Microbial eukaryote life in the new hypersaline deep-sea basin Thetis

Abstract

Only recently, a novel anoxic hypersaline (thalassic) basin in the eastern Mediterranean was discovered at a depth of 3,258 m. The halite-saturated brine of this polyextreme basin revealed one of the highest salt concentrations ever reported for such an environment (salinity of 348‰). Using a eukaryote-specific probe and fluorescence in situ hybridization, we counted 0.6 × 10(4) protists per liter of anoxic brine. SSU rRNA sequence analyses, based on amplification of environmental cDNA identified fungi as the most diverse taxonomic group of eukaryotes in the brine, making deep-sea brines sources of unknown fungal diversity and hotspots for the discovery of novel metabolic pathways and for secondary metabolites. The second most diverse phylotypes are ciliates and stramenopiles (each 20%). The occurrence of closely related ciliate sequences exclusively in other Mediterranean brine basins suggests specific adaptations of the respective organisms to such habitats. Betadiversity-analyses confirm that microeukaryote communities in the brine and the interface are notably different. Several distinct morphotypes in brine samples suggest that the rRNA sequences detected in Thetis brine can be linked to indigenous polyextremophile protists. This contradicts previous assumptions that such extremely high salt concentrations are anathema to eukaryotic life. The upper salinity limits for eukaryotic life remain unidentified.

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

“…The recently discovered Thetis DHAB was selected because a detailed description of the environmental setting, the diversity of its prokaryotes and eukaryotes, selected functional activities, and the genetic potential of the prokaryotic communities existed (La Cono et al, 2011;Ferrer et al, 2012;Stock et al, 2012). For the present study we examined the UI, from 7.0% to 16.3% salinity, and dissolved oxygen from 9.5 mmol l À 1 to 0, and the anoxic LI, with a salinity of 21.4% to 27.6%.…”

“…Based on SSU rRNA gene transcripts recovered within our metatranscriptome libraries, the eukaryotic community appears to be dominated by ciliates and dinoflagellates in the UI, and by ciliates and fungi in the LI (Figure 2). Previous studies of DHABs, including Thetis, based on SSU rRNA and microscopic observations have detected heterotrophic protists (particularly ciliates, dinoflagellates and fungi) in up to 36% salinity (see, for example, Alexander et al, 2009;Edgcomb et al, 2009;Stock et al, 2012). Phagotrophic protists are known to be successful along pelagic oxyclines where prokaryotes are abundant (see, for example, Behnke et al, 2006;Edgcomb et al, 2011a) and hence it follows that they are also successful within these haloclines.…”

“…Studies of Discovery, L'Atalante, Urania, Thetis and Bannock basins revealed that DHAB interfaces harbor abundant and diverse microbial communities that include numerous novel candidate divisions that are more productive than most pelagic marine systems (see, for example, Sass et al, 2001;Van Der Wielen et al, 2005;Yakimov et al, 2007;Edgcomb et al, 2009;Stock et al, 2012). Functional analyses of these communities based on PCR amplification of key functional genes as well as activity measurements revealed sulfur cycling and methanogenesis to be dominant prokaryotic metabolic processes supporting life in DHABs and contributing to observed elevated biomass in DHAB haloclines (Daffonchio et al, 2006;Yakimov et al, 2007;Borin et al, 2009).…”

“…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.…”

Exploring the uncultured microeukaryote majority in the oceans: reevaluation of ribogroups within stramenopiles

“…In addition, our extensive phylogenetic survey unveiled novel diversity within Ochrophyta. These novel groups were sometimes pointed out in the original publication but remained unnamed (Le Gall et al, 2008;Bachy et al, 2011;Stock et al, 2012). Here, we defined five novel ochrophyta ribogroups, and one of them (MOCH-2) seemed to be a quantitatively important member of oxic pico-and nanoplankton.…”