## 4 Discussion  
As a first approach to evaluate the presence of bacteria related to atrazine degradation processes, DNA was extracted from groundwater and the 16 SrRNA gene was amplified and cloned. Subsequently, 93 clones were partially sequenced and compared with 16 SrRNA databases to identify the bacteria. This is a quick and easy method to gain a view of the most abundant bacteria present in groundwater, although the information obtained is far from providing a complete characterization of the structure of the bacterial communities that inhabit groundwater, due to the low number of clones analyzed. In this sense, a higher number of clones sequenced from the sample would have resulted in greater species diversity. However, despite the low number of clones sequenced, it was possible to find bacteria that have been described to be involved in these bioremediation processes. Thus, the sequencing of 93 clones from deep groundwater in the Petrola lake-aquifer system must be enough to find the predominant bacteria and provided information about the possible biogeochemical processes carried out by microorganisms adapted to low oxygen and brackish groundwater ecosystems. This constituted the first approach to the knowledge of bacterial communities adapted to the deep zones of a hypersaline lake-aquifer system, in which environmental conditions may differ substantially from aquatic surface environments. The hypersaline lake-aquifer system may constitute a natural barrier to organic pollutants promoted by the existence of adequate biogeochemical conditions. It is noticeable the presence in deep groundwater zones of the aquifer of microorganisms belonging to families such as Pseudomonas, Comamonadaceae, Xanthomonadaceae, Bacillaceae, or Rhizobiaceae (69.7%), of which some of their members have been found related to nitrogen compounds degradation, including the banned herbicide atrazine in EU. Additionally, it is remarkable the presence of bacteria from families such as Sphingomonadaceae, Moraxellaceae, or the _Phenylobacterium_ genus from the Caulobacteraceae family (21.4%), which have been associated with mono- and multifunctional microorganisms related to xenobiotic compounds degradation. The transport and accumulation of contaminants in the groundwater derived from anthropic activities can cause processes of metabolism adaptation in the microbial community [56]. Regarding atrazine biodegradation, it has been reported to be a slow process due to its asymmetric substituent groups [57]. Nowadays, it is known that the acceleration of the biodegradation process is mainly due to the enzymatic-degrading pathway dispersed among a wide range of microorganisms. As a response to prolonged exposure of atrazine, microbial communities might be able to adapt to metabolize the compound, facilitating the biodegradation and thereby reducing toxicity in the contaminated sites [58; 59]. The microbial degradation pathways play the primary role in the reduction of toxic chemicals from contaminated aquatic environments. It has been described that atrazine catabolic pathways appear to be the result of the recruitment of different genes in broad microbial communities rather than a genetic element encoding the entire pathway. The predominance of the _Pseudomonas_ strain clones found in the analyzed groundwater sample suggests that their members could play a role in atrazine degradation within the aquifer bacterial community since it has been reported that some _Pseudomonas_ sp. strains are able to use atrazine as a nutrient source [50]. On the other hand, the members of the Sphingomonadaceae family have been described for their ability to adapt more rapidly by harboring different pathways for metabolic degradation of new compounds in the environment than members of other bacterial genera [48]. This report provides the first approach of the atrazine-degrading genetic potential of fourteen cultivable bacteria strains isolated from the regional groundwater flow.