Exploring the microbial diversity featuring the geochemical complexity of the quartzsandstone cave Imawarì Yeuta, Auyan Tepui, Venezuela

In the last three years, one of the longest quartz-sandstone caves, Imawarì Yeuta, has been explored in the Precambrian rocks of the Auyan Tepui massif in Venezuela. The uniqueness of this quartz-sandstone cave resides in its great age (estimated over 30 Ma) and its complete isolation from anthropogenic activities. Therefore, subsurface ecosystems might have been preserved from contamination and possible subsequent alteration. The cave contains a high level of microbial activity as demonstrated by the presence of silica stromatolite-like speleothems, biologically mediated sulphate-phosphate deposits and lakes covered by patinas of violaceous or other colourful biofilms. The high diversity of the environments, in terms of mineralogical substrate and geochemistry of the waters, suggests that niche-differentiation of the microbial communities could be mainly controlled by the specific chemical characteristics of each site rather than by light attenuation, i.e., the distance from the entrances of the cave toward the dark zones. To investigate this possibility, we characterised the microbial community composition in 17 samples collected during the expeditions of 2014 and 2016 to the cave system. In particular, the next-generation sequencing (NGS) approach was performed by using Illumina MiSeq technology targeting the 16S rRNA gene. The acquired data have then been correlated with geochemical (water pH, EC, major and minor elements) and mineralogical (predominant and secondary minerals) information from each sampling site. The results showed that the cave hosts three main types of microbial communities, each of these characteristic of a specific environment dominated by silica (crystalline or amorphous), iron hydroxides or hydrated sulphate-phosphates. This work presents a first set of results on the microbial characterisation of each sampling site and proposes a preliminary hypothesis on the functional biological factors supporting the biodiversity distribution in relation with the different geochemical environments described in this exceptional cave.