Lycopane (C
40H
82; 2,6,10,14,19,23,27,31-octamethyldotriacontane), a 40 carbon
alkane
In organic chemistry, an alkane, or paraffin (a historical trivial name that also has other meanings), is an acyclic saturated hydrocarbon. In other words, an alkane consists of hydrogen and carbon atoms arranged in a tree structure in which ...
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 "terpenes", ...
, is a widely present
biomarker that is often found in anoxic settings. It has been identified in anoxically deposited
lacustrine sediments (such as the
Messel formation
The Messel Formation is a geologic formation in Hesse, central Germany, dating back to the Eocene epoch (about 47 MaJens Lorenz Franzen (2005). "The implications of the numerical dating
of the Messel fossil deposit (Eocene, Germany) for mamma ...
and the Condor oil shale deposit). It has been found in sulfidic and anoxic hypersaline environments (such as the Sdom Formation). It has been widely identified in modern marine sediments, including the Peru upwelling zone, the Black Sea, and the Cariaco Trench. It has been found only rarely in crude oils.
Biological origins
The pathway for production of lycopane has not been conclusively identified. There are several theories for its origins/production.
Methanogenic archaea
Some of the earliest theories for the biosynthesis of lycopane center around it being anaerobically produced by
methanogenic archaea. Lycopane has been observed in recent
marine sediment
Marine sediment, or ocean sediment, or seafloor sediment, are deposits of insoluble particles that have accumulated on the seafloor. These particles have their origins in soil and rocks and have been transported from the land to the sea, mainly ...
s in contexts where methanogenic activity is occurring. In older sediments, methanogenic activity is harder to conclusively determine, as methane can migrate from other layers and not necessarily be a product of that geological time. It is possible that isoprenoid alkanes such as lycopane serve as biomarkers for
methanogenesis
Methanogenesis or biomethanation is the formation of methane coupled to energy conservation by microbes known as methanogens. Organisms capable of producing methane for energy conservation have been identified only from the domain Archaea, a group ...
and methanogenic archaea.
Lycopane has not yet been directly isolated in any biological organism, so its linkage to methanogenic archaea is conjecture. However, the process has been identified in a different isoprenoid alkane: squalane.
Squalane
Squalane is the organic compound with the formula . A colorless hydrocarbon, it is the hydrogenated derivative of squalene, although commercial samples are derived from nature. In contrast to squalene, due to the complete saturation of squalane ...
was not initially thought to be directly biologically synthesized, but was later determined to be present in
archaea
Archaea ( ; singular archaeon ) is a domain of single-celled organisms. These microorganisms lack cell nuclei and are therefore prokaryotes. Archaea were initially classified as bacteria, receiving the name archaebacteria (in the Archaebac ...
.
Some acyclic unsaturated
tetraterpenoid
Tetraterpenes are terpenes consisting of eight isoprene units and have the molecular formula C40H64. Tetraterpenoids (including many carotenoids) are tetraterpenes that have been chemically modified, as indicated by the presence of oxygen-contain ...
s (structurally similar to lycopane) have been detected in ''
Thermococcus hydrothermalis
''Thermococcus hydrothermalis'' is a hyperthermophilic archaeon. It is strictly anaerobic and coccus-shaped, and its cells range from 0.8 to 2.0 μm in diameter, with type strain AL662T. It was isolated from a hydrothermal vent in the East Pacif ...
'', a deep-sea
hydrothermal vent
A hydrothermal vent is a fissure 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 hotspot ...
archaea. Lycopane has also been found alongside archaeal ethers in certain marine sediments. These findings provide support for a methanogenic origin of lycopane, but it is not conclusive. Furthermore, lycopane has been identified in water columns that contain
sulfate, which is potentially an argument against lycopane having a methanogenic origin. Methanogens are generally not widespread in sulfate-rich environments.
Diagenesis of lycopene
Lycopane may be sourced from
diagenesis of an unsaturated precursor such as
lycopene, a
carotenoid
Carotenoids (), also called tetraterpenoids, are yellow, orange, and red organic compound, organic pigments that are produced by plants and algae, as well as several bacteria, and Fungus, fungi. Carotenoids give the characteristic color to pumpki ...
that is abundantly present in
photosynthetic organisms. In
cyanobacteria
Cyanobacteria (), also known as Cyanophyta, are a phylum of gram-negative bacteria that obtain energy via photosynthesis. The name ''cyanobacteria'' refers to their color (), which similarly forms the basis of cyanobacteria's common name, blu ...
, lycopene can be an important intermediate in the biosynthesis of other carotenoids. Diagenesis, broadly referring to physical and chemical changes that occur while biological material is undergoing fossilization, may include hydrogenation and transformation of unsaturated precursors to alkane derivatives. Some diagenetic time-dependent reduction of double bonds in carotenoids has been observed in marine sediments.
A direct
geochemical
Geochemistry is the science that uses the tools and principles of chemistry to explain the mechanisms behind major geological systems such as the Earth's crust and its oceans. The realm of geochemistry extends beyond the Earth, encompassing the e ...
diagenetic process for the transformation of lycopene to lycopane during sedimentation has not been determined. However, this process has been identified in other carotenoids (e.g.
carotene
The term carotene (also carotin, from the Latin ''carota'', "carrot") is used for many related unsaturated hydrocarbon substances having the formula C40Hx, which are synthesized by plants but in general cannot be made by animals (with the exc ...
to
carotane). Sulfur has been proposed as a general agent in the diagenesis of isoprenoid alkenes to alkanes. A sulfur polymer (with sulfur binding to unsaturated carbons) could eventually yield isoprenoid alkanes, as carbon-sulfur bonds are weaker than carbon-carbon bonds. Some experimental evidence in support of this theory has been gathered, but it has not been demonstrated in any sediment samples.
Marine photoautotrophs
It has also been theorized that lycopane is directly synthesized by marine
photoautotroph Photoautotrophs are organisms that use light energy and inorganic carbon to produce organic materials. Eukaryotic photoautotrophs absorb energy through the chlorophyll molecules in their chloroplasts while prokaryotic photoautotrophs use chlorophyll ...
s such as cyanobacteria or
green algae
The green algae (singular: green alga) are a group consisting of the Prasinodermophyta and its unnamed sister which contains the Chlorophyta and Charophyta/Streptophyta. The land plants (Embryophytes) have emerged deep in the Charophyte alga as ...
. Lycopene is abundantly present in marine photosynthetic organisms; possibly it is the precursor in a lycopene-to-lycopane pathway. The detection of lycopa-14(E),18(E)-diene in the green alga ''
Botryococcus braunii
''Botryococcus braunii'' is a green, pyramid-shaped planktonic microalga that is of potentially great importance in the field of biotechnology. Colonies held together by a lipid biofilm matrix can be found in temperate or tropical oligotrophic l ...
'' strengthens this theory, as the conversion of
lycopadiene to lycopane would be simpler and more feasible than that of lycopene to lycopane.
Measurement techniques
GC/MS
Gas chromatography-mass spectrometry
Gas is one of the four fundamental states of matter (the others being solid, liquid, and plasma).
A pure gas may be made up of individual atoms (e.g. a noble gas like neon), elemental molecules made from one type of atom (e.g. oxygen), or com ...
is a common tool for detecting and analyzing biomarkers. Depending on the stationary phase used in the column, lycopane tends to co-elute with the ''n''-C
35 alkane. Its tail-to-tail linkage yields diagnostic mass fragments. The mass spectrum has a periodic fragmentation pattern.
Raman spectroscopy
Raman spectroscopy
Raman spectroscopy () (named after Indian physicist C. V. Raman) is a spectroscopic technique typically used to determine vibrational modes of molecules, although rotational and other low-frequency modes of systems may also be observed. Raman sp ...
, a non-destructive analytical technique with no sample preparation, is a powerful tool for analyzing biomarkers. Lycopene, the unsaturated carotenoid that lycopane may be derived from, has a very characteristic Raman spectrum that is easily distinguishable. The spectrum of lycopane differs by a strong band at 1455 cm
−1 (CH
2 scissoring), a series of bands from 1390–1000 cm
−1 (C-C stretching), and some bands from 1000–800 cm
−1 (methyl in-plane rocking and C-H out-of-plane bending).
Stable isotope analysis
The amount of
carbon-13 present in lycopane found in sediment can give indications of its producer, particularly differentiating between methanogenic and algal origin. Lower levels of
13C suggest that the compound originated in methanogens, while higher levels support an algal origin. The high level of
13C found in the Messel shale lycopane (-20.8‰) suggests an algal producer.
Use as a biomarker (case study: Arabian Sea/Peru Upwelling region)
Recent work has proposed elevated levels of lycopane as a proxy for
anoxicity. When the C
35/C
31 n-alkane
In organic chemistry, an alkane, or paraffin (a historical trivial name that also has other meanings), is an acyclic saturated hydrocarbon. In other words, an alkane consists of hydrogen and carbon atoms arranged in a tree structure in whic ...
ratio was calculated both within and outside of the Oxygen Minimum Zone (OMZ) in the
Arabian Sea
The Arabian Sea ( ar, اَلْبَحرْ ٱلْعَرَبِيُّ, Al-Bahr al-ˁArabī) is a region of the northern Indian Ocean bounded on the north by Pakistan, Iran and the Gulf of Oman, on the west by the Gulf of Aden, Guardafui Channel ...
, ratios inside of the OMZ were approximately two to three times higher than they were outside of this zone. This increased ratio was determined to be due to the presence of lycopane, which coelutes with C
35 ''n''-alkane. Thus, it was determined that the lycopane/C
31 ratio is correlated with degree of anoxicity. Similar trends were observed in the Peru Upwelling region. This further solidifies the viability of lycopane abundance as an indicator of oxicitiy/anoxicity and provides additional support for a methanogenic origin of lycopane.
Astrobiological potential
One of the challenges involved in searching for
life on other planets is the practical limitations of instrumentation. While GC/MS or NMR may give unequivocal evidence of the existence of biomarkers, it is not practical to include these instruments on highly optimized spacecraft. Raman spectroscopy has emerged as a leading technique due to its sensitivity,
miniaturizability, and lack of sample preparation.
Carotenoids have long generated astrobiological interest given their diagnostic Raman spectra, their unlikelihood of being abiotically synthesized, and their high preservation potential. Recent work has indicated that the Raman spectrum of lycopane is sufficiently different from that of lycopene. The two molecules are distinguishable. While functionalized carotenoids in themselves are an attractive astrobiological biomarker, detecting their diagenetic products may be equally characteristic of extraterrestrial life. Detection of diagenetically reduced lycopane on other planetary bodies may be an unambiguous indication of life, as diagenesis occurs during biological fossilization.
References
{{Reflist
Alkanes
Biomarkers