Cryptobiosis: Key to Space-Traveling Organisms

Cryptobiosis: A reversible state in which an animal's metabolism has come to a virtual standstill. It is exhibited by various types of organism, including a small organism called the  tardigrade. Tardigrads are able to persist dry-out, freezing, and high temperatures by means of a physiological phenomenon called cryptobiosis. upon seccation of their environment, for example, the animals secrete water and shrink to immobile, knittered can-like structures which now only contain bound water. this way, the animals can persist for years and survive even experimental deep freezing in liquid oxygen. organs and metabolism maintain their function albeit at a very low rate (anabiosis). Rehydration  can lead to their revitalization to a normally lively specimen within half an hour.

Tardigrades [more info]

1. Anhydrobiosis occurs under conditions of dessication. The animal contracts, loses water, and takes on a shriveled, wrinkled appearance. This "tun" may survive in this state from four to seven years. Animals have been recovered from this state after immersion in liquid helium, absolute alcohol, brine, and ether.
2. Cysts may be formed when the animal undergoes stress in the form of damage, hunger, or abnormal environmental conditions. The animal withdraws into the cuticle and forms a dark, thick-walled cyst. The internal organs undergo some degeneration. The animal reconstitutes in favorable conditions.
3. When deprived of oxygen, tardigrades will enter an anoxybiotic state: the animal swells, the body becomes turgid and movement ceases.
4. Tardigrades have been revived after 120 years of dessication.
5. Tardigrades can survive the vacuum of space.

Life History Evolution in Cryptobiotic Organisms
Cryptobiosis is an ametabolic form of life utilised by some small invertebrates to survive periods of severe dessiccation or freezing. Cryptobiosis has been the subject of many biochemical studies, while very few ecological and evolutionary studies have been done.Cryptobiosis is known to occur in a wide variety of organisms including viruses, bacteria, fungi, seeds of higher plants, and even in animals—including tardigrades, eelworms (Keilin 1953), and eggs of some crustacea (Hinton 1960a). 

Cryptobiosis is of great interest in the study of cryogenics and tardigrades have been subjected to laboratory experiments which verified their ability to survive. Tardigrades have tolerated temperatures below freezing at 0.05K (-272.95 C) for 20 hours and -200 C for 20 months. They have survived 120 C, pressures of 1000 atmospheres, and high vacuums. In the cryptobiotic state, tardigrades have shown resistance to hydrogen sulfide, carbon dioxide, ultraviolet light, and X-rays (Kinchin 1994). We could speculate that tardigrades could be transported through outer space in their existing form.

One way in which tardigrades have adapted to various types of environments, has been to reversibly suspend their metabolism. This state is known as cryptobiosis and is a truly deathlike state. Metabolism lowers to 0.01% of normal or is entirely undetectable and the water content of the body decreases to less than 1%. The environmental extreme determines which of four crypto biotic pathways -- anhydrobiosis, cryobiosis, osmobiosis, and anoxybiosis---will occur.

The most intensely studied type of cryptobiosis is anhydrobiosis, a form of cryptobiosis initiated by desiccation. Living in a limnoterrestrial habitat, such as moss, requires that these organisms can survive periods of dryness. Anhydrobiosis is an almost complete loss of body water and the animal can stay in this state for an extended period of time. Tun formation, a vital part of the process, results in a body that is constricted and folded. The first step is invagination of the limbs, longitudinal contraction of the body, and infolding of the intersegmental cuticle. Wax extrusion covers the surface and may help to reduce transpiration (water loss by evaporation). The tun formation process requires active metabolism. The relative humidity required for tun formation to be successful varies between 70-95%, depending on the species. Once the tun is formed further desiccation can take place in 0% relative humidity and the tardigrade can still survive (Wright, 1989b). Revival from this state typically takes a few hours but is dependent on how long the tardigrade has been in the anhydrobiotic state (Somme, 1996).

Cryobiosis is a form of cryptobiosis which is initiated by a reduction in temperature and involves the ordered freezing of water within the cells. Recent studies done by Somme in 1995 and 1996 have helped to develop a greater understanding of the mechanism tardigrades use to survive extreme temperatures. John Wright (1992) claimed that organisms which live in polar regions must be able to withstand periods of freezing without becoming frozen themselves. However, certain animals that live in such environments are able to remain viable in the frozen state. These include some arthropod insects which may spend ten months in a completely frozen solid state (Storey, 1990). Cryobiosis allows tardigrades to tolerate rapid freezing and thawing cycles and allows for tardigrades in Arctic and Antarctic habits to withstand the temperature changes which occur (Wright, 1992). Recent work on two species, Adorybiotus coroniferandAmphibolus nebulosus found in the Arctic demonstrate the ability of tardigrades to survive super-cooling to &endash;6 Celcius.

Osmobiosis is a form of cryptobiosis initiated by a decreased water potential due to increased solute concentration in the surrounding solution. Osmobiosis has been poorly studied with only two studies (Collin and May, 1950 and Wright, 1987) concerning Tardigrada to date. Upon immersion in non-ambient saline solutions tardigrades commonly contract rapidly into a tun. However, this is not necessary since active animals can survive high salinity. Viability decreases with prolonged exposure. Some tardigrades are found in the marine intertidal zone and can tolerate changes in the salinity of the water. Echiniscoides sigismundi, species found on rocky shores, can tolerate tidal cycles of seawater and severe desiccation, combined with fluctuations in osmolality during evaporation and rainfall (Wright, 1992). The process by which osmobiosis occurs is not understood but does appear to involve the cessation of metabolism.

A reduction of oxygen tension initiates a suspended state in tardigrades, but is not really considered a form of cryptobiosis. Animals in this state remain extended, turgid, and immobile. Tardigrades are very sensitive to changes in oxygen tension and prolonged reduction of oxygen leads to osmoregulatory failure. Unlike other types of cryptobiosis, anoxybiosis involves the uptake of water and the animals become turgid. Revival time is directly proportional to duration of the dormant state. John Wright (1992) explained that the survival rate of a tardigrade in anoxybiosis is questioned because studies done by John Crowe (1975), show that specimens were only viable for up to 3-4 days, while Kristensen and Hallas (1980) reported survival for up to six months in closed vials.

Ability to Resist Environmental Extremes

While in a state of cryptobiosis organisms are able to resist environmental extremes that would be instantly lethal to the animal if in the active state. In a review of cryptobiosis, Crowe (1971) discusses some of the findings regarding the abilities of tardigrades to withstand these environmental extremes. In the 1920's P. G. Rahm of the University of Freidburg discovered tardigrades were able to withstand being heated for a few minutes in 151 degrees Celsius and survive being chilled for days in temperatures up to minus 200 degrees Celsius. While in this state the organisms are also greatly resistant to ionizing radiation as shown by Raul M. May from the University of Paris who found that 570,00 roentgens were required to kill 50% of exposed tardigrades (only 500 roentgens would be fatal to a human). Water bears are also resistant to vacuums. Specimens exposed to high vacuum and electron bombardment in a SEM for 0.5 hours were then revived and survived for a few minutes before dying. Why are organisms in the cryptobiotic state able to withstand extreme conditions? Crowe (1971) hypothesized that the importance of water, heat, and oxygen in destructive reactions may explain why the lack of at least one of these characters in animals in cryptobiosis provides resistance to such cellular breakdown. 

As science obtains a better understanding of biological processes we must at times re-examine previous beliefs or understandings. This is perhaps exemplified by cryptobiosis. The issue pertains to the question of whether or not tardigrades can die and come back to life. The answer is no. However, normally, the cessation of metabolic activity is associated with death and death is considered an irreversible state. The long-range implications may even include the ability to travel long distances in space. This could occur through suspended metabolism--cryptobiosis

 

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