Effect of saturation-level salinities on metabolic activities in a microbial mat from a saltflat

Microbial mats are densely populated, stratified ecosystems often performing a complete cycling of elements. Such metabolically diverse and self-sustaining communities can also be found at hypersaline conditions. In this study we investigated the biogeochemical activities and responsible microbial communities in salt-crust covered microbial mats growing at the verge of salt-saturation on a coastal sabkha in Oman. Thereby we explored the salinity limits for key microbial processes and potential protective properties of the salt crust with a combination of geochemical and molecular ecology methods, including microsensor profiles, hyperspectral imaging, pigment extraction, and fluorescence microscopy combined with 16S rRNA gene amplicon sequencing.While the salt crust did not reduce light intensities or heat, the microbial community remained active under these multi-stress conditions. Sulfate reduction, aerobic respiration and anoxygenic photosynthesis could be measured at saturation-level salinity (40%), while oxygenic phototrophs appeared to be more susceptible to such high salt concentrations. Oxygenic photosynthesis was still measurable at 30% salinity, but was completely inhibited at salt saturation and only resumed after significant dilution of the salt. High-resolution amplicon sequencing revealed a diverse microbial community including two layers of Cyanobacteria, high relative abundances of Chloroflexi and Archaea.Our study describes a microbial community adapted to exist at near salt-saturation, but also reveals a salinity limit for its carbon input via oxygenic photosynthesis. Thus, this community has to be depending on salt dilution by tides or floods to be truly self-sustaining.