Niche expansion for phototrophic sulfur bacteria at the Proterozoic-Phanerozoic transition

Lateral redox variability in ca. 1.9 Ga marine environments indicated by organic carbon and nitrogen isotope compositions

The stepwise oxygenation of Earth's surficial environment is thought to have shaped the evolutionary history of life. Microfossil records and molecular clocks suggest eukaryotes appeared during the Paleoproterozoic, perhaps shortly after the Great Oxidation Episode at ca. 2.43 Ga. The mildly oxygenated atmosphere and surface oceans likely contributed to the early evolution of eukaryotes. However, the principal trigger for the eukaryote appearance and a potential factor for their delayed expansion (i.e., intermediate ocean redox conditions until the Neoproterozoic) remain poorly understood, largely owing to a lack of constraints on marine and terrestrial nutrient cycling. Here, we analyzed redox-sensitive element contents and organic carbon and nitrogen isotope compositions of relatively low metamorphic-grade (greenschist facies) black shales preserved in the Flin Flon Belt of central Canada to examine open-marine redox conditions and biological activity around the ca. 1.9 Ga Flin Flon oceanic island arc. The black shale samples were collected from the Reed Lake area in the eastern part of the Flin Flon Belt, and the depositional site was likely distal from the Archean cratons. The black shales have low Al/Ti ratios and are slightly depleted in light rare-earth elements relative to the post-Archean average shale, which is consistent with a limited contribution from felsic igneous rocks in Archean upper continental crust. Redox conditions have likely varied between suboxic and euxinic at the depositional site of the studied section, as suggested by variable U/Al and Mo/Al ratios. Organic carbon and nitrogen isotope compositions of the black shales are approximately -23‰ and +13.7‰, respectively, and these values are systematically higher than those of broadly coeval continental margin deposits (approximately -30‰ for δ13Corg and +5‰ for δ15Nbulk). These elevated values are indicative of high productivity that led to enhanced denitrification (i.e., a high denitrification rate relative to nitrogen influx at the depositional site). Similar geochemical patterns have also been observed in the modern Peruvian oxygen minimum zone where dissolved nitrogen compounds are actively lost from the reservoir via denitrification and anammox, but the large nitrate reservoir of the deep ocean prevents exhaustion of the surface nitrate pool. Nitrogen must have been widely bioavailable in the ca. 1.9 Ga oceans, and its supply to upwelling zones must have supported habitable environments for eukaryotes, even in the middle of oceans around island arcs.

Potassium Chloride as an Effective Alternative to Sodium Chloride in Delaying the Thermal Aggregation of Liquid Whole Egg

The potential of potassium chloride (KCl) to be used as a substitute for sodium chloride (NaCl) was studied by monitoring the effects of salt treatment on thermal behavior, aggregation kinetics, rheological properties, and protein conformational changes. The results show that the addition of KCl can improve solubility, reduce turbidity and particle size, and positively influence rheological parameters such as apparent viscosity, consistency coefficient (K value), and fluidity index (n). These changes indicate delayed thermal denaturation. In addition, KCl decreased the content of β-sheet and random coil structures and increased the content of α-helix and β-turn structures. The optimal results were obtained with 2% KCl addition, leading to an increase in Tp up to 85.09 °C. The correlation results showed that Tp was positively correlated with solubility, α-helix and β-turn but negatively correlated with ΔH, turbidity, β-sheet and random coil. Overall, compared to NaCl, 2% KCl is more effective in delaying the thermal aggregation of LWE, and these findings lay a solid theoretical foundation for the study of sodium substitutes in heat-resistant liquid egg products.

Production of structurally diverse sphingolipids by anaerobic marine bacteria in the euxinic Black Sea water column

Microbial lipids, used as taxonomic markers and physiological indicators, have mainly been studied through cultivation. However, this approach is limited due to the scarcity of cultures of environmental microbes, thereby restricting insights into the diversity of lipids and their ecological roles. Addressing this limitation, here we apply metalipidomics combined with metagenomics in the Black Sea, classifying and tentatively identifying 1623 lipid-like species across 18 lipid classes. We discovered over 200 novel, abundant, and structurally diverse sphingolipids in euxinic waters, including unique 1-deoxysphingolipids with long-chain fatty acids and sulfur-containing groups. Sphingolipids were thought to be rare in bacteria and their molecular and ecological functions in bacterial membranes remain elusive. However, genomic analysis focused on sphingolipid biosynthesis genes revealed that members of 38 bacterial phyla in the Black Sea can synthesize sphingolipids, representing a 4-fold increase from previously known capabilities and accounting for up to 25% of the microbial community. These sphingolipids appear to be involved in oxidative stress response, cell wall remodeling, and are associated with the metabolism of nitrogen-containing molecules. Our findings underscore the effectiveness of multi-omics approaches in exploring microbial chemical ecology.

Geographic and Ecological Diversity of Green Sulfur Bacteria in Hot Spring Mat Communities

Three strains of thermophilic green sulfur bacteria (GSB) are known; all are from microbial mats in hot springs in Rotorua, New Zealand (NZ) and belong to the species Chlorobaculum tepidum. Here, we describe diverse populations of GSB inhabiting Travel Lodge Spring (TLS) (NZ) and hot springs ranging from 36.1 °C to 51.1 °C in the Republic of the Philippines (PHL) and Yellowstone National Park (YNP), Wyoming, USA. Using targeted amplification and restriction fragment length polymorphism analysis, GSB 16S rRNA sequences were detected in mats in TLS, one PHL site, and three regions of YNP. GSB enrichments from YNP and PHL mats contained small, green, nonmotile rods possessing chlorosomes, chlorobactene, and bacteriochlorophyll c. Partial 16S rRNA gene sequences from YNP, NZ, and PHL mats and enrichments from YNP and PHL samples formed distinct phylogenetic clades, suggesting geographic isolation, and were associated with samples differing in temperature and pH, suggesting adaptations to these parameters. Sequences from enrichments and corresponding mats formed clades that were sometimes distinct, increasing the diversity detected. Sequence differences, monophyly, distribution patterns, and evolutionary simulation modeling support our discovery of at least four new putative moderately thermophilic Chlorobaculum species that grew rapidly at 40 °C to 44 °C.

Genes and Pathway Reactions Related to Carotenoid Biosynthesis in Purple Bacteria

In purple bacteria, the genes of the carotenoid pathways are part of photosynthesis gene clusters which were distributed among different species by horizontal gene transfer. Their close organisation facilitated the first-time cloning of carotenogenic genes and promoted the molecular investigation of spheroidene and spirilloxanthin biosynthesis. This review highlights the cloning of the spheroidene and spirilloxanthin pathway genes and presents the current knowledge on the enzymes involved in the carotenoid biosynthesis of purple sulphur and non-sulphur bacteria. Mostly, spheroidene or spirilloxanthin biosynthesis exists in purple non-sulphur bacteria but both pathways operate simultaneously in Rubrivivax gelatinosus. In the following years, genes from other bacteria including purple sulphur bacteria with an okenone pathway were cloned. The individual steps were investigated by kinetic studies with heterologously expressed pathway genes which supported the establishment of the reaction mechanisms. In particular, the substrate and product specificities revealed the sequential order of the speroidene and spiriloxanthin pathways as well as their interactions. Information on the enzymes involved revealed that the phytoene desaturase determines the type of pathway by the formation of different products. By selection of mutants with amino acid exchanges in the putative substrate-binding site, the neurosporene-forming phytoene desaturase could be changed into a lycopene-producing enzyme and vice versa. Concerning the oxygen groups in neurosporene and lycopene, the tertiary alcohol group at C1 is formed from water and not by oxygenation, and the C2 or C4 keto groups are inserted differently by an oxygen-dependent or oxygen-independent ketolation reaction, respectively.

Late acquisition of the rTCA carbon fixation pathway by Chlorobi

The reverse tricarboxylic acid (rTCA) cycle is touted as a primordial mode of carbon fixation due to its autocatalytic propensity and oxygen intolerance. Despite this inferred antiquity, however, the earliest rock record affords scant supporting evidence. In fact, based on the chimeric inheritance of rTCA cycle steps within the Chlorobiaceae, even the use of the chemical fossil record of this group is now subject to question. While the 1.64-billion-year-old Barney Creek Formation contains chemical fossils of the earliest known putative Chlorobiaceae-derived carotenoids, interferences from the accompanying hydrocarbon matrix have hitherto precluded the carbon isotope measurements necessary to establish the physiology of the organisms that produced them. Overcoming this obstacle, here we report a suite of compound-specific carbon isotope measurements identifying a cyanobacterially dominated ecosystem featuring heterotrophic bacteria. We demonstrate chlorobactane is 13C-depleted when compared to contemporary equivalents, showing only slight 13C-enrichment over co-existing cyanobacterial carotenoids. The absence of this diagnostic isotopic fingerprint, in turn, confirms phylogenomic hypotheses that call for the late assembly of the rTCA cycle and, thus, the delayed acquisition of autotrophy within the Chlorobiaceae. We suggest that progressive oxygenation of the Earth System caused an increase in the marine sulfate inventory thereby providing the selective pressure to fuel the Neoproterozoic shift towards energy-efficient photoautotrophy within the Chlorobiaceae.

Collision-induced dissociation as "mass spectrometric filter" for rapid screening of tetrapyrrole derivatives and their chelated metal species in complex biological and environmental samples

RATIONALE

Cyclic tetrapyrroles, such as chlorophylls and their diagenetic derivatives, are structurally diverse and often chelated with certain metal species in the natural environment. A high throughput analytical method enabling quick tetrapyrrole screening in complex samples will promote the study of tetrapyrrole biogeochemistry and probably discoveries of new tetrapyrroles.

METHODS

Total lipid extracts of biological and environmental samples were injected onto a C18 column to separate compounds with a reverse-phase gradient. Collision-induced dissociation (CID) was performed at different energy levels, from 40 to 200 eV, on a quadrupole time-of-flight mass spectrometry (QTOF-MS) to identify cyclic tetrapyrroles in complex matrices.

RESULTS

Under 200 eV CID cyclic tetrapyrroles exhibit a unique fragmentation behavior, the production of fragments larger than 300 Da. Utilizing such feature as a filter to extract product ions in the range of 300-500 Da, various cyclic tetrapyrrole derivatives are readily recognized in all tested biological and environmental samples. The 200 eV CID setup also dissociates chelated to porphyrin metals, including Cu, Fe, Mn, Ni, and V, as single-charged ions for direct MS detection.

CONCLUSIONS

The 200 eV CID setup provides an efficient approach for the identification of cyclic tetrapyrroles, such as chlorophylls and fossil metalloporphyrins, in complex environmental samples. The direct detection of chelated to porphyrin metal ions with QTOF-MS shows the potential for compound-specific metal isotope analysis.

In Living Color: Pigment-Based Microbial Ecology At the Mineral-Air Interface

Pigment-based color is one of the most important phenotypic traits of biofilms at the mineral-air interface (subaerial biofilms, SABs), because it reflects the physiology of the microbial community. Because color is the hallmark of all SABs, we argue that pigment-based color could convey the mechanisms that drive microbial adaptation and coexistence across different terrestrial environments and link phenotypic traits to community fitness and ecological dynamics. Within this framework, we present the most relevant microbial pigments at the mineral-air interface and discuss some of the evolutionary landscapes that necessitate pigments as adaptive strategies for resource allocation and survivability. We report several pigment features that reflect SAB communities' structure and function, as well as pigment ecology in the context of microbial life-history strategies and coexistence theory. Finally, we conclude the study of pigment-based ecology by presenting its potential application and some of the key challenges in the research.

Chimeric inheritance and crown-group acquisitions of carbon fixation genes within Chlorobiales: Origins of autotrophy in Chlorobiales and implication for geological biomarkers

The geological record of microbial metabolisms and ecologies primarily consists of stable isotope fractionations and the diagenetic products of biogenic lipids. Carotenoid lipid biomarkers are particularly useful proxies for reconstructing this record, providing information on microbial phototroph primary productivity, redox couples, and oxygenation. The biomarkers okenane, chlorobactane, and isorenieratene are generally considered to be evidence of anoxygenic phototrophs, and provide a record that extends to 1.64 Ga. The utility of the carotenoid biomarker record may be enhanced by examining the carbon isotopic ratios in these products, which are diagnostic for specific pathways of biological carbon fixation found today within different microbial groups. However, this joint inference assumes that microbes have conserved these pathways across the duration of the preserved biomarker record. Testing this hypothesis, we performed phylogenetic analyses of the enzymes constituting the reductive tricarboxylic acid (rTCA) cycle in Chlorobiales, the group of anoxygenic phototrophic bacteria usually implicated in the deposition of chlorobactane and isorenieretane. We find phylogenetically incongruent patterns of inheritance across all enzymes, indicative of horizontal gene transfers to both stem and crown Chlorobiales from multiple potential donor lineages. This indicates that a complete rTCA cycle was independently acquired at least twice within Chlorobiales and was not present in the last common ancestor. When combined with recent molecular clock analyses, these results predict that the Mesoproterzoic lipid biomarker record diagnostic for Chlorobiales should not preserve isotopic fractionations indicative of a full rTCA cycle. Furthermore, we conclude that coupling isotopic and biomarker records is insufficient for reliably reconstructing microbial paleoecologies in the absence of a complementary and consistent phylogenomic narrative.

Phototrophy and carbon fixation in Chlorobi postdate the rise of oxygen

While most productivity on the surface of the Earth today is fueled by oxygenic photosynthesis, for much of Earth history it is thought that anoxygenic photosynthesis-using compounds like ferrous iron or sulfide as electron donors-drove most global carbon fixation. Anoxygenic photosynthesis is still performed by diverse bacteria in niche environments today. Of these, the Chlorobi (formerly green sulfur bacteria) are often interpreted as being particularly ancient and are frequently proposed to have fueled the biosphere during late Archean and early Paleoproterozoic time before the rise of oxygenic photosynthesis. Here, we perform comparative genomic, phylogenetic, and molecular clock analyses to determine the antiquity of the Chlorobi and their characteristic phenotypes. We show that contrary to common assumptions, the Chlorobi clade is relatively young, with anoxygenic phototrophy, carbon fixation via the rTCA pathway, and iron oxidation all significantly postdating the rise of oxygen ~2.3 billion years ago. The Chlorobi therefore could not have fueled the Archean biosphere, but instead represent a relatively young radiation of organisms which likely acquired the capacity for anoxygenic photosynthesis and other traits via horizontal gene transfer sometime after the evolution of oxygenic Cyanobacteria.

Lipid biomarkers: molecular tools for illuminating the history of microbial life

Fossilized lipids preserved in sedimentary rocks offer singular insights into the Earth's palaeobiology. These 'biomarkers' encode information pertaining to the oxygenation of the atmosphere and oceans, transitions in ocean plankton, the greening of continents, mass extinctions and climate change. Historically, biomarker interpretations relied on inventories of lipids present in extant microorganisms and counterparts in natural environments. However, progress has been impeded because only a small fraction of the Earth's microorganisms can be cultured, many environmentally significant microorganisms from the past no longer exist and there are gaping holes in knowledge concerning lipid biosynthesis. The revolution in genomics and bioinformatics has provided new tools to expand our understanding of lipid biomarkers, their biosynthetic pathways and distributions in nature. In this Review, we explore how preserved organic molecules provide a unique perspective on the history of the Earth's microbial life. We discuss how advances in molecular biology have helped elucidate biomarker origins and afforded more robust interpretations of fossil lipids and how the rock record provides vital calibration points for molecular clocks. Such studies are open to further exploitation with the expansion of sequenced microbial genomes in accessible databases.

Convergency and Stability Responses of Bacterial Communities to Salinization in Arid and Semiarid Areas: Implications for Global Climate Change in Lake Ecosystems

Climate change has given rise to salinization and nutrient enrichment in lake ecosystems of arid and semiarid areas, which have posed the bacterial communities not only into an ecotone in lake ecosystems but also into an assemblage of its own unique biomes. However, responses of bacterial communities to climate-related salinization and nutrient enrichment remain unclear. In September 2019, this study scrutinized the turnover of bacterial communities along gradients of increasing salinity and nutrient by a space-for-time substitution in Xinjiang Uyghur Autonomous Region, China. We find that salinization rather than nutrient enrichment primarily alters bacterial communities. The homogenous selection of salinization leads to convergent response of bacterial communities, which is revealed by the combination of a decreasing β-nearest taxon index (βNTI) and a pronounced negative correlation between niche breadth and salinity. Furthermore, interspecific interactions within bacterial communities significantly differed among distinct salinity levels. Specifically, mutualistic interactions showed an increase along the salinization. In contrast, topological parameters show hump-shaped curves (average degree and density) and sunken curves (modularity, density, and average path distance), the extremums of which all appear in the high-brackish environment, hinting that bacterial communities are comparatively stable at freshwater and brine environments but are unstable in moderately high-brackish lake.

Diversity and origin of carotenoid biosynthesis: its history of coevolution towards plant photosynthesis

The development of photosynthesis was a highlight in the progression of bacteria. In addition to the photosystems with their structural proteins, the photosynthesis apparatus consists of different cofactors including essential carotenoids. Thus, the evolution of the carotenoid pathways in relation to the functionality of the resulting structures in photosynthesis is the focus of this review. Analysis of carotenoid pathway genes indicates early evolutionary roots in prokaryotes. The pathway complexity leading to a multitude of structures is a result of gene acquisition, including their functional modifications, emergence of novel genes and gene exchange between species. Along with the progression of photosynthesis, carotenoid pathways coevolved with photosynthesis according to their advancing functionality. Cyanobacteria, with their oxygenic photosynthesis, became a landmark for evolutionary events including carotenogenesis. Concurrent with endosymbiosis, the cyanobacterial carotenoid pathways were inherited into algal plastids. In the lineage leading to Chlorophyta and plants, carotenoids evolved to their prominent role in protection and regulation of light energy input as constituents of a highly efficient light-harvesting complex.

The Archean origin of oxygenic photosynthesis and extant cyanobacterial lineages

The record of the coevolution of oxygenic phototrophs and the environment is preserved in three forms: genomes of modern organisms, diverse geochemical signals of surface oxidation and diagnostic Proterozoic microfossils. When calibrated by fossils, genomic data form the basis of molecular clock analyses. However, different interpretations of the geochemical record, fossil calibrations and evolutionary models produce a wide range of age estimates that are often conflicting. Here, we show that multiple interpretations of the cyanobacterial fossil record are consistent with an Archean origin of crown-group Cyanobacteria. We further show that incorporating relative dating information from horizontal gene transfers greatly improves the precision of these age estimates, by both providing a novel empirical criterion for selecting evolutionary models, and increasing the stringency of sampling of posterior age estimates. Independent of any geochemical evidence or hypotheses, these results support oxygenic photosynthesis evolving at least several hundred million years before the Great Oxygenation Event (GOE), a rapid diversification of major cyanobacterial lineages around the time of the GOE, and a post-Cryogenian origin of extant marine picocyanobacterial diversity.

Carotenoid biomarkers in Namibian shelf sediments: Anoxygenic photosynthesis during sulfide eruptions in the Benguela Upwelling System

Aromatic carotenoid-derived hydrocarbon biomarkers are ubiquitous in ancient sediments and oils and are typically attributed to anoxygenic phototrophic green sulfur bacteria (GSB) and purple sulfur bacteria (PSB). These biomarkers serve as proxies for the environmental growth requirements of PSB and GSB, namely euxinic waters extending into the photic zone. Until now, prevailing models for environments supporting anoxygenic phototrophs include microbial mats, restricted basins and fjords with deep chemoclines, and meromictic lakes with shallow chemoclines. However, carotenoids have been reported in ancient open marine settings for which there currently are no known modern analogs that host GSB and PSB. The Benguela Upwelling System offshore Namibia, known for exceptionally high primary productivity, is prone to recurrent toxic gas eruptions whereupon hydrogen sulfide emanates from sediments into the overlying water column. These events, visible in satellite imagery as water masses clouded with elemental sulfur, suggest that the Benguela Upwelling System may be capable of supporting GSB and PSB. Here, we compare distributions of biomarkers in the free and sulfur-bound organic matter of Namibian shelf sediments. Numerous compounds-including acyclic isoprenoids, steranes, triterpanes, and carotenoids-were released from the polar lipid fractions upon Raney nickel desulfurization. The prevalence of isorenieratane and β-isorenieratane in sampling stations along the shelf verified anoxygenic photosynthesis by low-light-adapted, brown-colored GSB in this open marine setting. Renierapurpurane was also present in the sulfur-bound carotenoids and was typically accompanied by lower abundances of renieratane and β-renierapurpurane, thereby identifying cyanobacteria as an additional aromatic carotenoid source.

A new constraint on the antiquity of ancient haloalkaliphilic green algae that flourished in a ca. 300 Ma Paleozoic lake

It is established that green algae and land plants progressively colonized freshwater and terrestrial habitats throughout the Paleozoic Era, but little is known about the ecology of Paleozoic saline lakes. Here, we report lipid biomarker and petrographic evidence for the occurrence of a green alga as a major primary producer in a late Paleozoic alkaline lake (Fengcheng Formation; 309-292 Ma). A persistently saline and alkaline lacustrine setting is supported by mineralogical and lipid biomarker evidence alongside extremely enriched δ¹⁵ N_bulk values (+16 to +24‰) for the lake depocenter. The prominence of C₂₈ and C₂₉ steroids, co-occurring with abundant carotene-derived accessory pigment markers in these ancient rocks, is suggestive of prolific primary production and elevated source inputs from haloalkaliphilic green algae. The high C₂₈ /C₂₉ -sterane ratios (0.78-1.29) are significantly higher than the typical marine value reported for late Paleozoic rocks (<0.5) and thus are associated with certain groups of chlorophytes. Adaptation to such extreme lacustrine environments, aided by enhanced biosynthesis of certain cell membrane lipids, likely played an important role in the evolution and physiological development of ancient green algae.

Molecular and isotopic evidence reveals the end-Triassic carbon isotope excursion is not from massive exogenous light carbon

The negative organic carbon isotope excursion (CIE) associated with the end-Triassic mass extinction (ETE) is conventionally interpreted as the result of a massive flux of isotopically light carbon from exogenous sources into the atmosphere (e.g., thermogenic methane and/or methane clathrate dissociation linked to the Central Atlantic Magmatic Province [CAMP]). Instead, we demonstrate that at its type locality in the Bristol Channel Basin (UK), the CIE was caused by a marine to nonmarine transition resulting from an abrupt relative sea level drop. Our biomarker and compound-specific carbon isotopic data show that the emergence of microbial mats, influenced by an influx of fresh to brackish water, provided isotopically light carbon to both organic and inorganic carbon pools in centimeter-scale water depths, leading to the negative CIE. Thus, the iconic CIE and the disappearance of marine biota at the type locality are the result of local environmental change and do not mark either the global extinction event or input of exogenous light carbon into the atmosphere. Instead, the main extinction phase occurs slightly later in marine strata, where it is coeval with terrestrial extinctions and ocean acidification driven by CAMP-induced increases in Pco₂; these effects should not be conflated with the CIE. An abrupt sea-level fall observed in the Central European basins reflects the tectonic consequences of the initial CAMP emplacement, with broad implications for all extinction events related to large igneous provinces.

Biomarker stratigraphy in the Athel Trough of the South Oman Salt Basin at the Ediacaran-Cambrian Boundary

The South Oman Salt Basin (SOSB) has been studied extensively for knowledge concerning the habitat of the enigmatic Ediacaran-Cambrian oils that are produced from that region. Geological, geochemical, geophysical, and geochronological investigations have all contributed to improved understanding of the range of late Neoproterozoic depositional environments recorded there. Of particular interest has been the deep Athel depocenter within the SOSB that features a silica-rich interval known as the Al Shomou Member or Athel Silicilyte and the co-eval A4 carbonate-evaporite sequence that straddles the Ediacaran-Cambrian boundary. The deep basin has been suggested to be anoxic and euxinic based on studies of sulfur isotopes, trace metal distributions and other proxies. Organic geochemistry has provided some clues concerning aspects of the depositional environments and microbial communities prevailing during this interval. However, ambiguities remain including a paucity of convincing molecular evidence for euxinia in the photic zone of the basin. Here, we present a comprehensive study of biomarker hydrocarbons, including steroids, triterpenoids, and carotenoids. Among the compounds detected is a distinctive array of aromatic carotenoids. Relatively low abundances of monoaromatic carotenoids, such as chlorobactane, okenane, and β-isorenieratane, suggest the possibility of transient photic zone euxinia with a shallow chemocline or, perhaps, exogenous inputs from microbial mats. However, it is the dominance of renieratane and renierapurpurane over isorenieratane in diaromatic carotenoids and their association with abundant C₃₈ and C₃₉ carotenoids that identifies cyanobacteria as major contributors to the inventory of carotenoids. Our results, based on multiple lines of molecular evidence and statistical analysis, also suggest that the Athel Silicilyte was biogeochemically distinct from the other units of the Ara Group. Overall, our study has important implications for understanding other late Neoproterozoic depositional environments.