Isorenieratene /ˌaɪsoʊrəˈnɪərətiːn/ is a

carotenoid Carotenoids () are yellow, orange, and red organic pigments that are produced by plants and algae, as well as several bacteria, archaea, and fungi. Carotenoids give the characteristic color to pumpkins, carrots, parsnips, corn, tomatoes, cana ...

light-harvesting pigment A photosynthetic pigment (accessory pigment; chloroplast pigment; antenna pigment) is a pigment that is present in chloroplasts or photosynthetic bacteria and captures the light energy necessary for photosynthesis. List of photosynthetic pigmen ...

produced exclusively by the genus ''

Chlorobium ''Chlorobium'' is a genus of green sulfur bacteria. They are photolithotrophic oxidizers of sulfur and most notably utilise a noncyclic electron transport chain to reduce NAD+. Photosynthesis is achieved using a Type 1 Reaction Centre using b ...

'', which are the brown-colored strains of the family of

green sulfur bacteria The green sulfur bacteria are a phylum, Chlorobiota, of obligately anaerobic photoautotrophic bacteria that metabolize sulfur. Green sulfur bacteria are nonmotile (except ''Chloroherpeton thalassium'', which may glide) and capable of anoxyg ...

( Chlorobiaceae). Green sulfur bacteria are

anaerobic Anaerobic means "living, active, occurring, or existing in the absence of free oxygen", as opposed to aerobic which means "living, active, or occurring only in the presence of oxygen." Anaerobic may also refer to: *Adhesive#Anaerobic, Anaerobic ad ...

photoautotrophic Photoautotrophs are organisms that can utilize light energy from sunlight, and elements (such as carbon) from inorganic compounds, to produce organic materials needed to sustain their own metabolism (i.e. autotrophy). Such biological activities ...

organisms, meaning they perform

photosynthesis Photosynthesis ( ) is a system of biological processes by which photosynthetic organisms, such as most plants, algae, and cyanobacteria, convert light energy, typically from sunlight, into the chemical energy necessary to fuel their metabo ...

in the absence of oxygen using

hydrogen sulfide Hydrogen sulfide is a chemical compound with the formula . It is a colorless chalcogen-hydride gas, and is toxic, corrosive, and flammable. Trace amounts in ambient atmosphere have a characteristic foul odor of rotten eggs. Swedish chemist ...

in the following reaction:

organic compounds Some chemical authorities define an organic compound as a chemical compound that contains a carbon–hydrogen or carbon–carbon bond; others consider an organic compound to be any chemical compound that contains carbon. For example, carbon-co ...

Such

anoxygenic photosynthesis Anoxygenic photosynthesis is a special form of photosynthesis used by some bacteria and archaea, which differs from the better known oxygenic photosynthesis in plants in the reductant used (e.g. hydrogen sulfide instead of water) and the byproduc ...

requires reduced

sulfur Sulfur ( American spelling and the preferred IUPAC name) or sulphur ( Commonwealth spelling) is a chemical element; it has symbol S and atomic number 16. It is abundant, multivalent and nonmetallic. Under normal conditions, sulfur atoms ...

and light; thus, this

metabolism Metabolism (, from ''metabolē'', "change") is the set of life-sustaining chemical reactions in organisms. The three main functions of metabolism are: the conversion of the energy in food to energy available to run cellular processes; the co ...

occurs only in strictly

photic The photic zone (or euphotic zone, epipelagic zone, or sunlight zone) is the uppermost layer of a body of water that receives sunlight, allowing phytoplankton to perform photosynthesis. It undergoes a series of physical, chemical, and biological ...

and

euxinic Euxinia or euxinic conditions occur when water is both wikt:anoxic, anoxic and wikt:sulfidic, sulfidic. This means that there is no oxygen (O2) and a raised level of free hydrogen sulfide (H2S). Euxinic bodies of water are frequently strongly strat ...

environments. Therefore, the discovery of isorenieratene and its derivatives in sediments and rocks are helpful

biomarkers In biomedical contexts, a biomarker, or biological marker, is a measurable indicator of some biological state or condition. Biomarkers are often measured and evaluated using blood, urine, or soft tissues to examine normal biological processes, p ...

to identify euxinic water columns in the

photic zone The photic zone (or euphotic zone, epipelagic zone, or sunlight zone) is the uppermost layer of a body of water that receives sunlight, allowing phytoplankton to perform photosynthesis. It undergoes a series of physical, chemical, and biological ...

Structure

Isorenieratene has the

chemical formula A chemical formula is a way of presenting information about the chemical proportions of atoms that constitute a particular chemical compound or molecule, using chemical element symbols, numbers, and sometimes also other symbols, such as pare ...

C₄₀H₄₈. It is a diaromatic

with a regularly-linked

isoprenoid The terpenoids, also known as isoprenoids, are a class of naturally occurring organic chemicals derived from the 5-carbon compound isoprene and its derivatives called terpenes, diterpenes, etc. While sometimes used interchangeably with "terpene ...

chain, except for a single tail-to-tail linkage in the middle of the molecule. Isorenieratene has a characteristic 1-alkyl-2,3,6-trimethyl substitution pattern on the aromatic rings, which helps identify the molecule. The nine conjugated double bonds on the isoprenoid backbone are all in the trans configuration and make the molecule highly reactive with reduced inorganic sulfur species. The molecule is

hydrophobic In chemistry, hydrophobicity is the chemical property of a molecule (called a hydrophobe) that is seemingly repelled from a mass of water. In contrast, hydrophiles are attracted to water. Hydrophobic molecules tend to be nonpolar and, thu ...

and insoluble in water, like most other carotenoids. Isorenieratene is generally non-toxic.

Biological Sources

Isorenieratene was first discovered when isolated from the orange-colored

sponge Sponges or sea sponges are primarily marine invertebrates of the animal phylum Porifera (; meaning 'pore bearer'), a basal clade and a sister taxon of the diploblasts. They are sessile filter feeders that are bound to the seabed, and a ...

''Reniera japonica.'' Marine sponges are brilliantly colored due to the occurrence of several carotenoids and their association with symbionts such as bacteria or

algae Algae ( , ; : alga ) is an informal term for any organisms of a large and diverse group of photosynthesis, photosynthetic organisms that are not plants, and includes species from multiple distinct clades. Such organisms range from unicellular ...

. Therefore, isorenieratene in sponges is assumed to originate from the

symbiosis Symbiosis (Ancient Greek : living with, companionship < : together; and ''bíōsis'': living) is any type of a close and long-term biological interaction, between two organisms of different species. The two organisms, termed symbionts, can fo ...

between sponges and

(Chlorobiaceae). Green sulfur bacteria live in euxinic environments, often at the

chemocline A chemocline is a type of cline, a layer of fluid with different properties, characterized by a strong, vertical chemistry gradient within a body of water. In bodies of water where chemoclines occur, the cline separates the upper and lower layers, ...

, where the light flux is present but low. To increase their metabolic efficiency, they have developed a

chlorosome A chlorosome is a photosynthetic antenna complex found in green sulfur bacteria (GSB) and many green non-sulfur bacteria (GNsB), together known as green bacteria. They differ from other antenna complexes by their large size and lack of protei ...

, a membrane-bound antenna with

bacteriochlorophyll Bacteriochlorophylls (BChl) are photosynthetic pigments that occur in various phototrophic bacteria. They were discovered by C. B. van Niel in 1932. They are related to chlorophylls, which are the primary pigments in plants, algae, and cyanobacte ...

''c, d,'' or ''e.'' The brown-colored strain of Chlorobiaceae has bacteriochlorophyll ''e'' in its chlorosome, which primarily makes isorenieratene. It is speculated that isorenieratene and other related carotenoids are adaptations that help organisms live under low-light conditions. Green sulfur bacteria fix carbon through the

reverse tricarboxylic acid cycle The reverse Krebs cycle (also known as the reverse tricarboxylic acid cycle, the reverse TCA cycle, or the reverse citric acid cycle, or the reductive tricarboxylic acid cycle, or the reductive TCA cycle) is a sequence of chemical reactions that ...

(TCA), resulting in the produced

biomass Biomass is a term used in several contexts: in the context of ecology it means living organisms, and in the context of bioenergy it means matter from recently living (but now dead) organisms. In the latter context, there are variations in how ...

, including isorenieratene, being anomalously enriched in

carbon-13 Carbon-13 (13C) is a natural, stable isotope of carbon with a nucleus containing six protons and seven neutrons. As one of the environmental isotopes, it makes up about 1.1% of all natural carbon on Earth. Detection by mass spectrometry A m ...

(¹³C) compared to other algal biomass by about 15 per mil. δ¹³C of green sulfur bacteria biomass ranges between –9 and –21 per mil. Isorenieratene is relatively uncommon but of great significance when encountered. It is a powerful proxy for euxinic conditions in the photic zone both today and in the

geologic record The geologic record in stratigraphy, paleontology and other natural sciences refers to the entirety of the layers of rock strata. That is, deposits laid down by volcanism or by deposition of sediment derived from weathering detritus (clays, sa ...

Environmental distribution

The combination of conditions in which green sulfur bacteria live and, thereby, where isorenieratene is found are limited today. Most of these locations are restricted water basins with highly stratified waters, allowing for anoxia development in the lower layers and H₂S accumulation. The

Black Sea The Black Sea is a marginal sea, marginal Mediterranean sea (oceanography), mediterranean sea lying between Europe and Asia, east of the Balkans, south of the East European Plain, west of the Caucasus, and north of Anatolia. It is bound ...

is one such water basin where the hydrogen sulfide interface, or the

, has moved up in the photic zone, and high concentrations of green sulfur bacteria and isorenieratene are found. Other modern-day environments include

meromictic lakes A meromictic lake is a lake which has layers of water that do not intermix. In ordinary, holomictic lakes, at least once each year, there is a physical mixing of the surface and the deep waters. The term ''meromictic'' was coined by the Austria ...

, restricted

fjords In physical geography, a fjord (also spelled fiord in New Zealand English; ) is a long, narrow sea inlet with steep sides or cliffs, created by a glacier. Fjords exist on the coasts of Antarctica, the Arctic, and surrounding landmasses of the no ...

, and some marine settings. Green sulfur bacteria are found to play a role in coral ecosystems and have been documented to live on coral and sponges as possible symbionts. Several cases have been found where green sulfur bacteria with bacteriochlorophyll ''e'' are abundant, but no isorenieratene was documented. Green sulfur bacteria were found to live near a deep-sea

hydrothermal vent Hydrothermal vents are fissures on the seabed from which geothermally heated water discharges. They are commonly found near volcanically active places, areas where tectonic plates are moving apart at mid-ocean ridges, ocean basins, and hot ...

off the coast of

Mexico Mexico, officially the United Mexican States, is a country in North America. It is the northernmost country in Latin America, and borders the United States to the north, and Guatemala and Belize to the southeast; while having maritime boundar ...

; however, the bacteria are no longer doing photosynthesis at this depth, and no isorenieratene was isolated. In Fayetteville Green Lake (New York), green sulfur bacteria and bacteriochlorophyll ''e'' were abundant below the chemocline, yet the sediments lacked isorenieratene. These unexpected absences of isorenieratene call for continued exploration of the microbial ecology of biomarker production in modern environments.

Preservation and measurement

Isorenieratene is generally poorly-preserved because its structure is susceptible to alteration and degradation. Upon

diagenesis Diagenesis () is the process of physical and chemical changes in sediments first caused by water-rock interactions, microbial activity, and compaction after their deposition. Increased pressure and temperature only start to play a role as sedi ...

and catagenesis, isorenieratene may be transformed and produce various related products that still indicate photic zone euxinia in the depositional environment. The two main transformation processes are the saturation of double bonds to form isorenieratane and the rupture of the carbon chain resulting in smaller molecular fragments. Other alterations include sulphurization,

cyclization A cyclic compound (or ring compound) is a term for a compound in the field of chemistry in which one or more series of atoms in the compound is connected to form a ring. Rings may vary in size from three to many atoms, and include examples where ...

, and

aromatization Aromatization is a chemical reaction in which an aromaticity, aromatic system is formed from a single nonaromatic precursor. Typically aromatization is achieved by dehydrogenation of existing cyclic compounds, illustrated by the conversion of cycl ...

Use as a biomarker

While euxinic conditions are rare today, In the early history of the Earth, these conditions were thought to be present in all

oceans The ocean is the body of salt water that covers approximately 70.8% of Earth. The ocean is conventionally divided into large bodies of water, which are also referred to as ''oceans'' (the Pacific, Atlantic, Indian, Antarctic/Southern, and ...

at depths of about 100 m (330 ft). The detection of isorenieratene and green sulfur bacteria in the mid-Proterozoic has been used as evidence for the long-term euxinic conditions remaining in oceans after the

Great Oxygenation Event The Great Oxidation Event (GOE) or Great Oxygenation Event, also called the Oxygen Catastrophe, Oxygen Revolution, Oxygen Crisis or Oxygen Holocaust, was a time interval during the Earth's Paleoproterozoic era when the Earth's atmosphere and ...

. For example, the 1.64-Gyr-old Barney Creek Formation in northern

Australia Australia, officially the Commonwealth of Australia, is a country comprising mainland Australia, the mainland of the Australia (continent), Australian continent, the island of Tasmania and list of islands of Australia, numerous smaller isl ...

hosts many biomarkers, including isorenieratene, that signify that these rocks were deposited in a marine basin with anoxic, sulphidic, and highly-stratified deep waters with colonies of green and

purple sulfur bacteria The purple sulfur bacteria (PSB) are part of a group of Pseudomonadota capable of photosynthesis, collectively referred to as purple bacteria. They are anaerobic or microaerophilic, and are often found in stratified water environments includi ...

. Isorenieratene derivatives have been identified in sedimentary rocks throughout the Paleozoic and Mesozoic, signifying that anoxygenic photosynthesis was a more common process in the past. Isorenieratene derivatives have also been isolated from many

petroleum Petroleum, also known as crude oil or simply oil, is a naturally occurring, yellowish-black liquid chemical mixture found in geological formations, consisting mainly of hydrocarbons. The term ''petroleum'' refers both to naturally occurring un ...

source rocks, suggesting euxinic conditions and anoxia are favorable for preserving organic matter, leading to forming of petroleum reservoirs. Additionally, the detection of isorenieratene derivatives during

mass extinctions An extinction event (also known as a mass extinction or biotic crisis) is a widespread and rapid decrease in the biodiversity on Earth. Such an event is identified by a sharp fall in the diversity and abundance of multicellular organisms. It occ ...

signifies that euxinic conditions may be common at such events. For example, the isolation of isorenieratene from rock units deposited during the Permian/Triassic Mass extinction, the deadliest mass extinction on Earth, was used as evidence for several pulses of widespread photic zone euxinia leading up to and during the extinction event.

References