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, we explored the microeukaryotic and prokaryotic community of anchialine caves in the Mediterranean region using high-throughput sequencing. Four anchialine caves located on nearby islands with a well-pronounced salinity gradient were sampled at the surface freshwater area, halocline area, and seawater area. Sequencing revealed a surprisingly wide diversity of the microeukaryotic and prokaryotic community with the relative abundance of major phyla differing within the salinity gradient and between the caves. Interestingly, microeukaryotic and prokaryotic communities clustered into four groups based on location, pointing out that sampled anchialine caves have different microbial community patterns and high microbial endemism. Our results indicate that even with the halocline acting as a selecting barrier, the salinity is not the only community structuring factor. Despite the short geographical distance, the isolation of anchialine caves facilitated high microbial community adaptation and endemism. Our study suggests that anchialine caves represent reservoirs of new biodiversity, maintaining unique and complex microbial diversity influenced by biotic interactions and abiotic environmental conditions. SUPPLEMENTARY INFORMATION: The online version contains supplementary material available at 10.1007/s00248-021-01760-5.