Unveiling microbial life in new deep‐sea hypersaline Lake Thetis. Part I: Prokaryotes and environmental settings

Abstract

In September 2008, an expedition of the RV Urania was devoted to exploration of the genomic richness of deep hypersaline anoxic lakes (DHALs) located in the Western part of the Mediterranean Ridge. Approximately 40 nautical miles SE from Urania Lake, the presence of anoxic hypersaline lake, which we named Thetis, was confirmed by swath bathymetry profiling and through immediate sampling casts. The brine surface of the Thetis Lake is located at a depth of 3258 m with a thickness of ≈ 157 m. Brine composition was found to be thalassohaline, saturated by NaCl with a total salinity of 348‰, which is one of highest value reported for DHALs. Similarly to other Mediterranean DHALs, seawater-brine interface of Thetis represents a steep pycno- and chemocline with gradients of salinity, electron donors and acceptors and posseses a remarkable stratification of prokaryotic communities, observed to be more metabolically active in the upper interface where redox gradient was sharper. [(14) C]-bicarbonate fixation analysis revealed that microbial communities are sustained by sulfur-oxidizing chemolithoautotrophic primary producers that thrive within upper interface. Besides microaerophilic autotrophy, heterotrophic sulfate reduction, methanogenesis and anaerobic methane oxidation are likely the predominant processes driving the ecosystem of Thetis Lake.

Inter-comparison of the potentially active prokaryotic communities in the halocline sediments of Mediterranean deep-sea hypersaline basins

“…Daffonchio et al 2006;Yakimov et al 2007a,b;La Cono et al 2011) and unicellular eukaryotes (e.g. Edgcomb et al 2009;Pachiadaki et al 2014) found in the seawater--brine interfaces and the brines of several of these systems.…”

“…While new cell morphologies and ecophysiological traits have been assigned to some of these microorganisms, only recently a more specific picture of the metabolic activities of DHAB microorganisms has started to emerge, e.g. S--oxidizing chemolithotrophy, microaerophilic autotrophy, heterotrophic sulfate reduction, methanogenesis and anaerobic methane oxidation (Borin et al 2009;La Cono et al 2011;Ferrer et al 2012;Pachiadaki et al 2014;Yakimov et al 2013;Alcaide et al 2015). In spite of these recent advances, we still know little about how these microorganisms survive and grow under the prevailing conditions of the DHABs.…”

Unveiling microbial activities along the halocline of Thetis, a deep-sea hypersaline anoxic basin

“…Chemical analysis of the brine body shows it is almost nine times more saline than sea water, with an ion composition reflecting late-stage evaporites. Based on this, the Thetis brine is argued to originate from ancient trapped interstitial brines rather than dissolution of halite (La Cono et al, 2011). A pronounced stratification of both prokaryotic and eukaryotic communities was observed over the B2 m thick interface, as was a distinction between halocline and brine communities (La Cono et al, 2011;Stock et al, 2012).…”

“…Based on this, the Thetis brine is argued to originate from ancient trapped interstitial brines rather than dissolution of halite (La Cono et al, 2011). A pronounced stratification of both prokaryotic and eukaryotic communities was observed over the B2 m thick interface, as was a distinction between halocline and brine communities (La Cono et al, 2011;Stock et al, 2012). A recent metagenomics study of the Thetis brine and halocline (Ferrer et al, 2012) provided increased understanding of the metabolic potential of microbial communities of these habitats.…”

“…A recent metagenomics study of the Thetis brine and halocline (Ferrer et al, 2012) provided increased understanding of the metabolic potential of microbial communities of these habitats. However, [ 14 C]-bicarbonate assimilation rates and reverse transcriptase-PCR of a few functional genes in samples from the same Thetis habitats detected activities not suggested by the metagenome (La Cono et al, 2011), including the expression of ribulose 1,5-bisphosphate carboxylase. Such inconsistency points to the need for metatranscriptomic information on a wider range of metabolic activities for a better understanding of community processes.…”