## 4 Discussion  
To our knowledge, this study provides the first effort to investigate the bacterial community structure along depth at Lake Alboraj, an ecosystem which is subjected to self-regulation due to the forgotten human care and nature preserving. Results showed that the diversity of bacterial communities, according to the phylogenetic analysis, gives rise to different lineages along spatial location and habitat. Phylogenetic diversity indicated a longer evolutionary history in water samples, due to a phylogenetic distancing between the microorganisms that coexist in the environment [41; 42; 43]. Moreover, a non-metric multidimensional scaling (NMDS) analysis revealed clear differences among the bacterial community structure between water samples and sediment (Supplemental Figure S2). So, both phylogenetic diversity measurement and NMDS analysis suggest the coexistence of different bacteria community structures into the water column and water-sediment interface. Under an environment with conditions of low-oxygen and anoxia, microorganisms are expected to adapt their requirements towards metabolic strategies diversification to fill the expected oxygen-limiting niche. For that reason, the bacterial community changes in their distribution between the epilimnion and hypolimnion layers, and the sediment, coinciding with noticeable oxygen depletion in the water column. DO concentrations shift to lower concentrations in a few decimeters at 2.5 m water depth, describing a sharp oxycline (2.5-3.5 m) which was not linked to the thermocline (Figure 1). This result is not in line with the accepted consensus which states that in oxygen stratified lakes is a common process of the development of a thermocline [44]. The small dimension of the lake, a water column of about 4.7 m depth and an extension near to 5500 m\({}^{2}\), may not promote the development of a temperature gradient during winter as well as an efficient oxygen exchange at the water-atmosphere interface. In fact, the existence of a complete anaerobic water column from late spring to late summer suggests that oxygen exchange with atmosphere can be limited to cold periods; during warm periods, a decrease in oxygen solubility matches with the augmentation of water temperature. Regarding to processes involved in the N-cycle, a greater abundance of bacterial families was found in the sediment and hypolimnion layer (W4), compared to epilimnion (W1). Despite of the similarity in the uses of nitrogen compounds, the bacterial communities found in the sediment and W4 samples are dissimilar. Noticeably, the greater abundance of denitrifiers, such as members belonging to _Pseudomonasaceae_, _Burkholderiaceae_ and _Haulanerobiaceae_, was detected in the sediment. In W4 sample, _Caulobacteraceae_ and some members clustering in _Rhodobacteraceae_ and _Hydrogenophilaceae_, were related to processes contributing to nitrogen turnover. Even though the presence of microorganisms identified as facultative aerobic denitrifiers were found in sample W1 (i.e., _Burkholderiaceae_, _Rhodobacteriaceae_), they have not such relative frequency such as in the deeper water layer and bottom lake sediment samples. This agrees with the fact that conventional biological nitrogen removal is achieved by two well-defined sequential stages, nitrification under aerobic conditions followed by denitrification under anoxic conditions. Therefore, in the oxic layer of the water column the denitrification is not such manageable by biological processes mediated by bacteria, which are commonly adapted to anaerobic conditions. Across the bacterial community clustered in W1, only _Burkholderiaceae_ has been identified within the denitrification processes. Even though little is known about the metabolic capacities, this finding suggests that some bacteria are capable of change their metabolism in agreement with the environmental conditions and N-species availability [11].