Cryptobiosis

Cryptobiosis is an ametabolic state of life entered by an organism in response to adverse environmental conditions such as desiccation, freezing, and oxygen deficiency. In the cryptobiotic state, all metabolic procedures stop, preventing reproduction, development, and repair. An organism in a cryptobiotic state can essentially live indefinitely until environmental conditions return to being hospitable. When this occurs, the organism will return to its metabolic state of life as it was prior to the cryptobiosis.

Anhydrobiosis
Anhydrobiosis is the most studied form of cryptobiosis and occurs in situations of extreme desiccation. The term anhydrobiosis derives from the Greek for "life without water" and is most commonly used for the desiccation tolerance observed in certain invertebrate animals such as bdelloid rotifers, tardigrades, brine shrimp, nematodes, and at least one insect, a species of chironomid (Polypedilum vanderplanki). However, other life forms, including the resurrection plant Craterostigma plantagineum, the majority of plant seeds, and many microorganisms such as bakers' yeast, also exhibit desiccation tolerance. Studies have shown that some anhydrobiotic organisms can survive for decades, even centuries in the dry state.

Invertebrates undergoing anhydrobiosis often contract into a smaller shape and some proceed to form a sugar called trehalose. Desiccation tolerance in plants is associated with the production of another sugar, sucrose. These sugars are thought to protect the organism from desiccation damage. In some creatures, such as bdelloid rotifers, no trehalose has been found, which has led scientists to propose other mechanisms of anhydrobiosis, possibly involving intrinsically disordered proteins.

In 2011, Caenorhabditis elegans, a nematode that is also one of the best-studied model organisms, was shown to undergo anhydrobiosis in the dauer larva stage. Further research taking advantage of genetic and biochemical tools available for this organism revealed that in addition to trehalose biosynthesis, a set of other functional pathways are involved in anhydrobiosis at the molecular level. These are mainly defense mechanisms against reactive oxygen species and xenobiotics, expression of heat shock proteins and intrinsically disordered proteins as well as biosynthesis of polyunsaturated fatty acids and polyamines. Some of them are conserved among anhydrobiotic plants and animals, suggesting that anhydrobiotic ability may depend on a set of common mechanisms. Understanding these mechanisms in detail may allow engineering of non-anhydrobiotic cells, tissues, organs or even organisms so that they can be preserved in a dried state of suspended animation over long time periods.

As of 2004, such an application of anhydrobiosis is being applied to vaccines. In vaccines, the process can produce a dry vaccine that reactivates once it is injected into the body. In theory, dry-vaccine technology could be used on any vaccine, including live vaccines such as the one for measles. It could also potentially be adapted to allow a vaccine's slow release, eliminating the need for boosters. This proposes to eliminate the need for refrigerating vaccines, thus making dry vaccines more widely available throughout the developing world where refrigeration, electricity, and proper storage are less accessible.

Based on similar principles, lyopreservation has been developed as a technique for preservation of biological samples at ambient temperatures.

Anoxybiosis
In situations lacking oxygen (a.k.a., anoxia), many cryptobionts (such as M. tardigradum) take in water and become turgid and immobile, but can still survive for prolonged periods of time just as with other cryptobiological processes. While survival rate studies of organisms during supposed anoxybiosis in anoxia have historically given some conflicting results, the current consensus in the scientific community seems to be in favor of the opinion that certain ectothermic vertebrates and some invertebrates (for example, sea monkeys (Clegg et al. 1999), copepods (Marcus et al., 1994), nematodes (Crowe and Cooper, 1971), and sponge gemmules (Reiswig and Miller, 1998)) are capable of successfully surviving in a seemingly inactive state during anoxic conditions for periods of time ranging from months to decades. Studies of the metabolic activity of these idling organisms during anoxia have been mostly inconclusive, primarily due to the technical difficulty of measuring very small degrees of metabolic activity with enough reliability to conclusively prove a cryptobiotic state is occurring rather than just an extreme case of the metabolic rate depression (MRD) phenomenon exhibited by all aerobic organisms to some degree (typically 1-10% of aerobic levels) when exposed to anoxia. Thus, anoxybiosis is not considered a legitimate form of naturally occurring cryptobiosis by some comparative biologists because of these inconclusive and conflicting experimental results regarding whether a truly complete metabolic standstill actually occurs in cryptobionts during anoxic conditions, or if the metabolism is merely such a tiny fraction of the aerobic metabolic rate that it falls below the detectable limits of modern analytical methods and/or technologies. Also, many experts are skeptical of the biological feasibility of anoxybiosis, because any preservation of biological structures during anoxic conditions would imply that a seemingly impossible situation is occurring, wherein the organism is managing to prevent damage to its cellular structures from the environmental negative free energy despite being both surrounded by plenty of water and thermal energy and, most remarkably, it does so without using any free energy of its own. However (as summarized by Professor James Clegg of the UC Davis Bodega Marine laboratory in a 2001 review article), while many studies have failed to find measurable quantities of metabolic activity in potential anoxybiotic organisms, and there is also some evidence that the stress-induced protein p26 may act as a protein chaperone that requires no energy in cystic Artemia franciscana embryos, more recent data suggest that most likely an extremely specialized and slow guanine polynucleotide pathway continues to provide metabolic free energy to the Artemia franciscana embryos during anoxic conditions. Although it is inherently difficult to prove that not a single organism in existence anywhere is capable of entering a truly anoxybiotic state, it seems to clearly be the case that most likely at least A. franciscana (a.k.a. sea monkeys, a well-known and commonly studied cryptobiont) is only capable of approaching, but not reaching, the above-mentioned implied violation of the usual biological conversion of environmental negative free energy into the negative entropy of cellular structures via metabolic processes.

Chemobiosis
Chemobiosis is the cryptobiotic response to high levels of environmental toxins.

Cryobiosis
Cryobiosis is a form of cryptobiosis that takes place in reaction to decreased temperature. Cryobiosis initiates when the water surrounding the organism's cells has been frozen, stopping molecule mobility and allowing the organism to endure the freezing temperatures until more hospitable conditions return. Organisms capable of enduring these conditions typically feature molecules that facilitate freezing of water in preferential locations while also prohibiting the growth of large ice crystals that could otherwise damage cells.

Osmobiosis
Osmobiosis is the least studied of all types of cryptobiosis. Osmobiosis occurs in response to increased solute concentration in the solution the organism lives in. Little is known for certain, other than that osmobiosis appears to involve a cessation of metabolism.

Examples
A commonly known organism that undergoes cryptobiosis is Artemia salina, also known as the brine shrimp or by its brand name Sea-monkeys. This species, which can be found in the Makgadikgadi Pans in Botswana, survives over the dry season when the water of the pans evaporates, leaving a virtually desiccated lake bed.

The tardigrade, or water bear, is a widely studied and notable example, partially because it can undergo all five types of cryptobiosis. While in a cryptobiotic state, the tardigrade's metabolism reduces to less than 0.01% of what is normal, and its water content can drop to 1% of normal. It can withstand extreme temperature, radiation, and pressure while in a cryptobiotic state.

Some nematodes and rotifers can also undergo cryptobiosis.