Abstract
Abstract. Benthic archaea comprise a significant part of the total prokaryotic biomass in marine sediments. Recent genomic surveys suggest they are largely involved in anaerobic processing of organic matter, but the distribution and abundance of these archaeal groups are still largely unknown. Archaeal membrane lipids composed of isoprenoid diethers or tetraethers (glycerol dibiphytanyl glycerol tetraether, GDGT) are often used as archaeal biomarkers. Here, we compare the archaeal diversity and intact polar lipid (IPL) composition in both surface (0–0.5 cm) and subsurface (10–12 cm) sediments recovered within, just below, and well below the oxygen minimum zone (OMZ) of the Arabian Sea. Archaeal 16S rRNA gene amplicon sequencing revealed a predominance of Thaumarchaeota (Marine Group I, MG-I) in oxygenated sediments. Quantification of archaeal 16S rRNA and ammonia monoxygenase (amoA) of Thaumarchaeota genes and their transcripts indicated the presence of an active in situ benthic population, which coincided with a high relative abundance of hexose phosphohexose crenarchaeol, a specific biomarker for living Thaumarchaeota. On the other hand, anoxic surface sediments within the OMZ and all subsurface sediments were dominated by archaea belonging to the Miscellaneous Crenarchaeota Group (MCG), the Thermoplasmatales and archaea of the DPANN (superphylum grouping Micrarchaeota, Diapherotrites, Aenigmarchaeota, Nanohaloarchaeota, Parvarchaeota, Nanoarchaeota, Pacearchaeota and Woesearchaeota). Members of the MCG were diverse, with a dominance of subgroup MCG-12 in anoxic surface sediments. This coincided with a high relative abundance of IPL GDGT-0 with an unknown polar head group. Subsurface anoxic sediments were characterized by higher relative abundance of GDGT-0, -2 and -3 with dihexose IPL types, GDGT-0 with a cyclopentanetetraol molecule and hexose, as well as the presence of specific MCG subgroups, suggesting that these groups could be the biological sources of these archaeal lipids.
Highlights
Archaea are ubiquitous microorganisms in the marine system (DeLong et al, 1994; Delong and Pace, 2001; Schleper et al, 2005)
Phylogenetic trees were constructed with the neighbor-joining method (Saitou and Nei, 1987) and evolutionary distances computed using the Poisson correction method with a bootstrap test of 1000 replicates. We analyzed both intact polar lipid (IPL) and DNA/RNA extracts from sediments previously collected along the Arabian Sea Murray Ridge within the oxygen minimum zone (OMZ) (885 m b.s.l.), just below the lower interface (1306 m b.s.l.), and well below the OMZ (2470 and 3003 m b.s.l.)
GDGT-0 with both an ether-bound cyclopentanetetraol moiety and a hexose moiety as head groups was identified (Fig. S2) in some sediments (Table 2). This IPL was previously reported as a glycerol dibiphytanyl nonitol tetraether (GDNT; de Rosa et al, 1988), but was later shown to contain a 2-hydroxymethyl-1(2,3-dihydroxypropoxy)-2,3,4,5-cyclopentanetetraol moiety by Sugai et al (1995) on the basis of NMR spectroscopy characterization
Summary
Archaea are ubiquitous microorganisms in the marine system (DeLong et al, 1994; Delong and Pace, 2001; Schleper et al, 2005) They occur in diverse environments, e.g., hydrothermal vents (Stetter et al, 1990), the marine water column (Karner et al, 2001; Massana et al, 2004), in the underlying sediments (Lloyd et al, 2013; Teske and Sørensen, 2008), and well below the seafloor (Biddle et al, 2006; Lipp et al, 2008), where they are considered key players in diverse biogeochemical processes (Offre et al, 2013, and references cited therein).
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