![Cryptoendolith](https://upload.wikimedia.org/wikipedia/commons/7/71/Cryptoendolith.jpg)
An endolith or endolithic is an
organism
In biology, an organism () is any living system that functions as an individual entity. All organisms are composed of cells (cell theory). Organisms are classified by taxonomy into groups such as multicellular animals, plants, and ...
(
archaeon
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 Archaebact ...
,
bacterium
Bacteria (; singular: bacterium) are ubiquitous, mostly free-living organisms often consisting of one biological cell. They constitute a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria were among ...
,
fungus
A fungus ( : fungi or funguses) is any member of the group of eukaryotic organisms that includes microorganisms such as yeasts and molds, as well as the more familiar mushrooms. These organisms are classified as a kingdom, separately from th ...
,
lichen
A lichen ( , ) is a composite organism that arises from algae or cyanobacteria living among filaments of multiple fungi species in a mutualistic relationship.[algae
Algae (; singular alga ) is an informal term for a large and diverse group of photosynthetic eukaryotic organisms. It is a polyphyletic grouping that includes species from multiple distinct clades. Included organisms range from unicellular mic ...]
or
amoeba
An amoeba (; less commonly spelled ameba or amœba; plural ''am(o)ebas'' or ''am(o)ebae'' ), often called an amoeboid, is a type of Cell (biology), cell or unicellular organism with the ability to alter its shape, primarily by extending and ret ...
) that is able to acquire the necessary resources for growth in the inner part of a
rock
Rock most often refers to:
* Rock (geology), a naturally occurring solid aggregate of minerals or mineraloids
* Rock music, a genre of popular music
Rock or Rocks may also refer to:
Places United Kingdom
* Rock, Caerphilly, a location in Wales ...
,
mineral
In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid chemical compound with a fairly well-defined chemical composition and a specific crystal structure that occurs naturally in pure form.John P. Rafferty, ed. ( ...
,
coral
Corals are marine invertebrates within the class Anthozoa of the phylum Cnidaria. They typically form compact colonies of many identical individual polyps. Coral species include the important reef builders that inhabit tropical oceans and sec ...
,
animal shell
An exoskeleton (from Greek ''éxō'' "outer" and ''skeletós'' "skeleton") is an external skeleton that supports and protects an animal's body, in contrast to an internal skeleton (endoskeleton) in for example, a human. In usage, some of the l ...
s, or in the
pore
Pore may refer to:
Biology Animal biology and microbiology
* Sweat pore, an anatomical structure of the skin of humans (and other mammals) used for secretion of sweat
* Hair follicle, an anatomical structure of the skin of humans (and other m ...
s between
mineral
In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid chemical compound with a fairly well-defined chemical composition and a specific crystal structure that occurs naturally in pure form.John P. Rafferty, ed. ( ...
grains of a rock. Many are
extremophile
An extremophile (from Latin ' meaning "extreme" and Greek ' () meaning "love") is an organism that is able to live (or in some cases thrive) in extreme environments, i.e. environments that make survival challenging such as due to extreme temper ...
s, living in places long imagined inhospitable to life. The distribution, biomass, and diversity of endolith microorganisms are determined by the physical and chemical properties of the rock substrate, including the mineral composition, permeability, the presence of organic compounds, the structure and distribution of pores, water retention capacity, and the pH. Normally, the endoliths colonize the areas within lithic substrates to withstand intense solar radiation, temperature fluctuations, wind, and desiccation.
They are of particular interest to
astrobiologists, who theorize that endolithic environments on
Mars
Mars is the fourth planet from the Sun and the second-smallest planet in the Solar System, only being larger than Mercury (planet), Mercury. In the English language, Mars is named for the Mars (mythology), Roman god of war. Mars is a terr ...
and other planets constitute potential
refugia for extraterrestrial microbial communities.
Subdefinitions
The term "endolith", which defines an organism that colonizes the interior of any kind of rock, has been further classified into five subclasses:
;Chasmoendolith: Colonizes fissures and cracks in the rock connected to the surface (''chasm'' = cleft)
;Cryptoendolith: Colonizes structural cavities within natural pore spaces within the rocks. These pores are usually indirectly connected to the rock surface; (''crypto'' = hidden)
;Euendolith: Penetrates actively into the interior of rocks forming channels and grooves that conform with the shape of its body, rock boring organism (''eu'' = true)
;Hypoendolith: Colonizes the pore spaces located on the underside of the rock and that make contact with the soil (''hypo'' = under)
;Autoendolith: Capable of rocks formation by mineral depositation (''auto'' = self)
Environment
Endolithic microorganisms have been reported in many areas around the globe. There are reports in warm hyper-arid and arid deserts such as Mojave and Sonora (USA), Atacama (Chile), Gobi (China, Mongolia), Negev (Israel), Namib (Namibia Angola), Al-Jafr basin (Jordan) and the Depression of Turpan (China),, also in cold deserts as Arctic and Antarctic, and deep subsoil and ocean trenches rocks. However, there are reports of endolithic microorganisms in inter-tropical zones, where humidity and solar radiation are significantly different from the above-mentioned biomes. Endoliths have been found in the rock down to a depth of 3 kilometers (1.9 mi), though it is unknown if that is their limit (due to the cost involved in drilling to such depths).
[ — Gold mines present "ideal environment" for geologists studying subsurface microbes] The main threat to their survival seems not to result from the pressure at such depth, but from the increased temperature. Judging from
hyperthermophile
A hyperthermophile is an organism that thrives in extremely hot environments—from 60 °C (140 °F) upwards. An optimal temperature for the existence of hyperthermophiles is often above 80 °C (176 °F). Hyperthermophiles are often within the doma ...
organisms, the temperature limit is at about 120 °C (
Strain 121 can reproduce at 121 °C), which limits the possible depth to 4-4.5 km below the
continent
A continent is any of several large landmasses. Generally identified by convention rather than any strict criteria, up to seven geographical regions are commonly regarded as continents. Ordered from largest in area to smallest, these seven ...
al crust, and 7 or 7.5 km below the
ocean
The ocean (also the sea or the world ocean) is the body of salt water that covers approximately 70.8% of the surface of Earth and contains 97% of Earth's water. An ocean can also refer to any of the large bodies of water into which the wo ...
floor. Endolithic organisms have also been found in surface rocks in regions of low humidity (
hypolith In Arctic and Antarctic ecology, a hypolith is a photosynthetic organism, and an extremophile, that lives
underneath rocks in climatically extreme deserts such as Cornwallis Island and Devon Island in the Canadian high Arctic. The community it ...
) and low temperature (
psychrophile
Psychrophiles or cryophiles (adj. ''psychrophilic'' or ''cryophilic'') are extremophilic organisms that are capable of growth and reproduction in low temperatures, ranging from to . They have an optimal growth temperature at . They are found in ...
), including the
Dry Valleys
The McMurdo Dry Valleys are a row of largely snow-free valleys in Antarctica, located within Victoria Land west of McMurdo Sound. The Dry Valleys experience extremely low humidity and surrounding mountains prevent the flow of ice from nearby ...
and
permafrost
Permafrost is ground that continuously remains below 0 °C (32 °F) for two or more years, located on land or under the ocean. Most common in the Northern Hemisphere, around 15% of the Northern Hemisphere or 11% of the global surface ...
of
Antarctica
Antarctica () is Earth's southernmost and least-populated continent. Situated almost entirely south of the Antarctic Circle and surrounded by the Southern Ocean, it contains the geographic South Pole. Antarctica is the fifth-largest contine ...
,
the
Alps
The Alps () ; german: Alpen ; it, Alpi ; rm, Alps ; sl, Alpe . are the highest and most extensive mountain range system that lies entirely in Europe, stretching approximately across seven Alpine countries (from west to east): France, Sw ...
, and the
Rocky Mountains
The Rocky Mountains, also known as the Rockies, are a major mountain range and the largest mountain system in North America. The Rocky Mountains stretch in straight-line distance from the northernmost part of western Canada, to New Mexico in ...
.
Metabolism and survival
The metabolism of endolithic microorganisms is versatile, in many of those communities have been found genes involved in
sulphur metabolism,
iron adquisition, and
carbon fixation
Biological carbon fixation or сarbon assimilation is the process by which inorganic carbon (particularly in the form of carbon dioxide) is converted to organic compounds by living organisms. The compounds are then used to store energy and as ...
. In addition, whether they metabolize these directly from the surrounding rock, or rather excrete an acid to dissolve them first is yet undetermined. According to Meslier & DiRuggiero there are found genes in the endolithic community involved in
nitrogen fixation
Nitrogen fixation is a chemical process by which molecular nitrogen (), with a strong triple covalent bond, in the air is converted into ammonia () or related nitrogenous compounds, typically in soil or aquatic systems but also in industry. Atmo ...
. The
Ocean Drilling Program
The Ocean Drilling Program (ODP) was a multinational effort to explore and study the composition and structure of the Earth's oceanic basins. ODP, which began in 1985, was the successor to the Deep Sea Drilling Project initiated in 1968 by th ...
found microscopic trails in
basalt
Basalt (; ) is an aphanite, aphanitic (fine-grained) extrusive igneous rock formed from the rapid cooling of low-viscosity lava rich in magnesium and iron (mafic lava) exposed at or very near the planetary surface, surface of a terrestrial ...
from the
Atlantic
The Atlantic Ocean is the second-largest of the world's five oceans, with an area of about . It covers approximately 20% of Earth's surface and about 29% of its water surface area. It is known to separate the " Old World" of Africa, Europe an ...
,
Indian
Indian or Indians may refer to:
Peoples South Asia
* Indian people, people of Indian nationality, or people who have an Indian ancestor
** Non-resident Indian, a citizen of India who has temporarily emigrated to another country
* South Asia ...
, and
Pacific ocean
The Pacific Ocean is the largest and deepest of Earth's five oceanic divisions. It extends from the Arctic Ocean in the north to the Southern Ocean (or, depending on definition, to Antarctica) in the south, and is bounded by the continen ...
s that contain
DNA.
Photosynthetic endoliths have also been discovered.
As water and nutrients are rather sparse in the environment of the endolith, water limitation is a key factor in the capacity of survival of many endolithic microorganisms, many of those microorganisms have adaptations to survive in low concentrations of water. Besides, the presence of pigments, especially 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 ...
and some
algae
Algae (; singular alga ) is an informal term for a large and diverse group of photosynthetic eukaryotic organisms. It is a polyphyletic grouping that includes species from multiple distinct clades. Included organisms range from unicellular mic ...
, such as;
beta carotenes and
chlorophyll
Chlorophyll (also chlorophyl) is any of several related green pigments found in cyanobacteria and in the chloroplasts of algae and plants. Its name is derived from the Greek words , ("pale green") and , ("leaf"). Chlorophyll allow plants to a ...
help them in the protection against dangerous radiation and is a way to obtain energy. Another characteristic is the presence of a very slow
reproduction
Reproduction (or procreation or breeding) is the biological process by which new individual organisms – "offspring" – are produced from their "parent" or parents. Reproduction is a fundamental feature of all known life; each individual or ...
cycle. Early data suggest some only engage in
cell division
Cell division is the process by which a parent cell (biology), cell divides into two daughter cells. Cell division usually occurs as part of a larger cell cycle in which the cell grows and replicates its chromosome(s) before dividing. In eukar ...
once every hundred years. In August 2013 researchers reported evidence of endoliths in the ocean floor, perhaps millions of years old and reproducing only once every 10,000 years. Most of their energy is spent repairing
cell damage
Cell damage (also known as cell injury) is a variety of changes of stress that a cell suffers due to external as well as internal environmental changes. Amongst other causes, this can be due to physical, chemical, infectious, biological, nutrition ...
caused by
cosmic ray
Cosmic rays are high-energy particles or clusters of particles (primarily represented by protons or atomic nuclei) that move through space at nearly the speed of light. They originate from the Sun, from outside of the Solar System in our own ...
s or
racemization In chemistry, racemization is a conversion, by heat or by chemical reaction, of an optically active compound into a racemic (optically inactive) form. This creates a 1:1 molar ratio of enantiomers and is referred too as a racemic mixture (i.e. con ...
, and very little is available for reproduction or growth. It is thought that they weather long
ice age
An ice age is a long period of reduction in the temperature of Earth's surface and atmosphere, resulting in the presence or expansion of continental and polar ice sheets and alpine glaciers. Earth's climate alternates between ice ages and gree ...
s in this fashion, emerging when the temperature in the area warms.
Ecology
As most endoliths are
autotroph
An autotroph or primary producer is an organism that produces complex organic compounds (such as carbohydrates, fats, and proteins) using carbon from simple substances such as carbon dioxide,Morris, J. et al. (2019). "Biology: How Life Works", ...
s, they can generate organic compounds essential for their survival on their own from inorganic matter. Some endoliths have specialized in feeding on their autotroph relatives. The micro-
biotope
A biotope is an area of uniform environmental conditions providing a living place for a specific assemblage of plants and animals. ''Biotope'' is almost synonymous with the term "habitat", which is more commonly used in English-speaking countrie ...
where these different endolithic species live together has been called a ''
Subsurface Lithoautotrophic Microbial Ecosystem'' (''SLiME''),
or ''endolithic systems'' within the subterranean lithic
biome
A biome () is a biogeographical unit consisting of a biological community that has formed in response to the physical environment in which they are found and a shared regional climate. Biomes may span more than one continent. Biome is a broader ...
.
''Endolithic systems'' are still at an early stage of exploration. In some cases its biota can support simple invertebrates, most organisms are unicellular. Near-surface layers of rock may contain blue-green algae but most energy comes from chemical synthesis of minerals. The limited supply of energy limits the rates of growth and reproduction. In deeper rock layers microbes are exposed to high pressures and temperatures.
Endolithic fungi and algae in marine ecosystems
Only limited research has been done concerning the distribution of
marine endolithic fungi and its diversity even though there is a probability that endolithic fungi could perhaps play an important role in the health of
coral reef
A coral reef is an underwater ecosystem characterized by reef-building corals. Reefs are formed of colonies of coral polyps held together by calcium carbonate. Most coral reefs are built from stony corals, whose polyps cluster in groups.
Co ...
s.
Endolithic fungi have been discovered in shells as early as the year 1889 by Edouard Bornet and Charles Flahault. These two French phycologists specifically provided descriptions for two fungi: ''Ostracoblabe implexis '' and ''
Lithopythium gangliiforme''. Discovery of endolithic fungi, such as ''Dodgella priscus'' and ''Conchyliastrum'', has also been made in the beach sand of Australia by George Zembrowski. Findings have also been made in coral reefs and have been found to be, at times, beneficial to their coral hosts.
In the wake of worldwide
coral bleaching
Coral bleaching is the process when corals become white due to various stressors, such as changes in temperature, light, or nutrients. Bleaching occurs when coral polyps expel the zooxanthellae (dinoflagellates that are commonly referred to as alg ...
, studies have suggested that the endolithic algae located in the skeleton of the coral may be aiding the survival of coral species by providing an alternative source of energy. Although the role that endolithic fungi play is important in coral reefs, it is often overlooked because much research is focused on the effects of coral bleaching as well as the relationships between
Coelenterate
Radiata or Radiates is a historical taxonomic rank that was used to classify animals with radially symmetric body plans. The term Radiata is no longer accepted, as it united several different groupings of animals that do not form a monophylet ...
and
endosymbiotic
An ''endosymbiont'' or ''endobiont'' is any organism that lives within the body or cells of another organism most often, though not always, in a mutualistic relationship.
(The term endosymbiosis is from the Greek: ἔνδον ''endon'' "within ...
Symbiodinia.
According to a study done by Astrid Gunther endoliths were also found in the island of
Cozumel
Cozumel (; yua, Kùutsmil) is an island and municipality in the Caribbean Sea off the eastern coast of Mexico's Yucatán Peninsula, opposite Playa del Carmen. It is separated from the mainland by the Cozumel Channel and is close to the Yucatán ...
(Mexico). The endoliths found there not only included algae and fungi but also included
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 ...
,
sponges
Sponges, the members of the phylum Porifera (; meaning 'pore bearer'), 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 ...
as well as many other microborers.
Endolithic parasitism
Until the 1990s
phototrophic endoliths were thought of as somewhat benign, but evidence has since surfaced that phototrophic endoliths (primarily
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 ...
) have infested 50 to 80% of midshore populations of the mussel species ''
Perna perna'' located in
South Africa
South Africa, officially the Republic of South Africa (RSA), is the southernmost country in Africa. It is bounded to the south by of coastline that stretch along the South Atlantic and Indian Oceans; to the north by the neighbouring countri ...
. The infestation of phototrophic endoliths resulted in lethal and sub-lethal effects such as the decrease in strength of the mussel shells. Although the rate of thickening of the shells were faster in more infested areas it is not rapid enough to combat the degradation of the mussel shells.
Endolithic fungi and the mass extinction of Cretaceous dinosaurs
Evidence of endolithic fungi were discovered within dinosaur eggshell found in central China. They were characterized as being “needle-like, ribbon-like, and silk-like."
Fungus is seldom fossilized and even when it is preserved it can be difficult to distinguish endolithic hyphae from endolithic cyanobacteria and algae. Endolithic microbes can, however, be distinguished based on their distribution, ecology, and morphology.
According to a 2008 study, the endolithic fungi that formed on the eggshells would have resulted in the abnormal incubation of the eggs and may have contributed to the mass extinction of these dinosaurs. It may also have led to the preservation of dinosaur eggs, including some that contained embryos.
Relationship with astrobiology
Endolithic microorganisms have been considered a model for the search for life on other planets by inquiring about what sort of microorganisms on
Earth
Earth is the third planet from the Sun and the only astronomical object known to harbor life. While large volumes of water can be found throughout the Solar System, only Earth sustains liquid surface water. About 71% of Earth's surfa ...
inhabit specific
minerals
In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid chemical compound with a fairly well-defined chemical composition and a specific crystal structure that occurs naturally in pure form.John P. Rafferty, ed. (2 ...
, which helps to propose those lithologies as life detection targets on an extra-terrestrial surface such as
Mars
Mars is the fourth planet from the Sun and the second-smallest planet in the Solar System, only being larger than Mercury (planet), Mercury. In the English language, Mars is named for the Mars (mythology), Roman god of war. Mars is a terr ...
. Several studies have been carried out in extreme places that serve as analogs for Mars's surface and subsurface, and many studies in
geomicrobiology
Geomicrobiology is the scientific field at the intersection of geology and microbiology and is a major subfield of geobiology. It concerns the role of microbes on geological and geochemical processes and effects of minerals and metals to microb ...
on Earth's hot and cold deserts have been developed. In these
extreme environments
An extreme environment is a habitat that is considered very hard to survive in due to its considerably extreme conditions such as temperature, accessibility to different energy sources or under high pressure. For an area to be considered an extrem ...
, microorganisms find protection against thermal buffering, UV radiation, and desiccation while living inside pores and fissures of minerals and rocks.
Life in these endolithic habitats might face similar stress due to the scarcity of water and high UV radiation that rule on modern
Mars
Mars is the fourth planet from the Sun and the second-smallest planet in the Solar System, only being larger than Mercury (planet), Mercury. In the English language, Mars is named for the Mars (mythology), Roman god of war. Mars is a terr ...
.
An excellent example of these adaptations is the non-hygroscopic but microporous translucent gypsum crusts, which are found as potential substrates that can mitigate exposure to UV radiation and desiccation and allow microbial colonization in hyper-arid deserts.
In the same way, the ability to grow under high water stress and oligotrophic conditions confer to endolithic microorganisms to survive in conditions similar to those found on
Mars
Mars is the fourth planet from the Sun and the second-smallest planet in the Solar System, only being larger than Mercury (planet), Mercury. In the English language, Mars is named for the Mars (mythology), Roman god of war. Mars is a terr ...
. There is evidence of the past existence of water on the red planet; perhaps, these microorganisms could develop adaptations found in current deserts on the
Earth
Earth is the third planet from the Sun and the only astronomical object known to harbor life. While large volumes of water can be found throughout the Solar System, only Earth sustains liquid surface water. About 71% of Earth's surfa ...
. Furthermore, The endolithic structures are a good way to find ancient or current biological activity (
biosignatures
A biosignature (sometimes called chemical fossil or molecular fossil) is any substance – such as an element, isotope, or molecule – or phenomenon that provides scientific evidence of past or present life. Measurable attribute ...
) on
Mars
Mars is the fourth planet from the Sun and the second-smallest planet in the Solar System, only being larger than Mercury (planet), Mercury. In the English language, Mars is named for the Mars (mythology), Roman god of war. Mars is a terr ...
or other rocky planets.
See also
*
Lithophile
Lithophiles are micro-organisms that can live within the pore interstices of sedimentary rock, sedimentary and even fractured igneous rock, igneous rocks to depths of several kilometers.
Some are known to live on surface rocks, and make use of p ...
*
Lithotroph
Lithotrophs are a diverse group of organisms using an inorganic substrate (usually of mineral origin) to obtain reducing equivalents for use in biosynthesis (e.g., carbon dioxide fixation) or energy conservation (i.e., ATP production) via aerobi ...
*
Astrobiology
Astrobiology, and the related field of exobiology, is an interdisciplinary scientific field that studies the origins, early evolution, distribution, and future of life in the universe. Astrobiology is the multidisciplinary field that investig ...
References
Notes
External links
Endoliths General Collection— This collection of online resources such as news articles, web sites, and reference pages provides a comprehensive array of information about endoliths.
— Compiled for professionals and advanced learners, this endolith collection includes online resources such as journal articles, academic reviews, and surveys.
{{Use dmy dates, date=June 2018
Lithophiles