# A Snapshot of the Microbiome of a Portuguese Abandoned Gold Mining Area
## 1 Introduction  
Despite its somewhat limited mainland territorial area, Portugal holds various important inorganic hotspots, mainly congregated in three large and distinct mineralogical clusters present across the country [1, 2]. When focusing on gold (Au)-rich areas/ores, multiple interesting sites have been identified, with some of them being known since the Roman occupation period [3, 4]. However, for the past thirty years, solely the Panasqueira (mostly Tungsten (W)), Aljustrel (mostly Copper (Cu) and Zinc (Zn)) and Neves Corvo (mostly Cu and Zn) mines have been actively operating. Thus, Portuguese gold mines or even interest presumed prospection areas, while explored and studied in the past, have now become largely forgotten. Throughout the ages, gold has served as a critical economiccurrency, shaping human civilization, society, history and development. Nonetheless, auriferous mining is known to contribute to the increase of toxic elements, heavy metals and metalloids in the environment [5; 6; 7]. Due to the high level of waste resulting from drilling and the consequent creation of dumps and tailings containing the ensuing waste stream, this activity is known for having serious detrimental environmental impacts. Arsenic (As), Zn, Cu and lead (Pb) are known to be released and accumulated during gold extraction processes, contributing to soil and water contamination and, consequently, posing several human health risks [8]. These include various skin disorders, respiratory issues and other long-term health effects (e.g., cancer) due to arsenic, mercury and cyanide food, water and land contamination, and human exposure. Moreover, these elements can also affect the microbiome of these areas and their various ecosystem functions/services. Microbial communities are known to be key players in geochemical cycles [9; 10; 11; 12; 13; 14]. Their presence, proliferation and activity contribute to various natural geomorphic roles, including weathering, protection from erosion and precipitation of particles and minerals [11; 12; 13]. Diverse microbial communities can act on fundamental geological processes, since they are able to conduct organic and inorganic transformations, element cycling, rock and mineral transformations, bioweathering, neo-mineral formation and metal/clay interactions [11; 15]. In fact, a large fraction of the microorganisms at the surface and subsurface biosphere constitutes a key component of biogeochemical cycles. The study of mining areas' soils microbiomes can contribute to (1) identifying environmental shifts; (2) recognizing key bioremediation species; (3) highlighting novel environmental metals biorerecovery strategies; (4) identifying microorganisms with relevant industrial physiological properties; (5) contributing to the development of circular economies; (6) and as for the case of fungi, acting as possible geological proxies for gold prospection [16; 17; 18; 19; 20; 21; 22; 23; 24; 25]. Thus, understanding the diversity and structure of microbial communities in rich metal areas, such as gold mines, is currently of high scientific, industrial and applied relevance. Nonetheless, microbial studies in such settings are still scarce, and while a considerable amount of information is available for prokaryotes, much less is known for fungal communities [6; 22; 26; 27; 28; 29; 30; 31; 32; 33; 34; 35; 36; 37; 38]. Commonly, the study of bacteria and archaea in gold mining environments focuses on their uniqueness and phylogenetic diversity in these settings [6; 26; 27; 30; 32; 33; 34; 36; 37; 38]. Some physiological peculiarities have also been explored, including bacteria contribution to the sulfur cycle, the identification of mercury-resistant strains, the evaluation of heavy metal immobilization potential, and the effects of toxic metals and plant diversity on the communities' organization [6; 27; 30; 34; 35; 36].