Ecotoxicology living in these environments might be limited as exposure (bioavailability) is small Also the distribution of different species in the environment is not random, but populations of a particular species are concentrated at particular sites (niche) For example, in soil some species are found in the litter that lays on the soil surface (epigeic and hemigeic species), while others frequent the deeper mineral soil layers (eudaphic species) It is clear that epigeic and hemigeic species are more likely to be affected by toxic chemicals that enter the soil environment from above than eudaphic species Besides distribution, exposure will be determined by feeding patterns Organisms that feed on prey that is likely to have accumulated the substance will be exposed to larger quantities of the toxicant than those that feed on substances that are relatively uncontaminated Especially recalcitrant, low water soluble chemicals such as PCBs and organochlorines (DDT, for example) can be biomagnified in the environment to lethal concentrations higher up the food chain Furthermore, toxicity of each hazardous chemical should be related to the metabolism of each major species present, as affected by the toxicant Therefore, signs of toxicity need to be sought by a battery of methods before meaningful conclusions can be drawn on the environmental risk of a potential chemical released in the environment; environmental pollutants at a specified dose may cause changes in finely balanced ecosystems due to differential toxicity to different species and differential exposure of different species that inhabit the environment Much of ecotoxicology relates to the toxic effects of chemicals in natural ecosystems, but it must be remembered that man is an integral part of these ecosystems and effects on the other biotic components of the ecosystems of which he is part can affect him directly or indirectly For example, in relation to bioaccumulation of toxic chemicals, substances like PCBs, chlorinated organopesticides, and heavy metals, are likely to accumulate (and cause toxic effects) in the human body because of the simple fact that man is high up in the food chain Indirectly, man is dependent on the biosphere for the maintenance of the climate, purification of water and air, and the provision of a sustained food supply Disruption of biological activity that impairs the capacity of the living world to fulfill these functions will ultimately affect humankind’s well-being and survival In this respect, therefore, human toxicology and ecotoxicology are of necessity closely interlinked Application of Different Scientific Disciplines to Ecotoxicology Chemistry Many different xenotoxicants of many chemical structures are now ubiquitous in aquatic, terrestrial, and atmospheric environments Most, however, are present at extremely low concentration For example, there may be 60,000 molecularspecies of ‘‘hormone-disrupters’’ such as the xenoestrogens that may be detected (albeit by remarkably sensitive detector systems that can respond to even a few hundred molecules for a positive identification) Nevertheless, most of these 119 xenoestrogens are extremely weak molecular-mimics of human estrogens, and many of these can, paradoxically, stop the estrogenic response of particular target tissues Proposed chemical approaches to ecotoxicology are often analytical in nature and seek to quantify the effect of each chemical species that can be identified and assayed so as to define the concentration-dependence of any hazardous substance and its consequent environmental or human risk Quick tests for biohazards are essential, such as that achieved in the famous Ames Test for mutagenicity (and related carcinogenicity) in selected bacteria, for example, by the polyaromatic hydrocarbon, benzo(a)pyrene Such mutagenicity (novel growth-behavior) due to genetic (DNA) damage may manifest as cancer in mammalian organs (because of loss of growth-control in particular tissues, due to subtle alterations in regulation of cellular growth by particular genes) Biochemistry Biochemistry studies the chemical processes that take place in living organisms The main pertinent principle of biochemistry that can be applied to ecotoxicology is the modern concept of enzyme (biocatalytic proteins) regulation by molecular-intermediates of tissue metabolism in cells Many xenobiotics (foreign chemicals unusual in nature) can interfere with the finely balanced biochemical reactions of living cells by perturbation of the web of molecular interactions necessary for life Additional problems are due to the molecular damage to DNA (deoxyribonucleic acid) and its related RNA (ribonucleic acid) along with cellular membranes (phospholipids that are readily destroyed by reactive oxygen species (ROS), generated by reactions of these chemicals with atmospheric molecularoxygen) The ecotoxicity of ROS may, however, be ameliorated by antioxidants in the environment (and by dietary antioxidants such as vitamins C, E, A, and D as well as phytoestrogens in the diet) Microbiology The activity of microorganisms is at the basis for most functions on which life on this planet depends Due to their almost limitless metabolic capacities, microbial communities are responsible for the transformation and recycling of organic and inorganic molecules in the environment This activity results in the maintenance of nutrient cycles, the maintenance of soil fertility, and the detoxification of toxic substances in the environment Furthermore, many microorganisms are intimately associated with every conceivable higher life form on this earth They perform functions without which higher organisms could not function Examples of this kind are organelles that have a microbial origin, including mitochondria, which are responsible for the energy generation in eukaryotic organisms (organisms with a true nucleus), and chlorophyll that allows algae and higher plants to convert carbon dioxide into sugars via photosynthesis Other symbiotic interactions between microorganisms and plants that are of crucial importance for plant growth and nutrition are dinitrogen fixing microorganisms and mycorrhizae, which have close