Water–Sediment Partitioning of a Neutrophilic Iron-Cycling Microbiome in the High-Altitude Dzhily-Su Mineral Springs (Mt. Elbrus)

Mineral springs of the Northern Caucasus remain largely uncharacterized by culture-independent approaches. At the high-altitude Dzhily-Su spring complex, where isotope data indicate deep-seated magmatic fluid inputs, planktonic and sediment-associated communities were analyzed using 16S rRNA gene profiling to determine whether these inputs shape the local microbiome. Rather than a distinct magmatically influenced community, the results revealed a pronounced water–sediment partitioning of taxa with known iron-cycling representatives, microaerophilic Gallionellaceae dominating the water fractions and facultatively anaerobic Aeromonas and Shewanella dominating the sediments. Furthermore, analysis of the fine-size (0.22 µm) water fraction highlighted a notable enrichment of candidate lineages and archaea, including a prominent Nanoarchaeota population (58.6% of archaeal reads) lacking identified mesophilic hosts. The predominance of a typical neutrophilic iron-cycling assemblage, together with the observed water–sediment partitioning, is consistent with an important role of local physicochemical gradients in structuring the microbial community despite the unusual geochemical setting of the parent aquifer.