Radioecology: Part 1
Ecology is the study the interactions of organisms with one another and with their non-living, physical environment of energy and matter. Ernst Haeckel, a biologist, began the science of ecology in 1869. The word ecology is derived from two Greek words: oikos, meaning “house” or “place to live” and logos, meaning study. Two fundamental categories of factors are involved in this study: biotic (living) and abiotic (non-living).
Radioecology is the study of the effects of radiation and radioactive substances on ecological communities. This multi-disciplinary science focuses on the analysis of the behavior and effects of radioactive substances in the biosphere. It encompasses the production, release, and transport of radionuclides through the biotic and abiotic parts of the biosphere and the uptake and distribution in humans and the effects of radiation on living organisms. One of the primary goals of radioecology is to provide a knowledge base of the radiation doses to humans, and suggest strategies and methodologies to reduce radiation dose.
There are three main divisions of radioecology. The first deals with radionuclide movement within ecological systems and accumulation within specific ecosystems such as soil, air, water and biota. The second is concerned with ionizing radiation effects on individual species, populations, communities and ecosystems. The third involves the use of radionuclides and ionizing radiation in studies of the structure and function of ecosystems and their component subsystems. In this article I will focus on the first of these after reviewing some basic ecology terminology.
Review of Ecology Terminology
Populations are groups of individuals belonging to the same species. Communities are composed of all the populations living and interacting together in a given area.
Ecosystems encompass both the biotic and abiotic components of communities and their physical environments within a given space or area. Complex biological, chemical and physical processes link all parts of an ecosystem. Adjacent ecosystems can influence each other when their components cross boundaries via wind, precipitation, water flow, gravity and animal movements.
Biomes are complexes of living communities covering large areas of the Earth that are maintained by the climate of a region and characterized by distinctive types of vegetation. Examples of biomes in North America include the tundra, desert, prairie, and the western coniferous forests. Biomes contain several ecosystems within their territory and are the largest recognizable assemblages of animals and plants on the Earth. The distribution of biomes is controlled mainly by climate. For a review of the world’s biomes, please see this site: HYPERLINK “http://www.blueplanetbiomes.org/world_biomes.htm” http://www.blueplanetbiomes.org/world_biomes.htm .
Food chains show the sequence of chemical energy (sunlight fixed in the form of glucose sugar) flow from the lowest level producers (photosynthetic organisms such as plants, algae and phytoplankton) through the primary consumers (herbivores such cows, deer and sheep) and secondary consumers (carnivores such as wolves, lions, and predatory birds) up to the tertiary consumers (omnivores such as humans). Producers are generally called autotrophs because they make their own food (glucose sugar) through photosynthesis. Consumers are known as heterotrophs because they must obtain their food by eating other organisms. Two other categories of organisms that live on other dead organisms are called detrivores (detritus feeders such as catfish, crabs and vultures) and decomposers such as fungi and bacteria. Decomposers play a critical role in the recycling of organic matter and nutrients in ecosystems. Food webs represent the flow of energy through cross-linked food chains resulting from the wide variety of possible sequences of producers and consumers.
Movement of Radioactivity In Ecosystems
One of the most important considerations involved in understanding the impact of radiation in the environment is the fact that radioisotopes are chemically identical to non-radioactive elements in their same “isotopic family” and can combine with other elements to make up the minerals nutrients that are taken up by plants from the soil-solution. Also, elements which occur in the same chemical “family” (columns or groups in the periodic table) act as analogs for and replace each other in these various compounds. These “radio-compounds” make their way “up” through food chains maintaining their radioactivity to varying degrees based on their half-lives. Not only do these compounds “flow” through ecosystems, but also their concentration can increase due to a process known as bio-magnification. In food webs, the amount of chemical energy which transfers to the next highest level is roughly 10%, so in order for the organisms in that level to obtain enough energy to maintain their life processes they must consume a large amount of biomass (living matter) from the level below. The “radio-compounds” which constitute a large percentage of the consumed biomass are incorporated into the biomass at each level so the amount of radioactivity effectively increases. The only way that radioactivity will decrease at higher trophic levels is by the combined effects of radioactive decay and biological decay which transports energy “down” the food web.
One very common example of these processes is the movement of strontium-90 (90Sr) through dairy “ecosystems” (artificial food chains involved in the dairy industry). 90Sr is a common radionuclide in the same chemical group (family) as calcium and has a half-life of 28 years. Because of this, 90Sr is concentrated by dairy cows in their bones and milk and as a result is concentrated in the bones and milk of organisms that consume that milk, namely humans and nursing mothers. In areas such as Chernobyl and Fukushima, where 90Sr contamination is significant, the migration of radioactivity through food webs into the human population is very problematic.
Another example, and potentially even more insidious, involves tritium (3H, containing two neutrons, one proton and one electron) which is a radioisotope of hydrogen (H), the major component of dihydrogen monoxide (H2O), more commonly known as water, the “universal solvent.” Tritium has a half-life of 12 years. Water is the single most important compound to the vast majority of living organisms because not only does it serve as the intra-cellular (within the cell) medium for all biochemical reactions but also because it serves as the solvent making up the soil-solution containing the mineral compounds (solutes) taken up by plants.
In addition to this, and even more importantly, water is utilized in conjunction with carbon-12 (12C, containing six neutrons, six protons and six electrons, 666, yikes!) in the process of photosynthesis whereby plants, using solar energy (visible light), “dismantle” the carbon dioxide (CO2) and water (H2O) molecules and use the resulting carbon, oxygen and hydrogen atoms to synthesize glucose sugar (C6H12O6), the primary energy molecule for the vast majority of living organisms. If you add to this the increased presence of carbon-14 (14C, containing two extra neutrons) in the atmosphere you have a potentially significant uptake of radionuclides into not only glucose (the energy molecule) but also into the very organic compounds such as amino acids (the building blocks of proteins) and the various nitrogen bases, which in conjunction with ribose sugar, make up the DNA (deoxyribonucleic acid) molecule. The DNA molecule is the genetic information storage and “transmitter” molecule for most living organisms. What are the implications for the “genetic future” of life on Earth if the fundamental information transmitter molecule is so drastically impacted by the presence of radionuclides known for their mutagenic (mutation inducing) properties? This is one of the most critical (and far reaching) concerns in understanding the ecological impacts of radiation. “There is only one constant in the Universe, change.” In part 2, I will explore the implications. Thanks again for reading! Peace!