Industrial, agricultural, and domestic activities originate several environmental aftermaths. Among them, the massive increase in undesirable inorganic nitrogen (N) loads within the aquatic environment causes serious threat to surface waters. Denitrification is a crucial part of N transformation, linking organic matter oxidation to benign dinitrogen molecule (N2) production from nitrate. In this sense, hypersaline environments provide unique conditions where redox processes mediated by microorganisms adapted to extreme conditions lead to nitrogen turnover. This is the case of the hypersaline Pétrola Lake (SE Spain), that constitutes the discharge zone of an endorheic basin of 42 km2. It is under the influence of various anthropic activities (agricultural and livestock activities, wastewater discharge without prior treatment), originating the continuous loading of different pollutants (i.e. pesticides, nitrate.). Hydrogeologically, surface waters are connected to groundwater throughout a complex density driven flow (DDF). One of the effects of this DDF is the transport of solutes to deep aquifer zones, playing an important role in the recycling of nutrients, such as N, and the distribution of anthropogenic and natural-derived dissolved organic carbon. Prior to the incorporation in the DDF, some of the transformations of N species take place in named sediment-water interface in which microbial activity promotes different redox processes such as denitrification. The objective of this work is the evaluation of the denitrifier communities of the organic-rich sediments of the hypersaline Pétrola Lake, which may be responsible for the attenuation processes of contaminants and N recycling.
To evaluate the presence of denitrifier communities, sediments were collected at 7 different points from the Pétrola Lake according to the different pollutant inputs (agricultural, livestock, wastewater). The bacterial genomic DNA was extracted from these samples and subjected to a metagenomic analysis by high-throughput amplification and sequencing of the V3-V4 region of the 16S rRNA gene. The results obtained were scanned to evaluate the presence of bacteria related to denitrification processes.
Pétrola Lake sediments showed a high diversity in extremophiles. Some of them are related to the occurrence of N recycling, like bacteria from the family Ectothiorhodospiraceae, which produce the complete autotrophic denitrification under halophillic conditions. Within this family, it is especially remarkable the presence of bacteria from the genus Thioalkalivibrio, accounting about 7 % of the total bacteria identified in such sediments. In addition, other microorganisms like some members of the Halomonadaceae family, present at all sampling points, can grow anaerobically using nitrate as electron acceptor and converting it into nitrite. Other families related to N cycle are also present in the sediments of Pétrola Lake, like Pseudomonadaceae or Nitrospinaceae, although in a minor proportion. This study contributes to our understanding of halophilic microbial communities in denitrification processes.