Microbial eukaryotes in the hypersaline anoxic L'Atalante deep‐sea basin

Abstract

The frontiers of eukaryote life in nature are still unidentified. In this study, we analysed protistan communities in the hypersaline (up to 365 g l(-1) NaCl) anoxic L'Atalante deep-sea basin located in the eastern Mediterranean Sea. Targeting 18S ribosomal RNA retrieved from the basin's lower halocline (3501 m depth) we detected 279 protistan sequences that grouped into 42 unique phylotypes (99% sequence similarity). Statistical analyses revealed that these phylotypes account only for a proportion of the protists inhabiting this harsh environment with as much as 50% missed by this survey. Most phylotypes were affiliated with ciliates (45%), dinoflagellates (21%), choanoflagelates (10%) and uncultured marine alveolates (6%). Sequences from other taxonomic groups like stramenopiles, Polycystinea, Acantharea and Euglenozoa, all of which are typically found in non-hypersaline deep-sea systems, are either missing or very rare in our cDNA clone library. Although many DHAB sequences fell within previously identified environmental clades, a large number branched relatively deeply. Phylotype richness, community membership and community structure differ significantly from a deep seawater reference community (3499 m depth). Also, the protistan community in the L'Atalante basin is distinctively different from any previously described hypersaline community. In conclusion, we hypothesize that extreme environments may exert a high selection pressure possibly resulting in the evolution of an exceptional and distinctive assemblage of protists. The deep hypersaline anoxic basins in the Mediterranean Sea provide an ideal platform to test for this hypothesis and are promising targets for the discovery of undescribed protists with unknown physiological capabilities.

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

“…Euglenozoan, jakobid, rhizarian and choanoflagellate signatures were only detected in the UI, indicating a decrease in diversity in the anoxic and more hypersaline layer. Other studies have found ciliates to dominate the eukaryotic microbial community in upper DHAB haloclines (Alexander et al, 2009;Edgcomb et al, 2009), but we observed a decrease in the relative abundance of ciliate rRNA as environmental conditions become more challenging in the LI, where Fungi appear equally abundant.…”

“…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.…”

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

“…Indeed, microbial life from all three domains has been shown to occur in such aquatic systems (e.g. van der Wielen et al 2005;Daffonchio et al 2006;Alexander et al 2009;Borin et al 2009;Yakimov et al 2007a). 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.…”

Biochemistry and Evolution of Anaerobic Energy Metabolism in Eukaryotes

“…6. Abbreviations: CI to CIV, respiratory complexes I to IV; UQ, ubiquinone; RQ, rhodoquinone; C, cytochrome c; A, ATPase; FRD, fumarate reductase; SQR, sulfide:quinone oxidoreductase; ST, sulfur transferase; SD, sulfur dioxygenase; [2B], acetate:succinate CoA-transferase (subfamily 1B); [3], succinyl-CoA synthetase; [5], malic enzyme; [6], pyruvate dehydrogenase complex; [8], fumarase; [9], pyruvate kinase; [10], phosphoenolpyruvate carboxykinase (ATP dependent); [11], malate dehydrogenase; [14], methylmalonyl-CoA mutase; [15], methylmalonyl-CoA epimerase; [16], propionyl-CoA carboxylase; [18], alanine aminotransferase; [19], aspartate aminotransferase; [20], alanopine dehydrogenase; [26] A strictly anoxic animal among the loriciferans. All animals with anaerobic metabolism discussed above do encounter oxygen permanently or from time to time.…”