial cells for chrysotile and crocidolite, but it released little iron from amosite (0.04 to 0.17 mg/L). The bacteria formed a biofilm on crocidolite fibers. The authors concluded that these molecules or the bacterial cells were able to weather all three fiber types, meaning chemical alteration measured here as iron removed from the fibers. They called for further work on asbestos wastes and noted that hand grinding was hard to keep uniform.

We wrote this plain-English summary from the full article with the help of AI, and a second AI model checked each sentence against the article. It is not the authors' wording; the abstract and the article are their own account.

What the study did and found

• Design: Laboratory (in vitro) experiment measuring iron dissolved from asbestos fibers by a purified bacterial siderophore, by EDTA, and by Pseudomonas aeruginosa cells and their culture supernatants, with transmission electron microscopy of fibers
• Who was studied: Samples (0.125 g each) of raw chrysotile, crocidolite and amosite, ground by hand or not. Per Table 1, the chrysotile came from the Natural History Museum of Paris and the crocidolite and amosite from South African mines. Bacteria were wild-type P. aeruginosa ATCC 15692 plus a pyochelin-deficient mutant and a pyoverdine-deficient mutant. Each assay was done in triplicate.
• Exposure: Fibers were incubated with purified pyoverdine or EDTA at 200 µM in succinate medium in repeated contact cycles (described as 48 h cycles plus a 96 h cycle). Ground fibers were also incubated with bacterial cells or with siderophore-containing bacterial supernatants over three 48 h cycles at 30°C.
• Main results:
  • Chrysotile: over the three incubation periods, pyoverdine extracted more iron than EDTA (pyoverdine 0.38 to 1.04 mg/L; EDTA 0.08 to 0.19 mg/L)
  • Crocidolite: pyoverdine extracted more iron than EDTA (pyoverdine 0.59 to 3.84 mg/L; EDTA 0.12 to 1.92 mg/L)
  • Amosite: pyoverdine and EDTA showed the same efficiency (pyoverdine 1.07 to 3.58 mg/L; EDTA 0.82 to 3.64 mg/L)
  • Grinding increased iron release with either EDTA or pyoverdine. The increase was 1.3 times for chrysotile and 2 times for amosite (as the authors word it), and 2 to 5 times for crocidolite.
  • For chrysotile and crocidolite, bacterial supernatants released more iron than bacterial cells at every cycle
  • Chrysotile, first 48 h cycle: the pyoverdine-containing supernatants (wild type and pyochelin-deficient mutant) dissolved iron 2 to 3 times more efficiently than the supernatant of the pyoverdine-deficient mutant, which produces only pyochelin
  • Second cycle: the result was reversed. The supernatant of the pyoverdine-deficient mutant (pyochelin only) extracted twice as much iron as the wild-type and pyochelin-deficient supernatants for chrysotile, and the authors state the same held for crocidolite.
  • Crocidolite, wild-type supernatant: the maximum iron level, 0.78 mg/L, came in the third cycle, compared with 0.41 mg/L (first cycle) and 0.34 mg/L (second cycle)
  • Amosite: all supernatants showed very low efficiency, between 0.04 and 0.17 mg/L
  • No changes were observed in control conditions: culture medium alone, or culture medium with only fibers or only bacteria
  • Electron microscopy showed crocidolite fibers incubated with bacteria covered with a biofilm, which explained why the fibers clumped together. Fibers kept in medium without bacteria showed no detectable alteration layer.
  • STEM-EDX analysis showed a slight decrease in the iron content of crocidolite fibers after 48 h of contact with bacteria
• The authors' conclusion: The authors concluded that either the siderophores or the Pseudomonas cells were able to weather all three types of asbestos fibers. They also stated that asbestos can serve as a source of nutrients for soil bacteria, and that biological chelators were more efficient at iron extraction than EDTA.
• Limitations the authors state:
  • Fibers were ground by hand and grinding was hard to keep uniform between experiments. The authors say this may have influenced iron dissolution and the lower values for supernatant-treated crocidolite and amosite.
  • The experiments used raw asbestos. The authors state that further work on asbestos wastes is needed before any bioremediation process could be developed.

Topics

The National Library of Medicine indexes this study under subjects our site covers: Amosite, Chrysotile, Crocidolite.

Medical terms in this study, explained in our Lexicon: Amosite, Crocidolite, Chrysotile, Transmission Electron Microscopy (TEM).

Related studies

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• Siderophore-mediated iron removal from chrysotile: Implications for asbestos toxicity reduction and bioremediation — S.K. Mohanty, C. Gonneau, et al. (2018)
• Engineering siderophore production in Pseudomonas to improve asbestos weathering — M. Lemare, H. Puja, et al. (2022)
• A Review of Asbestos Bioweathering by Siderophore-Producing Pseudomonas: A Potential Strategy of Bioremediation — S.R. David, V.A. Geoffroy (2020)
• Serpentine endophytic bacterium Pseudomonas azotoformans ASS1 accelerates phytoremediation of soil metals under drought stress — Y. Ma, M. Rajkumar, et al. (2017)
• Flocking asbestos waste, an iron and magnesium source for Pseudomonas — S.R. David, S. Fritsch, et al. (2020)

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