## 4 Discussion
The higher presence of denitrifier bacteria in the deeper zones of the water column, matches with a decrease in the concentration of nitrate (NO3), from 10.33 mg/L, in the epilimnion, to 4.70-0.60 mg/L, in the hypolimnion. The higher number of denitrification related families in sediments could be explained by low oxygen habitat and the availability of NO3 and organic energy sources, since NH4+ is rarely limiting in sediments [45; 46]. In W4, members belonging to _Saprospiraceae_, _Flavobacteriumaceae_, _Crocinitomicaceae_ and _Microbacteriaceae_ families, which can perform saprophytic processes linked to decomposition of organic matter and microbial biomass [24], have also been identified. The production and consumption of NH4+ can be driven under different microbially mediated processes linked to gradients of oxygen [47; 48; 49]. The development of an anaerobic hypolimnion is typical in eutrophic lakes and the processes can involve the increase of nutrients, such as NH4+. At the water-sediment interface, processes such as ammonification can release NH4+ during organic matter decay. In this sense, it has been proposed that in the hypolimnion NH4+ was the major N species identified, and sediment release can contribute nearly 85% hypolimnic NH4+ [47]. A decrease in NH4+ concentration is clearly detected from the hypolimnion (2.18 mg/L) to the epilimnion (0.09 mg/L). The nutrient availability found at hypolimnion may be stimulated by microbially-mediated processes at the water-sediment interface. Dissimilatory nitrate reduction to ammonium (DNRA), may produce NH4+ under strictly anaerobic conditions [45]. A wide distribution group of sediment bacteria is capable to perform DNRA [50], which acts in Alboraj sediment as facultative anaerbes, _Enterobacteriaceae_, to aerobes, _Pseudomonasaceae_. Moreover, bacteria belonging to _Desulfiromondaceae_ family, which have been found to be associated to DNRA were also found in W4 sample. Bacterial structure results could support these findings where DNRA processes could take place in sediment by related members from _Gammaproteobacteria_ and _Delatroetoobacteria_. Under anaerobic conditions, the oxidation of NH4+ can be driven under the presence of NO2- (anaerobic ammonium oxidation, anammox). According to present knowledge, anammox is carried out by a few anaerobic, autotrophic bacteria of a monophyletic clade in Planctomycetes [51]. Sediment gathered the conditions of anaerobic medium and substrate supplier necessary to keep families related to carried out anammox processes as _Pirellulaceae_, also poorly identified in W4 sample, within the phylum Plantomycetes. However, due to the limited information about factors controlling the processes in natural systems, knowledge about anammox in nature must continue as an ongoing effort to understand the distribution of the process that occur in the environment. Noticeably, in the epilimnion, the nitrogen fixers are abundant. Oxygenic photoautotrophic cyanobacteria (_Oxrophotobacteria_) together with populations of _Terasakiellaceae_ might be responsible for the primary productivity in the epilimnion. The proliferation of _Cyanobacteria_ is strongly linked to biosphere's environmental perturbations, including nutrient enrichment [52] and climatic changes (i.e., hydrologic changes, drought and precipitation events hardening) [53]. Their role as atmospheric nitrogen fixers, as well as their capacity of symbiotic cooperation with other microorganisms are well known. Nevertheless, cyanobacterial bloom results in a massive decomposition of dead cells which is associated to cyanotoxins release to the environment [53]. Whenever toxicity occurs, it can cause harmful effects to microorganisms, leading to an unbalanced development of those communities. Moreover, in epilimnion layer, NH4+ can be assimilated by biomass as well as transformed into NO3- by nitrifiers under aerobic conditions (aerobic ammonium oxidation, nitrification). Therefore, nitrification may explain the appearance of NO3- in the epilimnion.