In their article, Siwulski et al. (2017) studied the occurrence of the rare earth elements (REE; La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu), Sc, Y and further 46 elements in a large set of samples of the cultivable mushroom genus Pleurotus (P. citrinopileatus, P. eryngii, P. djamor, P. florida, P. ostreatus and P. pulmonarius). The current comment relates only to the data on REE elements that was reported by Siwulski et al. (2017).

In an effort to check the validity of the methodology used, Siwulski et al. (2017) analysed the certified reference materials: CRM NCSDC (73,349)—bush branches and leaves, CRM S-1—loess soil, CRM 2709—soil and two estuarine sediments—CRM 405 and CRM 667. Analytical recovery was reported to be in the range from 80 to 120%. However, the electronic supplementary material of the article also contained results for a fungal control material, CRM CS-M-1 (dried mushrooms powder Suillus bovinus), produced by the Institute of Nuclear Chemistry and Technology in Warsaw (Poland), which has no previous information on concentration of the REEs, Sc or Y. The CS-M-1 documentation contains reference data on As, Cd, Cu, Hg, Pb, Se and Zn (Polkowska-Motrenko et al. 2007; Saba et al. 2016).

The study on Pleurotus mushrooms (Siwulski et al. 2017) used an emission spectroscopy technique, i.e. inductively coupled plasma optical emission spectrometry (ICP-OES, using an Agilent 5100). In the “Electronic supplementary material”, the authors provided Tables S1, S2 and S3. Table S1 is with “Instrumental common parameters”. In Table S2 “Characteristic of analytical technique of ICP-OES” are detection limit (DL) (or MQL?; method quantification limit Rutkowska et al. 2022a, b) values for the elements including a limit of 0.01 mg kg−1 dry weight (dw) for La, 0.03 mg kg−1 for Nd, 0.03 mg kg−1 for Yb, 0.00 mg kg−1 for Er, 0.02 mg kg−1 for Ce, 0.03 mg kg−1 for Dy,  0.03 mg kg−1 for Sm, 0.01 mg kg−1 for Tb,  0.01 mg kg−1 for Ho, 0.01 mg kg−1 for Tm, 0.03 mg kg−1 for Lu, 0.03 mg kg−1 for Pr, 0.03 mg kg−1 for Eu and 0.02 mg kg−1 for Gd.

Siwulski et al. (2017) determined REE such as Nd, Er, Tm and Lu, which were quantified in each sample of Pleurotus spp. These elements were at mean concentrations of 410, 380, 50 and 20 (total 860) µg kg−1 dry weight (dw), respectively. Maximum concentrations per species were 7180 µg kg−1 dw for Nd in P. eryngii, and 1600 µg kg−1 dw for Er, 190 µg kg−1 dw for Tm and 50 µg kg−1 dw for Lu in P. ostreatus. A critical review and analysis of reported data on the occurrence of a given element or elements is a standard approach to evaluate the credibility of data that is used to understand the minerals and trace element composition of mushrooms, e.g. selenium (Se) (Falandysz 2008, 2012, 2013; Falandysz and Drewnowska 2014), and also to identify biased data in the reports (Ioannidis 2005; Jorhem et al. 2011).

There is no previous and independent data on the concentration of individual REE (also Sc and Y) in Pleurotus mushrooms such as P. citrinopileatus, P. eryngii, P. djamor, P. florida, P. ostreatus and P. pulmonarius. Koutrotsios et al. (2018) published results on REE, Sc, Y, Th and U in P. ostreatus from cultivation experiments using seven different plant-origin substrates. Nd, Er, Tm and Lu occurred in P. ostreatus in the range from 3.4 ± 1.2 to 13 ± 4 µg kg−1 dw (Nd), from 0.2 ± 0.0 to 0.5 ± 0.0 µg kg−1 dw (Er), from < 0.1 to 0.09 ± 0.02 µg kg−1 dw (Tm) and from 0.1 ± 0.0 to 0.2 ± 0.1 µg kg−1 dw (Lu), and sum (∑) of 14 REE was 90 ± 27 µg kg−1 dw (median 83 µg kg−1 dw), with total concentrations ranging from 61 to 130 µg kg−1 dw (Koutrotsios et al. 2018).

The wild Pleurotus dryinus was studied by Ivanić et al. (2021), who reported a Nd concentration of 2 µg kg−1 dw, while Er, Tm and Lu were not detected above the MQL. The ∑14REE concentration was 31 µg kg−1 dw. These concentration data of Nd, Er, Tm and Lu in P. ostreatus and P. dryinus and in several other, randomly selected studies on saprobic and ectomycorrhizal mushrooms for which credible data are available (Table 1) show that the REE results for P. citrinopileatus, P. eryngii, P. djamor, P. florida, P. ostreatus and P. pulmonarius reported by Siwulski et al. (2017) are relatively, highly elevated.

Table 1 Mean and maximum values of Nd, Er, Lu and Tm concentration in Pleurotus mushrooms from study by Siwulski et al. (2017) and in some mushrooms and certified reference material (BCR-670 – Duck weed) from other studies
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Rare earth elements are factually rare. They occur in mushrooms, plants, foods and tap water in small or very small concentrations (typically low or sub µg kg−1 levels). Also their analytical determination can be strongly affected by polyatomic molecular interferences during mass-spectroscopic measurement. Thus, the credible determination of REE in biological materials requires a high level of skill and experience from the analysts and supervisor (not to mention the laboratory status—“clean room conditions…”). The samples should be uncontaminated with soil (Bau et al. 2018; Stijve et al. 2004; Zocher et al. 2018) and advanced methodology, including instrumentation with the highest resolution, should be used, because of the problems with spectral interferences. For reliable analysis of REEs in biological materials, pre-concentration and matrix-separation are also carried out before ICP-MS measurement (Bau et al. 2018; Dulski 2001; Markert and Li 1991; Merschel and Bau 2015). Therefore, obtaining credible results using direct spectrometric analysis of all REE in mushrooms, plants and other biological materials requires among others ultra-clean laboratory conditions and a suitable mass spectrometer that is able to provide data not affected by spectral interferences (Ichihashi et al. 2001; Prohaska et al. 1999; Yamasaki et al. 1991). This kind of approach has been used in the study of REE in mushrooms by several authors (Bau et al., 2018; Borovička et al. 2011; Falandysz et al. 2001, 2017 and 2022; Ivanić et al. 2021; Koutrotsios et al. 2018; Zocher et al. 2018).

Early studies of REE in a few species of wild mushrooms used nitric acid-pressure for decomposition of the matrix followed by direct analysis using double focusing sector field spectrometry for measurement (PlasmaTrace, VG Elemental, UK), which allowed determination of La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu—all at very low concentrations (Falandysz et al. 2001). These decreased in Armillaria mellea, Boletus edulis, Laccaria amethistina, Suillus bovinus, Suillus luteus and Tricholoma equestre in the order Ce > La/Nd > Pr > Sm/Gd > Dy/Er/Yb > Ho > Eu/Tb > Tb/Lu and occurred at levels from sub- to low parts per billion [ppb; µg kg−1 dry weight (dw)] (Falandysz et al. 2001). This order and concentration levels of REE were the same as that reported in a set of other wild mushroom species collected from Poland, Belarus and Yunnan (China) around 25 years later and analysed after pre-concentration and matrix-separation before spectrometric measurement (Mędyk and Falandysz 2022).

The pattern of occurrence of REE in wild mushrooms in the early study (Fig. 1) and in some later studies (Fig. 2) follows the Oddo-Harkins rule, where REE elements with an even atomic number occur in higher abundance than REE elements with the adjacently larger and smaller odd atomic numbers. Graphically they present a typical sawtooth concentration pattern (Kwecko 2016; Migaszewski and Gałuszka 2015), which is natural for REE in soils, earth crust, shales and surface waters (Figs. 3 and 4). Koutrotsios et al. (2018) studying 7 batches of P. ostreatus and 7 batches of Cyclocybe cylindracea mushrooms cultivated in 7 kinds of plant substrate found this sawtooth pattern both in the substrates and mushrooms. Thus, the above data shows that saprobic mushrooms like Pleurotus spp. and others accumulate REE as a group and the same appears to be true for ectomycorrhizal mushrooms (Figs. 1 and 2).

Fig. 1
figure 1

Distribution pattern of REE—natural “zigzag” concentrations in mushrooms (Suillus luteus – slippery jack, Suillus bovinus – cow bolete, Laccaria amethistina – amethyst deceiver, Tricholoma equestre – yellow knight mushroom and Armillariella solidipes – honey mushroom) (adapted from Falandysz et al. 2001)

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Fig. 2
figure 2

Distribution pattern of REE—natural “zigzag” concentrations in the ectomycorrhizal (ECM) and saprotrophic (SAP) mushrooms, Boletus edulis and Suillus luteus (adapted from Bau et al. 2018; Borovicka et al. 2011 and Zocher et al. 2018)

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Fig. 3
figure 3

Distribution pattern of REE—natural “zigzag” concentrations in the shale (NASC—North American Shale Composite) and soils from Poland: Ol [organic (litter) and Ofh (fermentative-humic)], and mixed-humic-eluvial and humic-eluvial-illuvial (adapted from Dołęgowska and Migaszewski, 2013)

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Fig. 4
figure 4

Distribution pattern of REE—natural “zigzag” concentrations in surface and ground waters of Poland (adapted from Wysocka et al. 2018)

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The fruiting bodies of examined species seem to bio-exclude REE (BCF; quotient calculated from concentration of an element in mushroom and its soil or plant substrate is well below 1) (Mędyk and Falandysz, 2022). In a study of Pleurotus spp. by Siwulski et al. (2017), only Nd, Er, Tm and Lu were found in concentration above the MQL and ten other REE (including usually the highest contributors—Ce, La and Pr) were below the MQL. The pattern of their abundance in the mushrooms clearly did not follow the Oddo-Harkins rule. Apart from the highly elevated quantities of Nd, Er, Tm and Lu in Pleurotus spp. in the study (Siwulski et al. 2017), the quotients (ratios) between Nd/Er, Tm/Lu, Er/Lu, Nd/Tm and Nd/Lu were also unusual—if related to values obtained from other randomly selected mushroom studies as well as for duck weed, soils and European shale, which have been reported in the literature (Table 2). The comparison of these quotient values provides indirect but simple and helpful evidence on abnormal distribution REE patterns either for some elements (if data are available only for a few elements) or for all 14 REE. It also helps to find anomalies that may arise through anthropogenic pollution, e.g. with Gd in surface water and to identify the source of the pollution (Migaszewski and Gałuszka 2016). An example can be the element Er, which usually occurs in mushrooms at concentrations of a few µg kg−1 dw or less, and so is in very small proportion to the most abundant REE—La, Ce and Nd (Bau et al. 2018; Borovička et al. 2011; Falandysz et al. 2001; Ivanić et al. 2021; Zocher et al. 2018). Erbium in Pleurotus spp. was detected in the range from 60 µg kg−1 dw in P. pulmonarius to 1600 µg kg−1 dw in P. ostreatus (Siwulski et al. 2017).

Table 2 Values of quotients (ratios) from concentrations of Nd/Er, Tm/Lu, Er/Lu, Nd/Tm and Nd/Lu in Pleurotus mushrooms from a study by Siwulski et al. (2017) and in mushrooms, certified reference material (BCR-670 – duck weed) and soils from other studies
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Apart from data on REE in 7 species of Pleurotus mushrooms, Siwulski et al. (2017) also provided additional results (Electronic supplementary material; Table S3 Traceability studies for certified reference materials) on 14REE in another mushroom, Suillus bovinus (Cow bolete), which was the fungal Control Material (CS-M-1). Data for La, Pr, Nd, Sm and Er and other REE in the CS-M-1, which were below MQL, together with data on 13–14 REE in other Suillus mushrooms (including S. bovinus), that are available from the literature, are plotted in Fig. 5. The reported REE concentrations reported for the control material CS-M-1 were: La 100 and 100 µg kg−1 dw, Ce 200 and 300 µg kg−1 dw, Pr 100 and 100 µg kg−1 dw, Nd 500 and 600 µg kg−1 dw, Sm 100 and < 10 µg kg−1 dw, Er 1300 and 1200  µg kg−1 dw (sic!), and Dy, Eu, Gd, Ho, Lu, Tm and Yb were undetected (< 10 µg kg−1 dw) (after Siwulski et al. 2017) (Fig. 5). It appears that both, the pattern and very high concentration values of REE in the CS-M-1, do not agree with what was found in other Suillus spp. samples. Calculation of the quotients for Er and La, Ce, Pr and Nd and Sm in the CS-M-1 gives values of 12, 4.4, 13, 2,5 and 11, which are very high—typically for mushrooms these ratios are well below 1 (Table 3). A possible reason for this may lie with the methodology used by Siwulski et al. (2017), in particular the spectrometer used and the resulting high spectral interferences, and poor sensitivity for REE elements. Additionally, poor MQLs could also be a reason. These issues have been highlighted by scientists that have long experience of the analysis of REE in biological and other materials (Dulski 2001; Markert and Li, 1991; Merschel and Bau, 2015; Prohaska et al. 1999; Yamasaki et al. 1991; Zocher et al. 2018).

Fig. 5
figure 5

Concentrations of individual REE in mushrooms of the genus Suillus (µg kg−1 dry weight)—adapted from the references cited. Notes: *CS-M-1 (Control Material produced by the Institute of Nuclear Chemistry and Technology, Warsaw, Poland.—dried whole mushroom powder S. bovinus) (from Siwulski et al. 2017 in their “Electronic supplementary material”); 1 (Falandysz et al. 2001); 2 (Zocher et al. 2018); 3 (Mędyk and Falandysz 2022)

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Table 3 Values of quotients (ratios) from concentrations of Er/La, Er/Ce, Er/Pr, Er/Nd and Er/Sm in the control material CRM CS-M1 (Suillus bovinus) from study by Siwulski et al. (2017) and in Suillus mushrooms from other studies (1—Falandysz et al. 2001; 2—Zocher et al. 2018; 3—Mędyk and Falandysz, 2022)
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Obviously, apparent abnormalities cannot be useful for any kind of statistical evaluation or valid discussion on the REE composition of edible mushrooms or for inclusion in the valuable database on minerals and trace elements content of mushrooms.