Examination of prokaryotic communities and denitrification activity in riverbed sediments of Deba and its tributary Ego, in unaffected headwaters, moderately contaminated section and heavily affected (from residual and industrial contamination) sections.. Deba

Studying the dynamics of nitrogen and sulphur cycling bacteria in river surface sediments is essential to better understand their contribution to global biogeochemical cycles. Evaporitic rocks settled at the headwater of the Deba River catchment (northern Spain) lead to higher values of sulphate concentration in river water. Similarly, the discharge of effluents from untreated and treated residual (urban and industrial) and industrial wastewaters increases the concentration of metals, nutrients and organic compounds in its mid- and low-water courses. The aim of this study was to assess the impact of anthropogenic contamination from untreated and treated residual and industrial wastewaters on the structure and function of bacterial communities present in surface sediments of the Deba River catchment. The application of a quantitative approach (RT-qPCR) based on denitrification functional genes (nir: nirS + nirK; and nosZ), together with a 16S rRNA metabarcoding structural analysis, has revealed the (i) high relevance of the sulphur cycle at headwater surface sediments (as reflected by the high abundance of members of the Syntrophobacterales order, and the Sulfuricurvum and Thiobacillus genera) and (ii) predominance of sulphide-driven autotrophic denitrification over heterotrophic denitrification. Incomplete heterotrophic denitrification appeared to be predominant in surface sediments strongly impacted by treated and untreated effluents, as reflected by the lower values of the nosZ/nir ratio and the high abundance of members of the Thauera genus, thus favouring N2O emissions. Understanding nitrogen and sulphur cycling pathways has profound implications for the management of aquatic ecosystems, since this knowledge can help us determine whether a river is acting or not as a source of greenhouse gases (N2O).