## 1 Introduction  
Orchids belonging to the family _Orchidaceae_ are well known for their rarity and presence of mycorrhizal associations [1; 2]. This specialization contributes to the diversity and rarity of orchid species [3; 4]. They grow in specific habitats such as mining dumps. Mine heaps and mine wastes created by the mining industry are one of the extreme habitats made by anthropogenic activity. Interestingly, mine heaps create an environment with specific ecological conditions for plant adaptation. They are characterized by the lack of soil as well as nutrients and moisture, and the absence of a humus layer [5]. Unique species from the family _Orchidaceae_, such as narrow-leaved Helleborine (_Cephalanthera longifolia_; Figure 1D) and Pontic Helleborine (_Epipactis pontica_; Figure 1E) which grows on the Fe deposit in Sirk, dark-red Helleborine (_E. atrorubens_; Figure 1C) from the Ni-Co deposit Dobsina and Pb-Zn deposit Jasenie (_E. atrorubens_; Figure 1A) and Lesser Butterfly orchid (_Platanthera bifolia_; Figure 1B) from the Pb-Zn deposit Jasenie which grows on the initial soils (rankers) and tolerates high concentration of heavy metals, are well adapted to extreme conditions of the environment. These organisms germinate only after colonization with a suitable mycorrhizal fungus [6].  
The fungi provide the basic physiological function for the orchids in the uptake of needed nutrients and water, otherwise inaccessible to plants [7], facilitate the emergence and survival of vegetation under stressful conditions [8], stabilize waste material through a network of veneers and improve its structure by producing substances that bind to soil particles and lead to the formation of soil aggregates [9].  
The microorganisms are known to detoxy metals by several mechanisms including ion exchange, chelation, adsorption, crystallization, valence transformation, extra- and intracellular precipitation and active absorption [10]. The sorption and accumulation of metals depend on various factors such as pH, temperature, organic matter, ion species and the presence of other ions in solutions that may be in competition. They develop adaptation by temporarily altering their developmental patterns or modifying physiological characteristics depending on the toxicity of metals, which is affected by the concentration and form of salts in which the metal exists [11].  
Bacteria play a key role in the element biotransformation, biogeochemical cycling, metal and mineral transformation, biological weathering and formation of the soil and sediment [12]. They have high importance in industrial processes, bioremediation, and heavy metal tolerance [13].  
To identify fungi and bacterial communities the Illumina MiSeq platform was used. It is a scalable, ultra-high-throughput, cost-effective and a powerful tool to detect soil microbial communities at  
Figure 1: Unique species from the family _Orchidaeaae. E. atorubens_ (**A**) and _P. biglia_ (**B**) from the Pb-Zn deposit Jasenie, _E. atorubens_ (**C**) from the Ni-Co deposit Dobššná, _C. longifolia_ (**D**) and _E. pontica_ (**E**) from Fe deposit Sirk.  
greater depth and with more detailed sequence information [14; 15]. It has been successfully applied to identify microbial communities almost everywhere [16; 17], including heavy metal contaminated environments [18; 19].  
The aim of this paper is to identify the composition of bacterial and fungal communities present in the roots and the surrounding soil of the orchids growing on heavy metal contaminated areas of mining heaps. To detect these communities we used 16S rRNA gene and ITS next-generation sequencing approach. At the same time, our aim was to compare the data of fungal and bacterial communities with the pH value in the Pb-Zn mining heap Jasenie-Soviansko, Ni-Co mining heap Dobsina-Dobra nadej and Fe mining heap Sirk-Zeleznik. The broader goal of the study was to determine the unique microbial content of plant root collected from contaminated soil as potential biomarker applicable in further studies or monitoring efforts.