With a pinch of salt: metagenomic insights into Namib Desert salt pan microbial mats and halites reveal functionally adapted and competitive communities

Abstract

Playas, saline-rich springs surrounded by halite evaporates (also called salt pans) have played an essential role in landscape erosion during the formation of the Namib Desert and are numerous in its central region. In this study, we used shotgun metagenomics to investigate the phylogenetic and functional capacities of the microbial communities in two salt pans (namely, Eisefeld and Hosabes) located in the Central Namib Desert. We studied the source and sink sediment mat communities of the saline streams, as well as those from two halites (crystallized structures on the stream margins). The microbial assemblages and potential functions were distinct in both niches. Independently from their localization (Eisfeld vs Hosabes and source vs sink), the sediment mat communities were dominated by members of the Alphaproteobacteria and Gammaproteobacteria classes, while halites were Archaea-dominated (Euryarchaea phylum) and also contained high abundances of the extremely halophilic bacterium Salinibacter sp. (phylum Bacteroidetes). Photoheterotrophy and chemoheterotrophy were the principal lifestyles in both niches, with halite communities having a reduced diversity of metabolic pathways. Intense microbial-virus interactions in both niches were implied by the widespread detection of CRISPR-Cas defense systems. We identified a putatively novel clade of type II CRISPR-Cas systemsas well as novel candidate viral lineages of the order Caudovirales and of Euryarchaea-infecting haloviruses. Putative gene transfer agent-like sequences within the Alphaproteobacteria were identified in the sediment mat communities. These horizontal gene transfer elements have the potential to drive genome plasticity and evolution of the Alphaproteobacteria in the Namib Desert salt pan microbiomes.