Introduction

Element (toxic and non-toxic) deposition in soil and plants is one of the most pressing concerns in the science of food safety and food quality in Europe [1]. Many studies [2,3,4,5,6] show the impact of different metals/elements on human health as well as the mechanisms of uptake and bioaccumulation of heavy metals by plants and other foods for human nutrition. Some of these elements (lead, cadmium, mercury, and arsenic) are considered potential carcinogens and are associated with the etiology of a number of internal diseases [7]. On the other hand, some elements, such as Zn and Cu, are essential elements for human health [8].

Honey bees and their products can be contaminated with elements from different sources [9,10,11]. The origin of contamination can be the environment (air, water, plants, and soil), beekeeping practices [12], and then can be the bees themselves, which introduce contaminants into the beehive [13]. Also, the determination of elements in environmental samples including honey bees and their products is an important part of environmental pollution studies [13,14,15,16]. One of the most commonly examined honey bee products, due to its potential risk to human health, is honey (which can be floral-specific, e.g., acacia, linden, blossom) [17,18,19,20,21]. On the other hand, previous [10, 22] showed that honey bees can be better indicators of environmental element deposition than honey. This can be explained due to elements from the atmosphere, which firstly can deposit on the honey bees’ bodies, and then can be introduced into the beehive.

Thus, information about element concentrations in honeys and their potential dietary intake via this food is very important for assessing the risk to human health. During harvest time, bees are exposed to different sources of contaminants through their pollen and nectar that contains heavy metals of natural and/or anthropogenic origin [9]. Some toxic elements are bioaccumulative, and these can be measured in honey bee and different bee products (honey, wax, propolis, etc.). In this study, a detailed analysis of environmental element deposition in honey bees and their products is reported. The analysis covers the different toxic and non-toxic elements (As, Cd, Cr, Cu, Pb, Zn, Co, Ni, Fe, Zn, Mn, K, Na, Ca, and Mg). The aim of this study was to determine potential correlations between element concentrations in honey bees and their products.

Materials and Methods

Sample Collection

This study was performed in 2019, in three different locations in Serbia: central–Rudnik mountain (coordinates: 44° 07′ 53″ N 20° 32′ 25″ E), Lazarevac (coordinates: 44° 22′ N 20° 15′ E), and Ležimir in northern Serbia (coordinates: 45° 07′ N 19° 34′ E). Site I (Rudnik mountain) is mostly composed of wild areas, while site II (Lazarevac) is surrounded by industrial environments and site III (Ležimir) is near agricultural environments (Fig. 1). The study was conducted from April to July for each sampling site; live honeybees (worker bees, drone bees, and bee broods) were sampled five times. Honey bee samples were prepared according to Nuorteva [23]. Honeys (multifloral, 500 g) from each location were stored in glass containers at 4–8 °C until analysis. Sub-samples of wax (~ 4 g) were taken directly from freshly formed combs using sterile beekeeping knives and put in sterile plastic boxes.

Fig. 1
figure 1

Map of sampling

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Element Concentration Analyses

For analyses, approximately 0.5 g of homogenized sample was transferred into a Teflon vessel with 5 ml nitric acid (67% Trace Metal Grade, Fisher Scientific, Loughborough, UK) and 1.5 ml hydrogen peroxide (30% analytical grade, Sigma-Aldrich, St. Louis, MO, USA) for microwave digestion. The microwave (Start D, Milestone, Sorisole, Italy) program consisted of three steps: 5 min from RT to 180 °C, 10 min hold at 180 °C, and 20 min cooling. After cooling, the digested samples were quantitatively transferred into disposable flasks and diluted up to 100 ml with deionized water produced by a water purification system (Purelab DV35, ELGA, High Wycombe, Buckinghamshire, UK). Analysis of the elements (sodium, magnesium, potassium, calcium, chromium, manganese, iron, cobalt, nickel, copper, zinc, arsenic, cadmium, and lead) was performed by inductively coupled plasma mass spectrometry (ICP-MS) (iCap Q mass spectrometer, Thermo Scientific, Bremen, Germany).

Adjustment of physical and electronic parameters was performed before determining the elements, using calibration solution (Thermo Scientific Tune B). The calibration curve consisted of five points in two ranges (including zero). Multielemental internal standard was introduced into the ICP-MS during the measurements. Data analysis software automatically made corrections comparing internal standards. Quality control was performed using certified reference material (CRM) NIST SRM 1577c (Table 1).

Table 1 Quality control of element analysis
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Statistical Analysis

Statistical analysis was performed using the GraphPad Prism version 7.00 software. The concentrations of elements in honey bees and their products were expressed as mean and standard deviation (SD) and were subjected to analysis of variance (one-way ANOVA). The parameters were analyzed using the Student’s t test at the probability of 0.05. Pearson’s correlation analysis was applied to examine the relationship between element concentrations (P < 0.01 and P < 0.05).

Principal Component Analysis

The results of principal component analysis were used for presentation of the differences/similarities in the metal content of the bees, drones, broods, and products. The varimax method was performed for data rotation. For principal component analysis (PCA) analysis, determination statistic software JMP 10 (SAS Institute Inc., Cary, NC, USA) was employed.

Results and Discussion

Apiculture is very popular in Serbia [24]. According to the Statistical Yearbook оf the Republic of Serbia [25], the number of beehives exceeds 840,000, with total honey production of 2263 tons and 15 kg of honey per beehive (in 2015). The mean concentrations of fourteen elements measured in honey from the three different regions are presented in Fig. 2. Concentrations of elements were significantly different between locations (P < 0.05). The honey from Ležimir (III) had higher Na, Mg, K, and Ca (6.15 ± 0.34 mg kg−1, 38.45 ± 2.02 mg kg−1, 2824 ± 44.96 mg kg−1, and 1189.70 ± 5.44, respectively) concentrations compared with honey from the two other locations. Potassium was the most abundant element in honey, in agreement with previous studies [9, 17, 26]. According to Alqarni et al. [26], these elements promote fundamental functions in biological systems. The element concentrations in honey are related to the floral source and can vary widely among honeys of different origins and locations [27, 28]. Co was not detected in any of our honeys. Similar, Vukašinović-Pešić et al. [29] examined honey samples from eight different micro-regions of Montenegro, and the results confirm that, with the exception of cadmium in samples from two regions exposed to industrial pollution, none of the 12 elements (Pb, Cd, Cu, Zn, Fe, Cr, Sr, Ba, Ca, Na, K, Mg) analyzed exceeded the maximum allowable level. In Croatia, among the multifloral honeys, the following was determined (Fe, Na, Cu: mg kg−1; As, Pb: μg kg−1): orchard honey—highest of Fe 5.17 and As 276.1, lowest Pb 301; meadow honey—highest Na 36.1, lowest Cu 4.38. The average Ca, Cu, and Pb levels found in multifloral honey types were much higher than those reported in other European countries [30].

Fig. 2
figure 2

Element concentrations in honeys from three different regions (mean); concentrations of elements were significantly different between locations (P < 0.05)

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The toxic metal concentrations were compared with those established by the European Union [31] and Serbian legislation [32]. The As concentration ranged between 1.13 ± 0.12 μg/kg (location I) and 1.54 ± 0.27 μg/kg (location II). This difference was significant (P < 0.05) and can be explained by the nature of the different locations; in fact, location I is mostly wilder areas, while location II (Lazarevac) is surrounded by industrial environments. The mean As concentration in honeys was different in previous studies in Serbia [17] and Italy [19] (5.20 μg/kg, 6.96 ± 2.71 μg/kg, respectively). According to Serbian legislation, the maximum residue limit for As is 0.500 mg kg−1, and all tested samples of honey met this legislation. The highest Pb and Cd concentrations were in honey from location II (18.93 ± 1.24 μg/kg and 3.93 ± 0.63 μg/kg, respectively). Previous studies show that Pb contamination of honey in polluted and non-polluted areas is not significantly different [13, 33]. However, the highest Pb values were often found in honey from polluted areas. Similar results were presented in the study by Spirić et al. [17]. The contamination levels of the toxic elements Pb and Cd measured in our study were low, and the honeys were safe, according to EU Regulation (The MRL—maximum residue limit; proposal for EU is at 0.1 mg kg−1 for cadmium and 0.10 mg kg−1 for lead).

Element concentrations in waxes from the three different locations are presented in Fig. 3. The concentrations of elements were significantly different between locations (P < 0.05). The wax collected from all three locations (I, II, and III) contained, from the most abundant down, the highest level of K (238.70 ± 46.24 mg kg−1, 693.40 ± 28.41 mg kg−1, and 1572 ± 178.40 mg kg−1, respectively), Pb (29.25 ± 3.82 μg/kg, 111.80 ± 6.73 μg/kg, and 401.10 ± 16.02 μg/kg, respectively), Ca (36.96 ± 5.69 mg kg−1, 212.70 ± 22.71 mg kg−1, and 396.70 ± 21.18 mg kg−1, respectively), and Cr (131.20 ± 39.34 μg/kg, 85.77 ± 4.02 μg/kg, and 247.60 ± 33.59 μg/kg, respectively). Wax, as important honey bee product, was in the scope of large scientific studies as a potential source of heavy metal deposition [34,35,36,37].

Fig. 3
figure 3

Element concentrations in wax from three different regions (mean); concentrations of elements were significantly different between locations (P < 0.05)

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Generally, the relatively large amounts of trace elements (K, Ca, Pb, and Cr) in wax could be due to atmospheric pollutant deposition on plants and flowers, which might be absorbed together with flower nectar [34, 38]. It should also be noted that in most of our studied apiaries, higher Cd, Pb, Mg, K, and Ni concentrations were found in wax than in honey, with the general highest mean concentration in wax from Ležimir (location III). Similar results were present in a study by Formicki et al. [35].

One-way ANOVA indicated significant differences (P < 0.05) in element concentrations in the honey bees (worker bees, drone bees, and bee broods) (Figs. 4, 5, and 6).

Fig. 4
figure 4

Element concentrations in worker bees from different regions (mean); concentrations of elements were significantly different between locations (P < 0.05)

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Fig. 5
figure 5

Element concentrations in drone bees from different regions (mean); concentrations of elements were significantly different between locations (P < 0.05)

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Fig. 6
figure 6

Element concentrations in tested bee broods from different regions (mean); concentrations of elements were significantly different between locations (P < 0.05)

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Honey bees fly intensively in a radius of up to 7 km [13, 17]. For this reason, they are suitable to be used as bioindicators for environmental element deposition [33, 36, 38, 39]. In the present study, element concentrations in honey bees were affected by location. The high concentrations of Na, Mg, K, Fe, Ni, Cu, As, and Pb (215.20 ± 1.13 mg kg−1, 501.50 ± 1.38 mg kg−1, 4478 ± 103.10 mg kg−1, 64.50 ± 0.16 mg kg−1, 0.88 ± 0.01 μg/kg, 7.67 ± 0.12 μg/kg, 76.28 ± 1.61 μg/kg, 134.70 ± 0.52 μg/kg, respectively) were found in worker bees collected in the apiary from Lazarevac (location II). The results of the present study reinforce the idea that high concentrations of different elements (toxic) are reflected in the worker bees due to local environmental exposure (as for our Lazarevac bees, surrounded by industrial environments).

The concentrations of elements in drone bees were similar to those in worker bees. The high concentrations of Ca, Mn, Fe, Co, Ni, Cu, Zn, As, Cd, and Pb in drone bees ranged between 168.50 ± 1.38 and 377.20 ± 7.19 mg kg−1, 4.30 ± 0.22 and 52.35 ± 0.82 mg kg−1, 18.61 ± 1.57 and 54.26 ± 0.95 mg kg−1, 6.50 ± 0.09 and 70.23 ± 3.11 μg/kg, 0.04 ± 0.01 and 0.52 ± 0.14 μg/kg, 5.93 ± 0.09 and 7.41 ± 0.16 μg/kg, 24.15 ± 2.38 and 42.46 mg kg−1, 5.65 ± 0.24 and 73.63 ± 0.50 μg/kg, 5.12 ± 0.38 and 106.30 ± 1.48 μg/kg, and 9.12 ± 0.26 and 68.07 ± 1.35 μg/kg, respectively. Similarly, Conti and Botre [36] found the concentrations of Cd and Pb in honey worker bees ranged between 2.87 and 4.23 ppm for Cd and 0.61 and 1.25 ppm for Pb. In our study, these differences may be attributed to the varying degrees of heavy metal contaminations at each location.

According to the results obtained in the present study (Fig. 6), some of the Cd, Pb, As, Zn, Co, and Fe concentrations in bee broods collected from apiaries were similar to concentrations of these elements in worker bees and drone bees.

PCA components 1 and 2 explain 80.5% variability of data, cumulatively (Fig. 7). Principal component 1 (PC1) explained 64.4% of variability with corresponding eigenvalue of 9.01, and PC2 explained 16.1% with eigenvalue of 2.25. As it can be seen, data were separated in two distinctive groups: products on the one side and bees, drones and broods on the other. Grouping was mainly established by component 1 based on the differences in content of As, Fe, Ca, Ni, Zn, Co, Mg, Cd, Na, Cu, K, and Mn. The content of these metals was found higher in bees and broods. Higher amounts of Cr and Pb were found only in wax samples originated from Ležimir.

Fig. 7
figure 7

Principal component analysis

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Many previous studies found correlations between element concentrations in honey and/or in their products [34, 35]. Table 2 shows the correlations between element concentrations in the studied bees and their products. Mg had a significant (P < 0.01) positive correlation with K (r = 0.909). Also, Mg positively correlated (but not significant) with Fe (r = 0.901), Cu (r = 0.941), and As (r = 0.634). However, K had a significant (P < 0.05) negative correlation with Cr (r = − 0.393) and Pb (r = − 0.163). Cr had a significant (P < 0.05; P < 0.01) negative correlation with Mn (r = − 0.276), Fe (r = − 0.012), Co (r = − 0.507), Ni (r = − 0.205), Cu (r = − 0.398), and Cd (r = − 0.253).

Table 2 Pearson correlation coefficients for element concentrations in different honey bees and honey bee products
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Our results provide evidence that honey bees can be used as bioindicator for element deposition (including toxic elements). Also, higher element deposition was found in worker bees, drone bees, and bee broods rather than in honey and wax. Finally, honey bees (worker bees, drone bees, and bee broods) could be better bioindicators of the deposition of different elements than honey.