24-Isopropylcholestane

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24-isopropyl cholestane is an organic molecule produced by specific sponges, protists [1] and marine algae. [2] The identification of this molecule at high abundances in Neoproterozoic rocks has been interpreted to reflect the presence of multicellular life prior to the rapid diversification and radiation of life during the Cambrian explosion. [3] [4] In this transitional period at the start of the Phanerozoic, single-celled organisms evolved to produce many of the evolutionary lineages present on Earth today. [5] Interpreting 24-isopropyl cholestane in ancient rocks as indicating the presence of sponges before this rapid diversification event alters the traditional understanding of the evolution of multicellular life and the coupling of biology to changes in end-Neoproterozoic climate. However, there are several arguments against causally linking 24-isopropyl cholestane to sponges based on considerations of marine algae and the potential alteration of organic molecules over geologic time. [6] In particular the discovery of 24-isopropyl cholestane in rhizarian protists implies that this biomarker cannot be used on its own to trace sponges. [1] Interpreting the presence of 24-isopropyl cholestane in the context of changingglobal biogeochemical cycles at the Proterozoic-Phanerozoic transition remains an area of active research.

Contents

24-isopropyl cholestane

Chemical argument for Precambrian sponges

Figure 1: 24-isopropyl cholestane (left) and 24-n-propyl cholestane (right), two organic molecules produced by sponges and marine algae relevant for studying the evolution of multicellular life in the Precambrian and Phanerozoic. 24Iso2.svg
Figure 1: 24-isopropyl cholestane (left) and 24-n-propyl cholestane (right), two organic molecules produced by sponges and marine algae relevant for studying the evolution of multicellular life in the Precambrian and Phanerozoic.

24-isopropyl cholestane (figure 1, left) is a C30 sterane with chemical formula C30H54 and molecular mass 414.76 g/mol. The molecule has a cholestane skeleton with an isopropyl moiety at C24 and is the geologically stable form of 24-isopropyl cholesterol. [3] A related and important molecule is 24-n-propyl cholestane (figure 1, right), also with the cholestane skeleton, but with an n-propyl moiety at C24.

24-isopropyl cholestane is produced copiously by a particular group of sponges in the class Demospongiae within the phylum Porifera. [2] [7] Like other molecular fossils, the presence of 24-isopropyl cholestane in rocks may indicate whether demosponge were living in or near the rock's depositional environment. High abundances of 24-isopropyl cholestane are identified in the Precambrian rocks from the Hufq supergroup in Oman, suggesting the presence of sponges prior to the Cambrian explosion. [3] However, sponges are not the only organisms that produce 24-isopropyl cholestane, so the identification of this biomarker is not uniquely linked to the presence of demosponge.

While marine pelagophyte algae predominantly produce 24-n-propylcholestane, [8] they also produce 24-isopropyl cholestane. The two possible sources of 24-isopropyl cholestane to rocks, the demosponge and the algae, can be decoupled by considering the ratio of 24-isopropyl cholestane to 24-n-propyl cholestane. In many rocks, this ratio is 0.2-0.3. [3] However, in rocks from Oman, the ratio of steranes is 0.52-16.1, with an average value of 1.51, which strongly suggests input of sponge organic matter. [3] Notably, these elevated values disappear during the Cambrian, and the ratio of 24-isopropyl cholestane to 24-n-propyl cholestane is used an age-specific proxy for the Proterozoic-Phanerozoic transition. [9]

Recent research in molecular clocks has argued that the ability to produce 24-isopropyl cholesterol evolved independently in both the demosponge and algae. [4] However, it appears that the biosynthesis evolved earlier in the sponges, during the Neoproterozoic, and that the ability to perform the biosynthesis was not present in algae until the Phanerozoic. If correct, these results would give scientists much more confidence in interpreting elevated levels of 24-isopropyl cholestane in ancient rocks as reflecting the presence of sponges.

Observations of Precambrian sponges

Additional evidence for sponge evolution before the Cambrian explosion is found in bioclastic packstones from South Australia. [10] Through repeated grinding and photography, researchers constructed 3D models of asymmetric structures with ~1 mm-diameter interconnected channels contained within this rock. The complex network of tunnels appears inconsistent with fungi or algae, and the researchers tentatively suggested that they are primitive sponges. This interpretation is controversial because the structures pre-date the first appearance of other sponge fossils and the structures are only known to occur within a single sedimentary sequence.

Implications

While Love et al. (2009) argues for the presence of sponges in rocks below the Marinoan cap carbonate at ~635 Ma (millions of years ago), [3] Antcliffe (2013) estimates the age of the biomarker-bearing rock to be between 645 Ma and ~580 Ma. [6] Most recently, Gold et al. (2016) writes that the age of rocks containing 24-isoproylcholestane have an age between ~650 Ma and 540 Ma. [4] In all cases, estimates agree that the age of the rocks containing 24-isoproylcholestane pre-date the Cambrian explosion at ~541 Ma.

The presence of sponges before ~540 Ma has profound implications for the evolution of multicellular life and the coupling of the biosphere to Neoproterozoic climate. Climate change before the Cambrian explosion and the subsequent diversification of life are intricately intertwined with understanding the causes of Snowball Earth episodes, [11] the deposition of Banded Iron Formations, [12] and the second step in the rise of atmospheric oxygen. [13] In particular, the presence of sponges raises questions of the minimum dissolved O2 content of the oceans in the late Neoproterozoic and the transition from a euxinic Canfield ocean to the modern oxygenated deep-ocean. However, sponges appear to require very little O2 to survive, so their presence in the Precambrian may not provide strong constraints on Proterozoic O2 levels. [14]

Caveats

There are several lines of logic against interpreting 24-isopropyl cholestane as a biomarker for demosponge: [6]

  1. Much of the argument for Precambrian sponges is grounded in the observation that pelagophyte algae produce organic matter with a low ratio of 24-isopropyl cholestane to 24-n-propyl cholestane, but that this ratio is high in ancient rocks. However, the observed change in the sterane ratio could also be explained if algae changed the relative abundances in which they produce steranes over the past 600 million years. In a similar line of argument, it is possible that another extinct organism from which the algae descended produced organic matter with a higher ratio of 24-isopropyl cholestane to 24-n-propyl cholestane. As argued above, recent evidence has suggested that the algae's synthesis pathway only arose during the Phanerozoic, [4] which tempers this argument. More generally, these concerns address the issue of insufficient specificity in molecular fossils, which plagues many biomarker studies.
  2. 24-isopropyl cholestane can be formed through sedimentary diagenesis of other organic molecules, so the high ratio of 24-isopropyl cholestane to 24-n-propyl cholestane could simply reflect the post-depositional transformation of organic matter. Some research has addressed this concern by showing a lack of alteration in other organic molecules, [3] such as hopanes, but subsequent analysis has questioned whether the molecules with minimal alteration could have been contamination from modern petroleum-derived oil. [6]
  3. There may have been a group of bacteria living symbiotically with the sponges that also produced 24-isopropyl cholestane. [15] If these bacteria produced the biomarker throughout geologic time, its presence would not be strictly indicative of demosponge. However, as with the marine algae, analysis of the ratio of 24-isopropyl cholestane to 24-n-propyl cholestane may clarify the source of the compounds.
  4. It is strange to find sponge biomarkers before the Cambrian explosion without accompanying sponge fossils (although there is tentative evidence for sponge-like structures in the latest-Neoproterozoic [10] ). Moreover, once fossils of sponges do appear during the Paleozoic, the ratio of 24-isopropyl cholestane to 24-n-propylcholestane returns to its background value. This is surprising because we might expect the ratio to stay elevated or even to increase as metazoa diversified and sponges proliferated.

Related Research Articles

<span class="mw-page-title-main">Cambrian</span> First period of the Paleozoic Era, 539–485 million years ago

The Cambrian Period is the first geological period of the Paleozoic Era, and of the Phanerozoic Eon. The Cambrian lasted 53.4 million years from the end of the preceding Ediacaran Period 538.8 million years ago (mya) to the beginning of the Ordovician Period 485.4 mya. Its subdivisions, and its base, are somewhat in flux.

<span class="mw-page-title-main">Ediacaran</span> Third and last period of the Neoproterozoic Era

The Ediacaran period is a geological period of the Neoproterozoic era that spans 96 million years from the end of the Cryogenian period at 635 Mya, to the beginning of the Cambrian period at 538.8 Mya. It is the last period of the Proterozoic eon as well as the last of the so-called "Precambrian supereon", before the beginning of the subsequent Cambrian period marks the start of the Phanerozoic eon where recognizable fossil evidence of life becomes common.

The Precambrian is the earliest part of Earth's history, set before the current Phanerozoic Eon. The Precambrian is so named because it preceded the Cambrian, the first period of the Phanerozoic Eon, which is named after Cambria, the Latinised name for Wales, where rocks from this age were first studied. The Precambrian accounts for 88% of the Earth's geologic time.

<span class="mw-page-title-main">Sponge</span> Animals of the phylum Porifera

Sponges, the members of the phylum Porifera, are a basal animal clade as a sister of the diploblasts. They are multicellular organisms that have bodies full of pores and channels allowing water to circulate through them, consisting of jelly-like mesohyl sandwiched between two thin layers of cells.

<span class="mw-page-title-main">Proterozoic</span> Geologic eon, 2500–539 million years ago

The Proterozoic is the third of the four geologic eons of Earth's history, spanning the time interval from 2500 to 538.8 Mya, the longest eon of the Earth's geologic time scale. It is preceded by the Archean and followed by the Phanerozoic, and is the most recent part of the Precambrian "supereon".

<span class="mw-page-title-main">Acritarch</span> Microfossils

Acritarchs are organic microfossils, known from approximately 1800 million years ago to the present. The classification is a catch all term used to refer to any organic microfossils that cannot be assigned to other groups. Their diversity reflects major ecological events such as the appearance of predation and the Cambrian explosion.

<span class="mw-page-title-main">Doushantuo Formation</span>

The Doushantuo Formation is a geological formation in western Hubei, eastern Guizhou, southern Shaanxi, central Jiangxi, and other localities in China. It is known for the fossil Lagerstätten in Zigui in Hubei, Xiuning in Anhui, and Weng'an in Guizhou, as one of the oldest beds to contain minutely preserved microfossils, phosphatic fossils that are so characteristic they have given their name to "Doushantuo type preservation". The formation, whose deposits date back to the Early and Middle Ediacaran, is of particular interest because it covers the poorly understood interval of time between the end of the Cryogenian geological period and the more familiar fauna of the Late Ediacaran Avalon explosion, as well as due to its microfossils' potential utility as biostratigraphical markers. Taken as a whole, the Doushantuo Formation ranges from about 635 Ma at its base to about 551 Ma at its top, with the most fossiliferous layer predating by perhaps five Ma the earliest of the 'classical' Ediacaran faunas from Mistaken Point on the Avalon Peninsula of Newfoundland, and recording conditions up to a good forty to fifty million years before the Cambrian explosion at the beginning of the Phanerozoic.

<span class="mw-page-title-main">Microfossil</span> Fossil that requires the use of a microscope to see it

A microfossil is a fossil that is generally between 0.001 mm and 1 mm in size, the visual study of which requires the use of light or electron microscopy. A fossil which can be studied with the naked eye or low-powered magnification, such as a hand lens, is referred to as a macrofossil.

<span class="mw-page-title-main">Demosponge</span> Class of sponges

Demosponges (Demospongiae) are the most diverse class in the phylum Porifera. They include greater than 90% of all species of sponges with nearly 8,800 species worldwide. They are sponges with a soft body that covers a hard, often massive skeleton made of calcium carbonate, either aragonite or calcite. They are predominantly leuconoid in structure. Their "skeletons" are made of spicules consisting of fibers of the protein spongin, the mineral silica, or both. Where spicules of silica are present, they have a different shape from those in the otherwise similar glass sponges. Some species, in particular from the Antarctic, obtain the silica for spicule building from the ingestion of siliceous diatoms.

<span class="mw-page-title-main">Sterane</span> Class of tetracyclic compounds derived from steroids

Steranes constitute a class of tetracyclic triterpanes derived from steroids or sterols via diagenetic and catagenetic degradation, such as hydrogenation. They are found in sediments and sedimentary rocks in nature. Steranes are derivatives of gonane, the steroid nucleus which is also called "cyclopentanoperhydrophenanthrene". They have an androstane skeleton with a side chain at carbon C-17. The sterane structure constitutes the core of all sterols. Steranes are widely used as biomarkers for the presence of eukaryotes in past ecosystems because steroids are nearly exclusively produced by eukaryotes. In particular, cholestanes are diagenetic products of cholesterol in animals, while stigmastanes are diagenetic products of stigmasterols in algae and land plants. However, some bacteria are now known to produce sterols and it is inferred that the ultimate origin of sterol biosynthesis is in bacteria. Sterols are produced via protosterols that are direct cyclization compounds of squalene by the catalysis of oxidosqualene cyclase. All known sterols in eukaryotes are enzymatically extensively modified from protosterols, while organisms that only produce protosterols are not known. The oldest record of modified steranes are in sedimentary rocks deposited ca. 720–820 million years ago. In contrast, diagenetic products of protosterols are widely distributed in older Proterozoic rocks and imply the presence of extinct proto-eukaryotes and/or sterol-producing bacteria before the evolution of crown-group eukaryotes.

<span class="mw-page-title-main">Cholestane</span> Chemical compound

Cholestane is a saturated tetracyclic triterpene. This 27-carbon biomarker is produced by diagenesis of cholesterol and is one of the most abundant biomarkers in the rock record. Presence of cholestane, its derivatives and related chemical compounds in environmental samples is commonly interpreted as an indicator of animal life and/or traces of O2, as animals are known for exclusively producing cholesterol, and thus has been used to draw evolutionary relationships between ancient organisms of unknown phylogenetic origin and modern metazoan taxa. Cholesterol is made in low abundance by other organisms (e.g., rhodophytes, land plants), but because these other organisms produce a variety of sterols it cannot be used as a conclusive indicator of any one taxon. It is often found in analysis of organic compounds in petroleum.

Phytane is the isoprenoid alkane formed when phytol, a chemical substituent of chlorophyll, loses its hydroxyl group. When phytol loses one carbon atom, it yields pristane. Other sources of phytane and pristane have also been proposed than phytol.

<span class="mw-page-title-main">Ediacaran biota</span> All organisms of the Ediacaran Period (c. 635–538.8 million years ago)

The Ediacaranbiota is a taxonomic period classification that consists of all life forms that were present on Earth during the Ediacaran Period. These were enigmatic tubular and frond-shaped, mostly sessile, organisms. Trace fossils of these organisms have been found worldwide, and represent the earliest known complex multicellular organisms. The term "Ediacara biota" has received criticism from some scientists due to its alleged inconsistency, arbitrary exclusion of certain fossils, and inability to be precisely defined.

The small shelly fauna, small shelly fossils (SSF), or early skeletal fossils (ESF) are mineralized fossils, many only a few millimetres long, with a nearly continuous record from the latest stages of the Ediacaran to the end of the Early Cambrian Period. They are very diverse, and there is no formal definition of "small shelly fauna" or "small shelly fossils". Almost all are from earlier rocks than more familiar fossils such as trilobites. Since most SSFs were preserved by being covered quickly with phosphate and this method of preservation is mainly limited to the late Ediacaran and early Cambrian periods, the animals that made them may actually have arisen earlier and persisted after this time span.

The Cambrian explosion, Cambrian radiation,Cambrian diversification, or the Biological Big Bang refers to an interval of time approximately 538.8 million years ago in the Cambrian Period of early Paleozoic when there was a sudden radiation of complex life and practically all major animal phyla started appearing in the fossil record. It lasted for about 13 – 25 million years and resulted in the divergence of most modern metazoan phyla. The event was accompanied by major diversification in other groups of organisms as well.

<span class="mw-page-title-main">Sponge spicule</span> Structural element of sea sponges

Spicules are structural elements found in most sponges. The meshing of many spicules serves as the sponge's skeleton and thus it provides structural support and potentially defense against predators.

<span class="mw-page-title-main">Ergostane</span> Chemical compound

Ergostane is a tetracyclic triterpene, also known as 24S-methylcholestane. The compound itself has no known uses; however various functionalized analogues are produced by plants and animals. The most important of these are the heavily derivatised withanolides. However simpler forms do exist, such as the sterane campestane (24R-methylcholestane). Along with cholestane and stigmastane, this sterane is used as a biomarker for early eukaryotes.

24-<i>n</i>-Propylcholestane Chemical compound

24-n-Propylcholestane is a sterane biomarker molecule often found in marine source rocks. It is a C30 molecule, meaning that it is composed of thirty carbon atoms, and is one of the leading ways to distinguish a marine source rock from a terrigenous sample. It is composed of three six-carbon rings and one five-carbon ring, with two methyl groups and one eleven carbon side chain. 24-n-Propylcholestane has a molar mass of 414.76 g/mol.

<span class="mw-page-title-main">24-Norcholestane</span> Chemical compound

24-Norcholestane, a steroid derivative, is used as a biomarker to constrain the source age of sediments and petroleum through the ratio between 24-norcholestane and 27-norcholestane, especially when used with other age diagnostic biomarkers, like oleanane. While the origins of this compound are still unknown, it is thought that they are derived from diatoms due to their identification in diatom rich sediments and environments. In addition, it was found that 24-norcholestane levels increased in correlation with diatom evolution. Another possible source of 24-norcholestane is from dinoflagellates, albeit to a much lower extent.

<span class="mw-page-title-main">Stigmastane</span> Chemical compound

Stigmastane or 24R-ethylcholestane is a tetracyclic triterpene, along with cholestane and ergostane, this sterane is used as a biomarker for early eukaryotes.

References

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