Fig 1.
Map adapted from Böer [27] showing the approximate field location (red dot) of the Liwa Oasis Sabkhas.
White areas are hyper-arid, stippled grey areas are arid, dark areas are semi-arid and grey areas are sub-humid. Very light grey is the Arabian Gulf. The UAE boader is shown as a dotted line.
Fig 2.
Schematic of the aquifer contained within the dunes of the Liwa Oasis.
The base of the dunes is a relatively impermeable layer of carbonate close to sea level (striped layer). The solid line represents the dune profile, the dotted line the aquifer. Flow lines are shown as solid arrows and evaporation is indicated as wavy arrows. Discharge occurs at the lower elevations of the dune where the water table comes close to or intersects the surface. Evaporation occurs leaving behind salt. In the flat interdune spaces salt flats (sabkha) are formed. Figure adapted from Fig 2 of Wood and Imes [51].
Fig 3.
Satellite image of the inland sabkha studied here (dark area in the left side of image).
The red circle marks the sampling site, GPS coordinates of the sampling site in the center of the sabkha are N23.05088° E53.77005° elevation 86 m. Another interdune flat area is visible about 1 km to the east. North is upward. Image was taken on 4 Jan 2015. Image from Digital Globe to NASA with no restrictions on use or copying.
Fig 4.
Perspective image of the salt flat surface (background image) with insert of a sampling indentation made to access the water and sediments.
Fig 5.
Close up of an endoevaporite mat.
The uppermost white layer, about 5 mm thick, is the salt crust. Below that is a layer, almost 5 mm thick of pink halophilic bacteria and below that a layer of green photosynthetic organisms. Below the green layer the material is darker in color. Gas samples were acquired from sediments beneath this mat. The mat pictured above was sectioned from top to bottom as follows–top layer (T) composed of white and pink layer; middle layer (M) composed of green layer; bottom layer (B) composed of darker brown layer; sediment layer (S) mostly composed of fine sand attached to the bottom of the dark brown layer.
Fig 6.
Diagram of the water flow and salt layers of the Liwa inland sabkha.
Relatively fresh ground water flows upward into overtopping ancient salt layer. The salt layer inhibits evaporation, increases the salinity of the water near the surface, and allows some sunlight penetration.
Table 1.
Salinity and pH of water samples, and gas composition of released bubbles from the sabkha subsurface.
Fig 7.
Abundance of bacterial 16S rRNA genes for each phylum for the top, middle, bottom and sediment layers of the mat.
All sequences were classified using SINA program, with the exception of the plastids category, which was identified by BLASTN. A plastids category was included along with the different phyla, to indicate the presence of phototrophic eukaryotes.
Table 2.
Select identities of closest cultured matches based on longest sequences for the phyla identified in each layer, using the NCBI BLASTN program.
Proteobacteria representatives are additionally identified as belonging to the class Alphaproteobacteria (A), Deltaproteobacteria (D), or Gammaproteobacteria (G). The sequence similarity is indicated by ID% with similarities of 99% marked with bold type.
Table 3.
Abundance and distribution of Salinibacter OTUs.