## 4 Discussion
Nonetheless, in our case study, the importance of external inputs (i.e., discharge of polluted groundwater, surface runoff) in nutrient availability cannot either be confirmed or discarded. Biogeochemical S-cycle can be closely linked to microbial redox processes in aquatic systems [54; 55]. Sulphate is an important end-product during microbially-mediated sulphide oxidation processes but can also be derived from the dissolution of sulphate-bearing minerals, as gypsum. Gypsum has been identified in the Triassic evaporitic rocks outcropping in the study area as well as in detrital grains in the Holocene sediments in which the lake is located [56]. The difference in SO4- concentrations between the hypolimnion (6310 mg/L) and epilimnion (5426 mg/L) is important enough to dismiss the detection limit of the method. Nonetheless, a variety of sulphur related bacteria were identified in W1 and W4, which can perform coupled sulphate reduction-sulphide oxidation processes. At this interface, SO4- can be used during sulphate-reducing bacteria processes to produce H2S, which is re-oxidised in the epilimnion to SO4- in the presence of oxygen. In this sense, S-cycle bacteria were better represented in anoxic hypolimnion layer, W4, than in the oxic epilimnion layer, W1. In W4 sample bacteria belonging to _Desulfobulaceae_, _Rhodobacteraceae_, _Croscinitomicaceae_ families, which include various sulphate-reducing bacteria [30; 57] were found. High activity of sulphate-reducing bacteria is accompanied by production of H2S, which acts as an electron donor for the chemolithotropic sulphur-oxidising bacteria. Microorganisms within the families _Rhodobacteraceae_ and _Cyclobacteriaceae_ found in the epilimnion are known sulphate-oxidising bacteria [58; 59], which may participate in the activity of re-oxidising the bottom-produced H2S to SO4- in the upper oxic layer. Interestingly, both, the family _Methylomonadaceae_, identified in W4, and _Methylophagaceae_, identified in the sediment sample, are associated with H2S oxidation processes and methane (CH4) production. The availability of dissolved organic carbon is noticeable in the water column, but water samples do not show an important difference in DOC concentrations at W4 (14.9 mg/L) and W1 (12.9 mg/L). The consumption of organic matter by heterotrophs may lead to the production of CO2 (g) which, in turn, can be reduced to CH4 (g), under anaerobic conditions. The intensity of DO consuming processes by heterotrophic microorganisms may lead to the development of an oxygen deficient zone of different thickness in the lake. Depending on the rate of oxygen consumption processes and the renewal of oxygen at the water-atmosphere interface, a well-defined oxycline can be developed, with different thickness, in a poor ventilated water column, in absence of thermal stratification. The bacterial distribution found matched with changes in the observed dissolved oxygen concentration decrease, from epilimnion to hypolimnion, chlorophyll concentrations and the distribution of some dissolved chemicals. These results support the idea that microbial structure plays an important role in the distribution and fate of chemical species. The bacterial distribution in the bottom lake sediment and the stratified water column can promote the development of different interfaces in which redox conditions lead to the recycling of N, S and C-species. Despite the importance of inherent interplay between native microorganisms and the environmental conditions in the development of a chemical stratification, there is still a paucity of evidence on what biochemical processes are being conducted by bacteria communities in Lake Alboraj. The interfaces, in turn, may offer the opportunity of learning about the distribution and fate of some pollutants, as well as deepening in the knowledge of the processes conducting to eutrophication of water bodies in which an oxycline is developed independently of the water volume stored and morphology of the lake.