Paleoecology of Neoproterozoic hypersaline environments: Biomarker evidence for haloarchaea, methanogens, and cyanobacteria

Abstract

While numerous studies have examined modern hypersaline ecosystems, their equivalents in the geologic past, particularly in the Precambrian, are poorly understood. In this study, biomarkers from ~820 million year (Ma)-old evaporites from the Gillen Formation of the mid-Neoproterozoic Bitter Springs Group, central Australia, are investigated to elucidate the antiquity and paleoecology of halophiles. The sediments were composed of alternating laminae of dolomitized microbial mats and up to 90% anhydrite. Solvent extraction of these samples yielded thermally well-preserved hydrocarbon biomarkers. The regularly branched C isoprenoid 2,6,10,14,18-pentamethylicosane, the tail-to-tail linked C isoprenoid squalane, and breakdown products of the head-to-head linked C isoprenoid biphytane, were particularly abundant in the most anhydrite-rich sediments and mark the oldest current evidence for halophilic archaea. Linear correlations between isoprenoid concentrations (normalized to n-alkanes) and the anhydrite/dolomite ratio reveal microbial consortia that fluctuated with changing salinity levels. Halophilic archaea were the dominant organisms during periods of high salinity and gypsum precipitation, while bacteria were prevalent during stages of carbonate formation. The irregularly branched C isoprenoid 2,6,10,15,19-pentamethylicosane (PMI), with a central tail-to-tail link, was also abundant during periods of elevated salinity, highlighting the activity of methanogens. By contrast, the irregularly branched C isoprenoid 2,6,11,15-tetramethylhexadecane (crocetane) was more common in dolomite-rich facies, revealing that an alternate group of archaea was active during less saline periods. Elevated concentrations of isotopically depleted heptadecane (n-C ) revealed the presence of cyanobacteria under all salinity regimes. The combination of biomarkers in the mid-Neoproterozoic Gillen Formation resembles lipid compositions from modern hypersaline cyanobacterial mats, pointing to a community composition that remained broadly constant since at least the Neoproterozoic. However, as a major contrast to most modern hypersaline environments, the Gillen evaporites did not yield any evidence for algae or other eukaryotes.

Multigene phylogeny and cell evolution of chromist infrakingdom Rhizaria: contrasting cell organisation of sister phyla Cercozoa and Retaria

“…However, if the root of the eukaryote tree were between Eozoa and neokaryotes (all eukaryotes except Euglenozoa, Percolozoa and Eolouka: Cavalier-Smith 2018 ) not within Eozoa, then crown eukaryotes might be no older than 800–850 My as previously argued (Cavalier-Smith 2002b , 2006a ); so far, the oldest biomarker age for archaebacteria (likely sisters of eukaryotes, not their ancestors: Cavalier-Smith 2014 ) is ~ 820 My (Schintele and Brocks 2017 ) but earlier evidence is wanting. Irrespective of the position of the root of the eukaryotic tree, it is likely that Rhizaria are substantially more than half as old as eukaryotes and could be over three quarters of their age, significantly older than animals and other opisthokonts.…”

Cyanobacteria evolution: Insight from the fossil record

“…Molecular fossils include complex organic molecules produced only by biology and, in some cases, are indicative of particular metabolisms or lineages [83]. Cyanobacteria produce lipids (2-methyl-hopanes – [83,84] and pigments that can potentially be preserved in the unmetamorphosed geological record [35,36]. So far, the lipids 2-methyl-hopanes were extracted from bitumen in black shales as old as 1.6 Ga (McArthur basin, Australia) [83,84].…”

“…Biomarkers (fossil molecules) can indicate the presence of metabolisms such as oxygenic photosynthesis, but they are preserved only in well-preserved unmetamorphosed rocks, and contamination is a challenge [35]. Among those, lipids such as 2-methyl-hopanes are produced by cyanobacteria [36] but not only [37], and pigments such as porphyrins with N isotope composition [38]. Other geochemical (redox and isotopic) proxies can also inform on the presence of molecular oxygen in the water column or biologically-induced isotopic fractionation due to oxygenic photosynthesis, but their interpretation is often debated.…”

Membrane Lipid Composition and Amino Acid Excretion Patterns of Methanothermococcus okinawensis Grown in the Presence of Inhibitors Detected in the Enceladian Plume

“…For example, isoprenoid hydrocarbons of methanotrophic archaea have been found in authigenic limestones from the late Pennsylvanian (approx. 300 Ma) in southern Namibia [100] and lipids of methanogenic and halophilic archaea in mid-Neoproterozoic (~820 Ma old) evaporites from central Australia [101]. The lipid biomarker approach may therefore offer an opportunity to search for signs of life in extraterrestrial worlds in the future.…”