Weathering of chrysotile asbestos by the serpentine rock-inhabiting fungus Verticillium leptobactrum

Our plain-English summary of Weathering of chrysotile asbestos by the serpentine rock-inhabiting fungus Verticillium leptobactrum, by S. Daghino, F. Turci, M. Tomatis, et al (FEMS Microbiology Ecology, 2009).

The study

• Authors: S. Daghino, F. Turci, M. Tomatis, M. Girlanda, B. Fubini, S. Perotto
• Journal: FEMS Microbiology Ecology
• Published: 2009;69(1):132-41
• Cited by: 16 articles, as counted by NIH iCite
• Read it: PubMed · Publisher

Abstract

Verticillium leptobactrum, a rare fungal species, has repeatedly been isolated from serpentinic rocks in the Western Alps, thus suggesting that it adapts easily to this selective mineral substrate. The rRNA internal transcribed spacer region of several isolates has been sequenced to confirm their identity and taxonomic position within Verticillium, a recently revised polyphyletic entity. Isolates of V. leptobactrum have also been investigated to establish their ability to weather asbestos chrysotile, the most common mineral in the isolation sites. The results of solubilization assays on magnesium and silicon, as well as measurement of the Mg/Si ratio in the asbestos fibres after exposure to fungal mycelia, indicate a high bioweathering activity of V. leptobactrum towards chrysotile. Comparison with data on Fusarium oxysporum shows differences among species, with V. leptobactrum being more active than F. oxysporum in removing structural ions from chrysotile. Asbestos weathering by fungi could be envisaged as a bioremediation strategy for hazardous asbestos-rich soils (e.g. abandoned mines). Fungi that have adapted to live in serpentine sites could be good candidates for this purpose.

In plain English

The authors collected soil and rock samples from five serpentine sites in the Italian Western Alps, including two abandoned mines, and grew the fungi they contained. At three of these sites with little or no human impact, the fungus Verticillium leptobactrum was found in rock samples from every site, at isolation frequencies of 68% to 100%. Genetic sequencing of 18 isolates confirmed the identification. In a laboratory experiment, one V. leptobactrum strain from the Balangero mine and one Fusarium oxysporum strain from agricultural soil were grown for up to 20 days with Balangero chrysotile fibres. V. leptobactrum removed 22.3% of the magnesium and 11.7% of the silicon in the fibres. F. oxysporum removed 11.8% and 2.0%, not statistically different from medium alone. With V. leptobactrum, magnesium was lost faster than silicon. The surface magnesium-to-silicon ratio fell to 0.46, against 1.33 for fibres in medium alone. The authors concluded that V. leptobactrum is active in weathering chrysotile and could be a candidate for asbestos bioremediation. They noted that in the field, other organisms would likely influence this ability.

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: Field isolation of fungi with molecular (rRNA ITS sequencing) identification, plus an in vitro laboratory bioweathering experiment
• Who was studied: Fungi isolated from soil and rock samples at five serpentine sites in the Aosta Valley and Lanzo Valley (Turin), Italy: Vallone delle Acque Rosse (1 sample), Col de la Croix (11), Emarese mine (4), Balangero mine (19) and Fubine (8). Eighteen V. leptobactrum-like isolates were sequenced. The weathering experiment used one V. leptobactrum strain from Balangero mine rock debris and one F. oxysporum strain from agricultural soil.
• Exposure: Chrysotile asbestos fibres from the Balangero mine, held in a suspension inside a dialysis membrane in fungal culture medium for up to 20 days. Mg and Si removal was measured in the medium and mycelia, and the Mg/Si ratio was measured in the fibre bulk and on the fibre surface.
• Main results:
  • In the three sites with little or no human impact investigated for occurrence in this work (sites 1, 2 and 5), V. leptobactrum was isolated from rock samples at all sites, with frequencies in the isolation plates of 68% to 100%. At site 1, 95% of 1249 CFUs belonged to this species.
  • ITS sequences from the serpentine isolates formed a clade with 99% bootstrap support and clustered with V. leptobactrum reference sequences, confirming the morphological identification.
  • With fibres present, V. leptobactrum cultures, but not F. oxysporum cultures, showed significant removal of Mg (10.44 mg) and Si (4.65 mg) compared with medium containing only fibres (4.73 mg and 0.52 mg).
  • As a share of the ions in the fibres, V. leptobactrum extracted 22.3 wt% of Mg and 11.7 wt% of Si.
  • F. oxysporum extracted 11.8 wt% of Mg and 2.0 wt% of Si, which was not significantly different from the medium alone.
  • In V. leptobactrum cultures, most of the mobilized ions stayed in the medium and little was taken up by the fungus.
  • With V. leptobactrum, bulk analysis showed uneven (incongruent) dissolution, with Mg removed faster than Si.
  • Surface Mg/Si ratio was 0.46 for fibres exposed to V. leptobactrum, 1.15 for F. oxysporum and 1.33 for medium alone. The F. oxysporum effect was limited to the uppermost atomic layers, with no significant bulk difference from control fibres.
• The authors' conclusion: The authors concluded that V. leptobactrum is active in weathering chrysotile fibres, that its abundance suggests it has adapted to serpentine substrates, and that it could be a good candidate for studies on mineral weathering and for asbestos bioremediation.
• Limitations the authors state:
  • The experiments were in vitro. The authors state that in the field, the fungus's weathering ability is likely influenced by interactions with other fungi, plant roots, soil animals and bacteria.
  • No genetic clustering by site of origin was found. The authors note that more sensitive techniques could be used to investigate this.
  • The fungal compounds responsible for the weathering were not identified in this study and were still under investigation.

Mines named here

Asbestos mines this study is about. Each mine's page says who ran it, when it worked, and what it produced.

• Balangero mine, Balangero, about 20 km north of Turin, Piedmont

All asbestos mines

Topics

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

Medical terms in this study, explained in our Lexicon: Chrysotile, Serpentine Group.

Related studies

• Soil fungi reduce the iron content and the DNA damaging effects of asbestos fibers — S. Daghino, F. Turci, et al. (2006)
• Fungal diversity is not determined by mineral and chemical differences in serpentine substrates — S. Daghino, C. Murat, et al. (2012)
• Microbial diversity and mineral composition of weathered serpentine rock of the Khalilovsky massif — I.V. Khilyas, A.V. Sorokina, et al. (2019)
• Occurrence of heavy metal-resistance in microflora from serpentine soil of Andaman — A. Pal, S. Dutta, et al. (2005)
• Bioweathering of chrysotile by fungi isolated in ophiolitic sites — S. Daghino, E. Martino, et al. (2008)
• Serpentine endophytic bacterium Pseudomonas azotoformans ASS1 accelerates phytoremediation of soil metals under drought stress — Y. Ma, M. Rajkumar, et al. (2017)

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