Prokaryotic Diversity and Community Distribution in the Complex Hydrogeological System of the Añana Continental Saltern.

The Añana Salt Valley (northern Spain) is a continental saltern consisting of a series of natural springs that have been used for salt production for at least 7000 years. This habitat has been relatively understudied; therefore, prokaryotic diversity was investigated through Illumina-based 16S rRNA gene sequencing to determine if the waters within the valley exhibit distinctive microbiological characteristics. Two main types of water were found in the valley: salty (approximately 200 g/L salinity) from the diapiric structure and brackish (≤ 20 g/L salinity) from shallow streams. The beta diversity indices showed that salinity was the primary factor influencing the prokaryotic distribution. However, a niche-specific influence was observed between waters of the same origin, with significant differences in the relative abundance of the ASVs. The microbiome of the saltern revealed that the archaeal domain was mainly restricted to salty waters, while the bacterial domain was ubiquitous throughout the saltern, with a notable prevalence in brackish waters. The main bacterial and archaeal phyla identified were Pseudomonadota and Halobacterota, respectively. The genus Halorubrum was abundant and widespread in salty waters, while Pseudomonas was a significant part of the prokaryote community, mainly in brackish waters. The relative abundance of the genera Haloplanus and Salinibacter increased in the salt ponds used for salt production. The taxa involved in chemoheterotrophy and fermentation were widespread, sharing the same niche. Overall, the location of this saltern on a diapiric structure favors the occurrence of waters with different origins that affect the prokaryotic distribution beyond the niche location in the valley.