Microeukaryotic and prokaryotic diversity of anchialine caves from Eastern Adriatic Sea islands

Anchialine ecosystems in the eastern Adriatic Sea are diverse both morphologically and biologically. In this study, for the first time in this geographic area, we explored the microbial community of anchialine caves using high-throughput sequencing. The four anchialine caves, each on nearby islands located in National Park Kornati, had pronounced stratification due to a strong salinity gradient. The microeukaryotic and prokaryotic community clustered into four groups based on the cave of origin, demonstrating high microbial diversity within caves. Freshwater (brackish) and marine microbial communities differed in the relative abundance of major phyla and contained habitat-specific lineages, yet many shared taxa were observed in both environments. Sequencing revealed a surprisingly wide diversity of the microeukaryotic and prokaryotic community within the salinity gradient of the well-pronounced halocline. Our results indicated that the strong salinity gradient recorded in all studied anchialine caves is not the only community structuring factor. Interestingly, anchialine caves within close geographical proximity to each other have different microbial community patterns. Anchialine caves represent unique ecosystems and poorly described biodiversity reservoirs, maintaining unique and complex microbial diversity influenced by an interplay of biotic interactions and abiotic environmental conditions.