Heroic exertion of radiation-resistant extremophiles

25 February 2014

Extremophiles are bacteria and fungi that can survive under harsh environmental conditions such as high and low pressures in deep-sea and high-altitude zones, extremely high and low temperatures (above 45°C or below 15°C), high-salt and acidic conditions, and infrared and thermal radiation, including ionising (gamma) and non-ionising (UV) radiation. This article provides an overview of these ‘super’ radiation-resistant extremophiles and their potential uses in biotechnology and medicine.

The radioresistants – a gang of radiation-resistant superheroes!

Radiation is the emission of energy that comes from a source, travels through space, and is able to penetrate various materials. Sunlight includes both ionising and non-ionising radiation, which can be distinguished from each other by the length of their waves. The ultraviolet (UV) radiation (UVR) that contains UVA, UVB and UVC types (Fig. 1), is the most basic form of radiation in sunlight to reach the Earth’s surface. UVA radiation is the lowest-energy type in the UV spectrum; its wavelengths range from 315 to 400 nm, and it carries 3.1–3.94 eV per photon. It is also the most abundant radiation type that reaches the Earth’s surface, due to continuous depletion of the ozone layer, and is known to cause damage to living organisms.

UVA can penetrate deep into human skin, causing alterations in vital biomolecules, including nucleic acids and proteins. It forms pyrimidine dimers in DNA, which ultimately cause mutations; if these types of mutations occur in a cell cycle regulatory gene such as p53, they can result in normal cells transforming into cancerous cells. However, there may be some extremophiles – micro-organisms that thrive in extreme environments – that give us a key to developing treatments that prevent the effects of radiation (Fig. 1).

FIG. 1. SCHEMATIC REPRESENTATION OF THE ORIGIN OF DIFFERENT TYPES OF RADIATION ON EARTH AND THEIR EFEECTS ON MICROBIAL SURVIVAL (I.E. EXTREMOPHILES) AND NON-SURVIVAL. ADAPTED WITH PERMISSION, AND OM THE ARTICLE BY GABANI & SINGH (2013) APPL MICROBIOL BIOTECHNOL 97, 512–555.

Extremophiles are placed in different categories based on their growth characteristics, as shown in Table 1. Extremophiles that can survive high levels of radiation are commonly called radioresistants. The metabolic products (extremolytes) and enzymes (extremozymes) they secrete are optimised to help the organisms survive high-radiation conditions. It may be possible to use these compounds (such as proteins, enzymes, antioxidants, anti-radiation agents, and pigments) to develop radioprotective drugs that prevent skin damage from UVR; they have enormous potential for use in space programmes and medical applications.

TABLE 1. EXTREMOPHILES IN DIFFERENT CATEGORIES AND THEIR GROWTH CHARACTERISTICS

CATEGORY	GROWTH CHARACTERISTICS	TERMINOLOGY
Radiation	Varying types of radiation	Radioresistant
Temperature	High: 90-130ºC Low: 0-12ºC	Thermophiles Psychrophiles
pH	High: pH 8.5-12 Low: pH 0.06-4	Alkaliphiles Acidiphiles
Pressure	High: >1,000 atm Low: 500 atm	Barophiles
Salt (NaCl)	15-32%	Halophiles

Life under radiation

Outer space, with its vacuum, temperature fluctuations, a full spectrum of extraterrestrial solar electromagnetic radiation and cosmic ionising radiation, is one of the most harsh and hostile environments in existence. However, microbial life is transported across the globe via atmospheric strata, and a variety of micro-organisms can be found in different atmospheric layers, including outer space. UVR is one of the most limiting abiotic factors for microbial communities at higher altitudes, and as a result it could be anticipated that micro-organisms isolated from higher elevations will have UVR resistance.

COLOURED SCANNING ELECTRON MICROGRAPHS (SEMS) OF FOUR DEINOCOCCUS RADIODURANS FORMING A TETRAD

Indeed, many micro-organisms isolated at higher elevations are UVR-resistant. Microbes have been found at altitudes of up to 85 km. In one study, samples of Anabaena cylindrica and Chroococcidiopsis survived high UV exposure for 548 days in low Earth orbit. Scientists have reported epilithic lichens and cryptoendolithic microbial communities surviving on the outer surface of the International Space Station, and a number of microbial species have been isolated from NASA’s Jet Propulsion Laboratories Spacecraft Assembly Facility and the Mars Odyssey spacecraft.

Another extremophile genus, Deinococcus, is extremely radioresistant and has been found in deserts, oceans, lakes and marine fish. One strain of Deinococcus reticulitermitis survived UVR up to an intensity of 100 J m–2. Elevated resistance to gamma radiation has been demonstrated by a micro-organism isolated from the desert in China, Hymenobacter xinjiangensis, which survives by producing pink pigment. A broader overview of UVR-resistance in extremophiles has been summarised in Table 2.

TABLE 2. ULTRAVIOLET RADIATION-MEDIATED RADIORESISTANCE IN VARIETY OF MICRO-ORGANISMS

ORGANISM	INTENSITY OF UVR RESISTANCE
Bacillus horneckiae	UVR resistance up to 1000 J m^–2
Acinetobacter sp., Bacillus sp., Exiguobacterium sp., Micrococcus sp., Sphingomonas sp.	For time period: 0.5, 3, 6, 12, 24 h with half bandwidth of 300–325 nm
Stenotrophomonas maltophilia, Exiguobacterium sp., Staphylococcus sp.	For time period: 1, 3, 6, 12 and 24 h UV-B exposure at intensity 1.9 W m^–2
Deinococcus guangriensis, D. wulumuqiensis, D. grandis, D. misasensis, D. xibeiensis, D. gobiensis	UVR resistance >600 J m^–2
Hymenobacter tibetensis	UV-resistant – brick-red-pigmented strain
Prochlorococcus MED4	UV-hyper-resistant 9.2 J m^–2
Microbacterium maritypicum	UV radiation 0.14 J m^–2 s^–1

What makes extremophiles super?

Studies indicate that extremophiles have developed a variety of ingenious strategies for survival in high-radiation conditions. One such strategy is synthesising organic osmolytes to protect biological macromolecules and cells; these low-molecular-mass compounds accumulate in response to extreme environmental changes and minimise the denaturation of biopolymers (Fig. 1). For example, halophilic (salt-loving) bacteria produce ectoines, which have proved their value as cell protectants in skin care products and as stabilisers of proteins in other biological products. Ectoines have also been found to protect human keratinocyte cells from UVA damage.

Some bacteria secrete pigments that shield them from UVR, including the cyanobacterium Tolypothrix byssoidea, found on the exposed rock surface of an Indian temple, and Chroococcidiopsis from the desert (Chroococcidiopsis is also desiccation-tolerant). Deinococcus depolymerans, isolated from radioactive sites in Japan, produces a red pigment that may make it gamma- and UVR-resistant. Scytonemin, a pigment synthesised by many strains of cyanobacteria including Calothrix sp. and Lyngbya aestuarii, blocks UVA radiation and has antiproliferative and anti-inflammatory properties.

COLOURED SCANNING ELECTRON MICRORAPH (SEMS) OF LYNGBYA SP.

Mycosporine-like amino acids (MAAs) are known to absorb UVR and can be found in a wide range of micro-organisms, including cyanobacteria and eukaryotic algae. MAAs protect DNA against UVR-induced damage by preventing the formation of DNA dimers. Currently, they are being used in sunscreens in the cosmetics industry. One formulation containing MAA was found to prevent sunburn and other structural and morphological alterations to the skin.

In the human body, proteins are the ultimate downstream regulators of various metabolic, cellular and molecular reactions, and various stresses, including radiation exposure, can affect the folding or misfolding of proteins. Some extremolytes are being investigated that may protect proteins by increased (preferential) hydration of the protein, which favours the original state of the protein.

How far from reality?

Currently, there is a technological gap that prevents extremophiles from being more commonly used in biotechnology. Five major technical steps must be completed before extremophiles can take their place as heroes saving lives in reality.

Simulation of extreme environmental conditions in the laboratory in order to study specific extremophiles.
Assembling complete sets of genes, proteins, and metabolites in order to study the molecular cascades of microbial metabolic pathways in extreme environments.
Understanding the nutritional requirements of specific extremophiles in controlled culture conditions.
Developing bioreactors that simulate specific extreme conditions.
Building downstream processing (extraction, purification and storage) systems that maintain the integrity of microbial metabolic products for specific therapeutics.

Conclusion

The compounds produced by radioresistant extremophiles have vast potential for use in human therapeutics as well as for nuclear waste remediation. However, although advancements have been made in recent years, knowledge in this field is still limited, and the rate of progress largely depends on its economic appeal to industry. More research efforts are necessary to fully investigate the possible therapeutic and biotechnological applications of these organisms.

OM V. SINGH

Division of Biological and Health Sciences, University of Pittsburgh, Bradford, PA 16701, USA
Tel: +1 814 362 7562
[email protected]