The waterbodies of the Dallol volcano: A physico-chemical and geo-microbial survey
Authors: Moors Hugo , Honty Miroslav , Smolders Carla , Provoost Ann , De Craen Mieke , Leys Natalie 
Published: March 2020

&lt;p&gt;The geological extreme Dallol region, located around the Dallol volcano in the north-east of Danakil depression (Ethiopia), is considered as one of the harshest and hottest places on Earth. The geology is made up of years and years of evaporates accumulation. Volcanic activity generates ascending brines that may cross and mix with aquifers from inflowing meteoric water originating from the Ethiopian highlands on the east of the Danakil depression. When these mixtures reach the surface they can generate hydrothermal springs giving rise to waterbodies in the form of small ponds or lakes. During the Europlanet 2018 Danakil field expedition, ten of these saline waterbodies were extensively studied by &lt;em&gt;in situ&lt;/em&gt; measurements and &lt;em&gt;ex situ&lt;/em&gt; geo&amp;#8211;physico-chemical and &amp;#8211;microbiological analyses of collected samples, liquids as well as sediments.&lt;/p&gt;&lt;p&gt;The &lt;em&gt;in situ&lt;/em&gt; physico-chemical measurements clearly indicated the extreme nature of all ten investigated lakes. Laboratory analyses of the collected batch samples of liquids and sediments confirmed the extreme character of the waterbodies and complements our geological survey of the region with valuable geo&amp;#8211;chemical and &amp;#8211;microbiological data.&lt;/p&gt;&lt;p&gt;Based on our analytical results, the relative small Dallol region can still be subdivided into three geological smaller areas: the outcrop zone, the volcanic base region and the distant south area. The outcrop zone is dominated by sodium, iron and potassium. Oxidation processes in the outflowing superheated ferrous and sulfidic rich brine give rise to some of the most acidic ponds on our planet. In the ponds and lakes of the volcanic base region, incredible high amounts of calcium and/or magnesium can remain in their dissolved form as the most dominant and quasi only available anion is chloride. This region is host for the most saline water body on Earth. Chemical analysis of the lakes of the distant south area show that sodium is by far the most dominant cation. It is therefore no surprise that the large Karum Lake in the south region is economically exploited for the mining of sodium chloride.&lt;/p&gt;&lt;p&gt;Our mineralogy analyses render results that are completely in line with the observed geochemistry of the waterbodies. Halite and sylvite are the most present minerals in the Dallol outcrop zone associated with some gypsum and in one case with anhydrite. The geology around the waterbodies of volcanic base zone are a little bit more divers. On the shores of the Gaet&amp;#8217;ale Pond tachyhydrite, chloromagnesite, halite and sylvite is determined, while the Black Lake is surrounded by bischofite and carnalite. Logically, the mineralogy of the south area, the salt mining area, is dominated by halite and sylvite.&lt;/p&gt;&lt;p&gt;Apparently, the geochemistry of the outcrop zone and volcanic base region is so harsh that no extremophilic organism is able to survive in these areas. Only in the distant south area did we find indications of the presence of halophiles. Besides the bacterial genus &lt;em&gt;Salinibacter&lt;/em&gt;, our 16S rDNA microbiological fingerprinting indicates the presence of halophilic archaea like:&amp;#160; &lt;em&gt;Halobaculum sp., Halobellus sp., Halomicroarcula sp., Halorientalis sp.&lt;/em&gt; with the majority of the population being &lt;em&gt;Candidatus Nanosalina sp&lt;/em&gt;.&lt;/p&gt;