Abstract
The Um Alhool area in Qatar is a dynamic evaporative ecosystem that receives seawater from below as it is surrounded by sand dunes. We investigated the chemical composition, the microbial activity and biodiversity of the four main layers (L1–L4) in the photosynthetic mats. Chlorophyll a (Chl a) concentration and distribution (measured by HPLC and hyperspectral imaging, respectively), the phycocyanin distribution (scanned with hyperspectral imaging), oxygenic photosynthesis (determined by microsensor), and the abundance of photosynthetic microorganisms (from 16S and 18S rRNA sequencing) decreased with depth in the euphotic layer (L1). Incident irradiance exponentially attenuated in the same zone reaching 1% at 1.7-mm depth. Proteobacteria dominated all layers of the mat (24%–42% of the identified bacteria). Anoxygenic photosynthetic bacteria (dominated by Chloroflexus) were most abundant in the third red layer of the mat (L3), evidenced by the spectral signature of Bacteriochlorophyll as well as by sequencing. The deep, black layer (L4) was dominated by sulfate reducing bacteria belonging to the Deltaproteobacteria, which were responsible for high sulfate reduction rates (measured using 35S tracer). Members of Halobacteria were the dominant Archaea in all layers of the mat (92%–97%), whereas Nematodes were the main Eukaryotes (up to 87%). Primary productivity rates of Um Alhool mat were similar to those of other hypersaline microbial mats. However, sulfate reduction rates were relatively low, indicating that oxygenic respiration contributes more to organic material degradation than sulfate reduction, because of bioturbation. Although Um Alhool hypersaline mat is a nutrient-limited ecosystem, it is interestingly dynamic and phylogenetically highly diverse. All its components work in a highly efficient and synchronized way to compensate for the lack of nutrient supply provided during regular inundation periods.
Highlights
In nature, microorganisms tend to aggregate forming assemblages of different levels of complexity ranging from mono-species biofilms to well-defined laminated microbial mat ecosystems [1,2]
Mat structure The mat exhibited conspicuous vertical stratification, with a characteristic green layer (L1) of variable thickness (1–2 mm) at the mat surface. This distinct appearance was mainly due to cyanobacteria as indicated by the presence of pronounced absorption at the wavelengths of maximal absorption by chlorophyll a (675 nm) and phycocyanin (625 nm) detected by hyperspectral imaging (Fig. 2D)
The presence of algae and diatoms in that layer cannot be excluded, especially because of the presence of intense Chlorophyll a (Chl a) signal (Fig. 2B), which is a general pigment found in all oxygenic photosynthetic organisms
Summary
Microorganisms tend to aggregate forming assemblages of different levels of complexity ranging from mono-species biofilms to well-defined laminated microbial mat ecosystems [1,2]. These arrangements provide better defensive capabilities as well as efficient transfer of resources and energy from one layer to the other. Microbial mats provide several important ecological roles especially to the marine intertidal ecosystem [4]. In addition to their contribution to the global primary productivity and CO2 fixation, these communities interact with the sediment, ‘‘glue’’ particles, and enhance precipitation and lithification [2,5,6]. The physical conditions include light, temperature, and pressure, whereas the chemical parameters are oxygen, pH, redox potential, salinity, and available electron acceptors and donors [2]
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