Microbial and Chemical Characterization of Underwater Fresh Water Springs in the Dead Sea

Due to its extreme salinity and high Mg concentration the Dead Sea is characterized by a very low density of cells most of which are Archaea. We discovered several underwater fresh to brackish water springs in the Dead Sea harboring dense microbial communities. We provide the first characterization of these communities, discuss their possible origin, hydrochemical environment, energetic resources and the putative biogeochemical pathways they are mediating. Pyrosequencing of the 16S rRNA gene and community fingerprinting methods showed that the spring community originates from the Dead Sea sediments and not from the aquifer. Furthermore, it suggested that there is a dense Archaeal community in the shoreline pore water of the lake. Sequences of bacterial sulfate reducers, nitrifiers iron oxidizers and iron reducers were identified as well. Analysis of white and green biofilms suggested that sulfide oxidation through chemolitotrophy and phototrophy is highly significant. Hyperspectral analysis showed a tight association between abundant green sulfur bacteria and cyanobacteria in the green biofilms. Together, our findings show that the Dead Sea floor harbors diverse microbial communities, part of which is not known from other hypersaline environments. Analysis of the water’s chemistry shows evidence of microbial activity along the path and suggests that the springs supply nitrogen, phosphorus and organic matter to the microbial communities in the Dead Sea. The underwater springs are a newly recognized water source for the Dead Sea. Their input of microorganisms and nutrients needs to be considered in the assessment of possible impact of dilution events of the lake surface waters, such as those that will occur in the future due to the intended establishment of the Red Sea−Dead Sea water conduit

Locations of the sampling sites on the west coast of the Dead Sea, showing the northern and southern spring systems.

<p>Underwater springs with the corresponding reference site are marked with blue and red circles, whereas shore springs together with their reference site are marked with green and red squares, respectively. The open-water reference site for the shore springs was used only for comparison of dissolved organic matter (DOM) and total dissolved nitrogen (TDN). The open blue circle is located in the center of an underwater spring upwelling and was sampled for DOM and TDN analysis. The contour lines on the left panel represent the yearly drop in the lake level and are a close approximation of the areal topography. The satellite image was created using Google Earth.</p

Non Metric Multidimensional Scaling (NMDS) plots derived by the DICE algorithm from the (A) 454 pyrosequencing and (B) ARISA data, using stress values of 13% and 7%, respectively.

<p>Clustering of the pyrosequencing data was performed on the data matrix produced by the NGS system at a taxonomic depth of 5 (Family level). Duplicate samples represent biological replicates. Data for the 1992 and 2007 analyses where obtained from Rhodes et al <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0038319#pone.0038319-LaronneBenItzhak2" target="_blank">[31]</a> and Bodaker et al <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0038319#pone.0038319-Mller1" target="_blank">[30]</a> respectively.</p

Seawater normalized (C_sw) concentrations of major ions in waters from the underwater springs (A) and from reference sites (B).

<p>The concentrations are listed in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0038319#pone-0038319-t002" target="_blank">Table 2</a> and in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0038319#pone.0038319.s008" target="_blank">Table S1</a>. The ions are arranged along the x-axis based on their natural behavior: heavy alkalis Cs and Rb are mainly controlled by surfaces such as those of clay minerals; K, Na, Cl and Br stand for brines and salt minerals (halides); SO₄, Ca, Sr, Mg, Ba and HCO₃ represent dissolved species from carbonate-sulfate minerals (e.g., anhydrite/gypsum, aragonite and barite). All these minerals are abundant in the Dead Sea sediments. (C) Ratios between the measured ion concentrations and those calculated by a two-component mixing model (see <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0038319#pone-0038319-t002" target="_blank">Table 2</a> for the estimated mixing coefficients) using the Jericho 5 freshwater and either the Dead Sea pore water or the Qedem brine as end-members.</p

Graphical representation of the sequence frequency in the studied Dead Sea samples, showing major detected phyla and families of different functional groups of Bacteria.

<p>PSB and PNSB in panel B refer to purple sulfur and non sulfur bacteria, respectively. The different genera within the families Chlorobiaceae,Campylobacteraceae and Helicobacteraceaeare shown to facilitate a more specific sample comparison. The symbols and sample naming are explained in detail in Fig. 8. Note different legends for OTU/path for each panel, and scale-bars for relative sequence frequency for several combined panels.</p

(A) Total counts of DAPI-stained cells and (B) percentage abundance of Archaea and Bacteria within the total cell counts in water samples collected from different underwater springs and from the reference Dead Sea water.

<p>Wn denotes water sample from spring n. Error-bars indicate standard error (N = 10); NA = data not available.</p

Water chemistry of underwater springs and auxiliary sites.

*<p>Taken from Stein et al. <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0038319#pone.0038319-Stein1" target="_blank">[93]</a>;</p>†<p>Taken from Gross <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0038319#pone.0038319-Barkan1" target="_blank">[47]</a>;</p>#<p>Taken from Stiller and Nissenbaum <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0038319#pone.0038319-Stiller1" target="_blank">[76]</a>.</p><p>Classification based on REY: L- Limestone group;D1- Dead Sea group, subgroup 1;D2- Dead Sea group, subgroup 2.</p><p>Chemical composition and other characteristics of the waters from the Dead Sea, underwater springs, shore springs and additional auxiliary sites. Mixing coefficients represent the calculated percentages of brine (either porewater or the Qedem brine) admixed to the freshwater from the Jericho 5 well that best explain the measured concentrations of K, Cl and Br as well as the ratios of Cl/Br and Ca/Mg. PW/− denotes mixture of the porewater and freshwater, −/Q denotes mixture of the Qedem brine and freshwater.</p