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. 2018 Mar 7:6:e4478.
doi: 10.7717/peerj.4478. eCollection 2018.

Cd and Zn interactions and toxicity in ectomycorrhizal basidiomycetes in axenic culture

Vinicius H De Oliveira  1 , Mark Tibbett  1
Affiliations

Cd and Zn interactions and toxicity in ectomycorrhizal basidiomycetes in axenic culture

Vinicius H De Oliveira et al. PeerJ. .

Abstract

Background: Metal contamination in soils affects both above- and belowground communities, including soil microorganisms. Ectomycorrhizal (ECM) fungi are an important component in belowground community and tolerant strains have great potential in enhancing plant-based remediation techniques. We assessed cadmium and zinc toxicity in five ECM species in liquid media (Hebeloma subsaponaceum; H. cylindrosporum; H. crustuliniforme; Scleroderma sp.; Austroboletus occidentalis) and investigated the potential of Zn to alleviate Cd toxicity. Due to highly divergent results reported in the literature, liquid and solid media were compared experimentally for the first time in terms of differential toxicity thresholds in Cd and Zn interactions.

Methods: A wide range of Cd and Zn concentrations were applied to ectomycorrhizal fungi in axenic cultures (in mg L-1): 0; 1; 3; 9; 27; 81; 243 for the Cd treatments, and 0; 1; 30; 90; 270; 810; 2,430 for Zn. Combined Zn and Cd treatments were also applied to H. subsaponaceum and Scleroderma sp. Dry weight was recorded after 30 days, and in case of solid medium treatments, radial growth was also measured.

Results and discussion: All species were adversely affected by high levels of Cd and Zn, and A. occidentalis was the most sensitive, with considerable biomass decrease at 1 mg L-1 Cd, while Scleroderma sp. and H. subsaponaceum were the most tolerant, which are species commonly found in highly contaminated sites. Cd was generally 10 times more toxic than Zn, which may explain why Zn had little impact in alleviating Cd effects. In some cases, Cd and Zn interactions led to a synergistic toxicity, depending on the concentrations applied and type of media used. Increased tolerance patterns were detected in fungi grown in solid medium and may be the cause of divergent toxicity thresholds found in the literature. Furthermore, solid medium allows measuring radial growth/mycelial density as endpoints which are informative and in this case appeared be related to the high tolerance indices found in H. subsaponaceum.

Keywords: Ectomycorrhizal fungi; Heavy metal toxicity; Metal interaction; Toxicity threshold.

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Conflict of interest statement

Mark Tibbett is an Academic Editor for PeerJ.

Figures

Figure 1
Figure 1. Toxicity thresholds of Cd and Zn in five ectomycorrhizal species.
Dry weight of five ECM species (Austroboletus occidentalis, Hebeloma cylindrosporum, H. crustuliniforme, H. subsaponaceum, Scleroderma sp.) after 30 days under a range of Cd (A–E) or Zn (F–J) concentrations in liquid media. Asterisks represent the first concentration from which fungal growth starts to be adversely affected, LOAEC, determined by Dunnett’s test (p < 0.05). LOAEC for Cd and Zn (in mg L−1) were, respectively, 1 and 30 in A. occidentalis; 1 and 270 in H. cylindrosporum; 1 and 90 in H. crustuliniforme; 3 and 90 in H. subsaponaceum; 9 and 270 in Scleroderma sp.
Figure 2
Figure 2. Tolerance index for five ectomycorrhizal fungi exposed to Cd and Zn.
Metal tolerance indices (TI%) for five ECM species under increasing concentrations of Cd: 0; 1; 3; 9; 27; 81 and 243 mg L−1 (A) or Zn: 0; 1; 30; 90; 270; 810 and 2,430 mg L−1 (B) in liquid media. X axes are in logarithmic scale. TI% = DW treated/DW control × 100.
Figure 3
Figure 3. Effect of Zn addition on ectomycorrhizal cultures exposed to Cd.
Effects of Zn concentrations on dry weights (mean, n = 4) of Hebeloma subsaponaceum (A–B) and Scleroderma sp. (C–D) under two Cd concentrations (0 and 9 mg L−1). Data for other species were not significantly different and therefore are not shown. Different letters represent significant differences by Tukey test (p < 0.05).
Figure 4
Figure 4. Contour plots showing different tolerance patterns of ectomycorrhizal fungi grown in solid and liquid media contaminated by Zn and Cd.
Contour plots: tolerance indices (TI%) for H.  subsaponaceum (A–B) and Scleroderma sp. (C–D) exposed to Cd and Zn in vitro in two types of Modified Melin-Norkrans media, liquid (left) and solid (right). TI% = DW treated/DW control × 100. The reference value (100%) was considered as the treatment which produced the most biomass (dry weight). Contour plots produced by linear interpolation. High TI% (orange and red) are associated with lower toxicity, while low TI% (purple and blue) with higher toxicity.
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Grants and funding

This research was supported by CAPES (Coordination for the Improvement of Higher Education Personnel - Brazil) with the Science without Borders Programme, project: 13462-13-0. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.

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