## 1 Introduction  
Mines shape ecosystems that harbor oligotrophic microorganisms adapted to an extreme environment. In most cases, microbes have evolved mechanisms to resist high heavy metal concentrations [1; 2]. Microbial communities in mines are largely represented by _Proteobacteria_, followed by _Acidobacteria_, _Firmicutes_, and _Bacteroides_[3]. Among the _Proteobacteria_, the genus _Pseudomonas_ is commonly retrieved in mines [2; 4], as well as in other subterranean environments such as caves [5; 6; 7]. Lousal mine (Azinheira dos Barros, Portugal) is located in the Iberian Pyrite Belt, one of the largest massive sulfide deposits in the world. The mine was active from 1900 to 1988 and exploited mainly for pyrite. However, mining was abandoned due to the low content of zinc and copper in the ores, making the activity not viable economically. After the mine closure, the rather well-preserved surface infrastructures were adapted for a Mining Museum and a center for education in science and technology. The neighboring contaminated area was rehabilitated, and a program of geo-tourism was launched [8]. In the old Lousal mine, the Waldemar Gallery is open to visitors and a sampling campaign was carried out for isolating airborne bacteria. Aerobiological studies in caves were recently adopted as a mean to forecast and control microbial outbreaks [9; 10]. The data showed that species of _Pseudomonas_ were commonly isolated from the air [11], but studies on airborne bacteria are infrequent in mines. There, we isolated an interesting _Pseudomonas_ sp., strain L5B5, in Lousal mine. _Pseudomonas_ is a versatile genus of bacteria widely found in environments such as soils and waters, as well as involved in cooperative relations or, in contrast to beneficial interactions, causing infections in other organisms [12]. Well known cases of beneficial bacteria are some strains of _P. fluorescens_ and _P. protogens_[13; 14], whereas _P. syringae_ or the extensively studied opportunistic human pathogen _P. aeruginosa_ could represent faithfully the best examples of dangerous members in this genus for plants and humans, respectively [15]. _Pseudomonas_ includes a group of bacteria synthesizing valuable secondary metabolites for medicine and biotechnology. For instance, the polykeletible antibiotic muiprocin produced by _P. fluorescens_ NCIMB 10586 was originally isolated in 1971 and, it is still used to control infection of _Staphylococcus aureus_ on the skin and nasal cavity [16]. Likewise, phenazines stand out among other bacterial secondary metabolites, being one of the most abundant and the most studied compounds. Phenazines-producing bacteria are usually associated with host organisms, presenting antagonistic roles depending on the related organism. This is the case of pyocyanin, produced by _P. aeruginosa_, which was originally isolated from human wounds and, in consequence, linked to diseases. However, phenazines synthesized by fluorescent plant pseudomonads were described as a beneficial activity for plant growth and pathogens protection [17]. More recently, during the first decade of the 21 st century, a biosurfactant compound, oramide, was discovered in _P. protogens_ Pf-5 [18]. Afterwards, different metabolites were produced by some _Pseudomonas_, and their antimicrobial activities described [19]. Siderophores are also very common secondary metabolites in _Pseudomonas_. Iron is an essential element for many redox reactions in bacteria, but since this metal is not bioavailable under aerobic conditions, siderophores represent the best strategy that microorganisms have developed to acquire this element. Pyochelin and pyyoverdine are the two most usual siderophores in _Pseudomonas_ to chelate ferric iron and release it in the cytoplasm under iron-limiting conditions. Siderophores were extensively studied in _P. aeruginosa_, due to their involvement in virulence factors [20].