Deciphering the Geochemical Constrains Influencing Sediment Microbiomes in Gypsum Caves

Abstract Considerable research efforts have been dedicated to the geomicrobiology of limestone and volcanic caves all over the world but comparatively evaporite (gypsum and anhydrite) karst has been rarely investigated. We studied the sediments from Gypsum, C3, and Water caves, located in the Gypsum Karst of Sorbas, Almeria, SE Spain. Geochemical analyses revealed a high heterogeneity among sediments within caves, especially with regard to nitrogen, ammonium, nitrates, phosphorus, magnesium and calcium contents, depending on the sampling location. According to PERMANOVA analyses, geochemical variables such as pH, carbonate content, magnesium, calcium and strontium showed significant influences on bacterial community structure, explaining from 15.8 to 21.3% of the variance. These results indicate that specific geochemical composition of sediments are relevant drivers of bacterial communities. The gypsum cave sediments exhibited different microbial communities characterized by a large variability in abundance of phyla and genera, which were also commonly found in other caves worldwide. The highest bacterial richness and diversity was observed in Water Cave while a few samples from C3 and Gypsum caves showed relatively low richness and diversity. The resulting bacterial communities from sediments were affiliated to 38 phyla, 16 of them with relative abundances > 1%. The dominant phylum was Actinomycetota in Gypsum Cave, but lower abundances were present in Water Cave and C3 Cave samples. Remarkably, in four sediments from C3 and Water caves Pseudomonadota replaced Actinomycetota as dominant phylum. Other relatively abundant phyla were Bacillota , Planctomycetota , Acidobacteriota , Bacteroidota , Gemmatimonadota , and Chloroflexota . The two most abundant genera were Crossiella and wb1-P19. Overall, gypsum caves contain highly heterogeneous microhabitats in which microbial community structure is primarily controlled by the cave environment, as denoted the finding of similar microbial assemblages with respect to other limestone and volcanic caves, and secondly by the geochemical composition of the sediments.