ECOLOGICAL BIOCHEMISTRY - Study Guide - V. M. Isaienko 2005

Chapter 9. RADIORESISTANCE OF ORGANISMS

9.5. Radioresistance of Animals

The resistance of various animals to ionizing radiation varies within a wide range. Some species approach microorganisms in their Radioresistance, whereas others are comparable to humans or are even less resilient (Table 9.6).

Class="center">Table 9.6

RADIORESISTANCE OF ANIMALS

(after Hrodzynskyi, 2000; Kutlakhemadov et al., 2003)

PROTOZOA. Protozoa are the most radioresistant among animals. Amoebae and Ciliates are the most thoroughly studied in this regard.

When exposed to ionizing radiation, amoebae experience both premitotic and postmitotic Cell death, with species containing fewer Chromosomes being more radioresistant than polyploid ones. Surviving amoebae frequently form cell lines (so-called clones) characterized by slow development. Their progeny exhibit an increased frequency of spontaneous lethality. The radioresistance of certain specific amoeba species is presented in Table 9.7.

Table 9.7

RADIORESISTANCE OF CERTAIN AMOEBA SPECIES TO γ- AND X-RAY IRRADIATION

(after Hrodzynskyi, 2000)

Although ciliates are more complex in Structure than amoebae, they also exhibit high radioresistance. This is primarily attributed to significant redundancy of genetic material within The Genome as well as The Development of DNA Repair systems. They reproduce both asexually and sexually, with their radioresistance being significantly higher during asexual reproduction. For example, for the slipper Ciliate (Paramecium caudatum), the LD₅₀ value reaches several thousand grays, and they are capable of recovering from potentially lethal damage.

Ciliate Cells may die upon exposure to high Doses of ionizing radiation either without division or after a few divisions. The progeny of irradiated ciliates are less resistant to various adverse environmental factors.

INVERTEBRATES. Numerous data regarding the radioresistance of invertebrates indicate that although they are significantly more radioresistant than vertebrates, they are nevertheless affected by ionizing radiation. Studies conducted in forest areas contaminated with radioisotopes show shifts in the Abundance of soil invertebrates. For instance, in the first year of the study in areas with a soil specific activity for ¹³⁷Cs of 500—1600 Bq/kg, insects were the dominant group among the mesofauna (medium-sized soil invertebrates), whereas earthworms had predominated prior to irradiation; the population of spiders decreased significantly. Two years after irradiation, the abundance of stone centipedes (order Lithobiida) and soil centipedes (order Geophilida) increased.

It is worth noting that earthworms belong to highly radioresistant animals. Specifically, individuals that received an absorbed dose of approximately 1000 Gy over a year do not differ significantly in Anatomical Structure from those inhabiting less contaminated areas.

Studies of microfauna (small-sized soil invertebrates) indicate that During the first year of exposure to elevated doses of ionizing radiation, the population of armored mites (oribatid mites) decreased, whereas the number of springtails (collembolans) and predatory gamasid mites that feed on them increased. Two years later, the abundance of oribatid mites partially recovered, whereas the numbers of gamasid mites, Acaridid mites, small insects (staphylinids), and springtails declined.

A stable increase in mortality with increasing dose was observed for certain invertebrates, such as oribatids. At the same time, uropodid mites did not perish until the dose reached a threshold value of 500 Gy.

Thus, soil invertebrates react distinctly to elevated doses of ionizing radiation: after a certain period, the population size of some groups may recover, yet the species STRUCTURE OF THE community is capable of changing.

Insects also exhibit high radioresistance. For example, the LD₅₀ for wasps is 1000 Gy. At the same time, their sensitivity to ionizing radiation is significantly higher at the larval stage than in adults. Various insect representatives at this stage perish from acute exposure to doses ranging from a few grays to several tens of grays. However, under chronic irradiation at doses ranging from a few rads to tens of rads per day, no significant damage is observed.

Although adult insects are considerably more radioresistant than larvae, they are still damaged by ionizing radiation. Their most sensitive components are the epithelial Cells of the gut, hemocyte-producing cells in the hemolymph, and cell populations of the Gonads.

Aquatic invertebrates are also resistant to ionizing radiation. For instance, Water fleas (Daphnia magna) inhabiting water with an activity of 3.7 × 10³—3.7 × 10⁵ Bq/L suffered no severe damage. Mollusks are even more resistant to ionizing radiation. For Lymnaea stagnalis, a slight decrease in larval hatching from eggs is observed only at water activities of 3.7 × 10⁵—3.7 × 10⁷ Bq/L.

VERTEBRATES. The radioresistance of vertebrate animals has been studied across numerous species belonging to various classes. Reptiles and amphibians are more radioresistant than fish. Birds occupy an intermediate position between fish and mammals.

Reptiles and amphibians. Like other animals, the Embryonic Stage of development in these creatures is the most sensitive to ionizing radiation. The number of larvae hatching from amphibian eggs vastly exceeds the threshold required to sustain the population. Therefore, radiation-induced mortality of a certain fraction of these organisms may have no significant impact on population viability. It has been established that doses of approximately 0.1 Gy under acute (single) exposure and 10 Gy/year under chronic (long-term) exposure do not affect the viability of populations of various amphibians (frogs, axolotls, newts, etc.).

Reptiles (crocodiles, snakes, lizards, turtles) are more radioresistant than amphibians. It is estimated that a dose of 10 Gy results in the death of 10–50% of embryos when reptile eggs are irradiated, while for adult animals, the lethal doses are 100 Gy under acute exposure and 1,000–10,000 Gy under chronic exposure.

Fish. In fish, fish roe (eggs) is the stage most sensitive to ionizing radiation. Under acute irradiation of fish eggs, such as tench eggs at the two-blastomere stage, mortality is observed at an absorbed dose of 2 Gy and above, while a slight increase in the yield of abnormal forms occurs at 0.5 Gy and higher.

Irradiation of eggs at doses of 0.25–2.5 Gy does not lead to an increased radiosensitivity in fry. When subjected to subsequent irradiation at higher doses (40 Gy and above), the most radiosensitive individuals are those that hatch from non-irradiated eggs or eggs

that were irradiated at doses of 2.5 Gy and higher. The most resistant to ionizing radiation are fry hatched from eggs irradiated at doses of 0.25–1.0 Gy.

Chronic irradiation of fish, as in the case of other organisms, results in less pronounced damage than acute exposure. Adult fish are more resilient to ionizing radiation than eggs. Therefore, the presence of adult fish in water bodies contaminated with radioisotopes at levels that do not hinder egg development is harmless. It can be assumed that under chronic fish irradiation at doses of 1 Gy, the viability of the fish population remains unimpaired.

Birds. Under acute irradiation, 50% mortality in birds is observed at doses of 8–20 Gy (Table 9.6). Notably, eggshells provide robust protection for embryos against α- and β-radiation.

The effects of elevated levels of ionizing radiation have been most thoroughly studied in poultry, particularly chickens. It has been found that embryonic irradiation at doses of 0.5 Gy induces certain developmental anomalies. Higher doses can lead to an increased incidence of abnormalities and reduced survival rates. Ionizing radiation also suppresses reproductive Functions in birds. For instance, irradiating laying hens at a dose of 4 Gy over 10 days results in a significant drop in egg production.

Birds feed on plant fruits and seeds, insects, or other animals capable of accumulating radioisotopes. These substances can affect the birds and their offspring, and be transported over considerable distances via avian excreta. Their contribution to the overall radioactivity of an area is negligible compared to the cycling of radioisotopes involving other organisms.

Mammals. Among animals, mammals are the most sensitive to ionizing radiation. Within this group, donkeys, sheep, horses, and pigs exhibit the highest radiosensitivity under acute exposure (LD₅₀ is 1.5–2.0 Gy), followed by monkeys (2.0–5.0 Gy), goats, dogs, humans, and guinea pigs (2.5–5.0 Gy), while rats, mice (6.0–15.0 Gy), hamsters, and rabbits (7.5–11.0 Gy) are more resistant. Chronic exposure at doses equivalent to acute ones causes lesser damage. As a rule, mammalian radiosensitivity to chronic radiation decreases in inverse proportion to the absorbed dose rate.

Among mammals, mice and rats have been studied the most extensively. It is believed that the mechanisms of radiation-induced injury in these species are similar to those in humans. Observations of mice inhabiting areas contaminated with radioisotopes following the Chornobyl Nuclear Power Plant accident indicate that chronic exposure (for a year or more) at doses of 1.0–2.0 Gy/year does not significantly affect population viability. At the same time, evidence suggests that chronic irradiation at a rate of 10 mGy/day already induces certain changes within the population.