2
Circular Versus Linear

CHAPTER MENU (NOTE: ENVIRONMENTAL TOXICOLOGY INCLUDES AQUATIC AND TERRESTRIAL TOXICOLOGY)

1. 2.1 Ecotoxicology and Environmental Toxicology
2. 2.1.1 Environmental Toxicology: In the Beginning
3. 2.1.2 What was the Context for Rachel Carson's Landmark Publication Which Led to the Development of Environmental Toxicology?
4. 2.2 Forgotten Chapter of DDT
5. 2.3 The Origins of Toxicity Testing as the Keystone to Environmental Toxicology
6. 2.3.1 What are the Toxicants Subject to Toxicity Testing?
7. Problem Set
8. Recommended Reading

2.1 Ecotoxicology and Environmental Toxicology

To detangle the science of ecotoxicology from the applied methods of environmental toxicology, the context for each is required. As defined in Chapter 1, ecotoxicology is the study of anthropogenic impacts on ecosystem structure and function. It is circular and regenerative. It has its genesis in an attempt to deal with the vast human alterations of our planet, from ocean acidification to terrestrial habitat loss within an ecosystem context. Ecotoxicology, through the study of ecosystem structure and function, strives to understand how the ecosystem has responded to anthropogenic activities such as climate change, and by doing so, implement restoration strategies to offset the impact. By contrast, environmental toxicology with the application of LD50s and single-species toxicity tests using death as the most extreme end point to determine contaminant safety guidelines is linear and terminal (Table 2.1, Figure 2.1). Environmental toxicology had its roots in pharmacology, and its approach has not changed in 50?years (Bhattacharya et al. 2011), although there are attempts to move away from animal testing and more humane practices (Burden et al. 2020).

2.1.1 Environmental Toxicology: In the Beginning

Some would place the beginnings of environmental toxicology with the publication of Silent Spring in 1962 by Rachel Carson (Figure 2.2, timeline of the development initiation of the Society of Environmental Toxicologists and Chemists).

Table 2.1 Commonly used toxicity tests in aquatic toxicology. (For a more comprehensive list, see https://www.canada.ca/en/environment-climate-change/services/wildlife-research-landscape-science/biological-test-method-publications.html.)

1. Acute Toxicity Tests (LC50/EC50): Short-term tests to determine the concentration of a substance that causes lethal effects (LC50) or 50% effect (EC50) in a specified percentage of tested organisms within a set duration, typically 48-96?hours. Fish, invertebrates, and algae are the most common test organisms.
2. Fish Early-Life Stage (FELS) Toxicity Test: This test evaluates the effects of substances on fish from the embryonic stage through early development and into juvenile stages. It assesses parameters like survival, growth, and developmental abnormalities over a period that can extend up to several weeks.
3. Chronic Daphnia Reproduction Test: It is used to determine the effects of substances on the reproduction of Daphnia magna (a type of water flea). This test measures reproductive output, growth, and survival over a period of 21?days, providing insights into both lethal and sublethal effects of toxicants.
4. Algae Growth Inhibition Test: This test measures the effect of substances on the growth rate of algae over a period of 72?hours.
5. Amphibian Metamorphosis Assay: This test evaluates the effects of substances on the hormonal regulation of amphibian development, particularly thyroid-mediated processes. This assay is applied for screening chemicals that may disrupt endocrine systems during the sensitive stages of amphibian life cycles.
6. Sediment Toxicity Tests: These tests assess the effects of contaminated sediments on benthic organisms. Test organisms, such as oligochaetes, amphipods, or chironomid larvae, are exposed to sediment samples, and end points, such as survival, growth, and reproduction, are measured.
7. Microtox ® Test: A rapid screening test that uses bioluminescent bacteria (usually Vibrio fischeri) to assess the toxicity of water samples. The decrease in bioluminescence after exposure to toxic substances serves as a measure of acute toxicity.
8. Biochemical Oxygen Demand (BOD) Test: Though not a direct measure of toxicity, the BOD test is used to evaluate the potential of organic pollutants to deplete oxygen levels in water bodies, indirectly assessing the impact on aquatic life.
9. Whole Effluent Toxicity (WET) Testing: WET tests assess the combined effects of all pollutants in wastewater (effluent) on aquatic organisms. This approach evaluates the overall potential impact of effluent discharge into aquatic environments.
10. High-Content Analysis (HCA): This technique combines automated microscopy with quantitative image analysis to assess multiple biological parameters simultaneously, allowing for a detailed understanding of toxicological effects at the cellular level.

Widespread use of dichlorodiphenyltrichloroethane (DDT) had commenced in the early 1940s; six years later, the consequences of the widespread use of DDT were being reported. In 1946, Clarence Cottam and Elmer Higgins of the US Fish and Wildlife Service warned in the Journal of Economic Entomology that the pesticide was a potential menace to mammals, birds, fishes, and other wildlife and that special care should be taken to avoid its application to streams, lakes, and coastal bays because of the sensitivity of fishes and crabs. It was, however, the publication of Silent Spring written by the journalist, Rachel Carson, that brought people's attention to the consequences of the indiscriminate use of pesticides, in this case, DDT. As noted by Woodwell in his seminal paper published in Science in 1967, "That DDT is a serious ecological hazard was recognized from the beginning of its use." Woodwell further comments, "Because of the wide-spread distribution of DDT the effects of the substance on a species of animals can be more damaging than hunting or the elimination of a habitat."

Figure 2.1 A comparison of the approaches taken in ecotoxicology as compared to environmental toxicology. Ecotoxicology is circular and regenerative, and environmental toxicology is linear and terminal.

Source: Bendell, L.I. 2024 ©.

Figure 2.2 Timeline of the development of the Society of Toxicologists and Chemists that was initiated to help direct the development of environmental toxicology and its use for the regulation of toxic chemicals.

Source: Setac/https://www.setac.org/static/e1cd428f-9901-4ec8-afdd95a675048a5f/2019-SETAC-Timeline-Large.png/Last accessed on December 6, 2024.

Figure 2.3 Dichlorodiphenyltrichloroethane (DDT) in linear and as a spatial model. The two benzene rings provide the stability of the compound. DDT is derived from coal tar, which is carbon, the backbone of life. The ability of carbon to create four bonds allows for the synthesis of highly stable compounds as found in nature.

Source: Ben Mills/Public domain/https://en.wikipedia.org/wiki/DDT#/media/File:DDT-from-xtal-3D-vdW.png/Last accessed on December 6, 2024.

2.1.2 What was the Context for Rachel Carson's Landmark Publication Which Led to the Development of Environmental Toxicology?

It begins c. 1942, toward the end of the Second World War and the beginning of what has been referred to as the "Anthropocene" or as historically stated our entry into a "brave new world." Perhaps the pesticide DDT, the subject of Carson's book, is the best example to follow through the timeline of a panacea-turned poison (Figure 2.3).

2.2 Forgotten Chapter of DDT

Stapleton (2005) provides an early history of DDT and begins his dialogue of events in late 1943, c. 2?years before the end of the Second World War. Typhus, a disease caused by bacteria carried by lice and spread through contact with body lice, was considered a "threat" to military personnel, and through a secret telegram, then President Eisenhower demanded that a shipment of a "secret new chemical" that controlled louse-borne typhus be given the highest priority (Stapleton, 2005). The DDT became critical to the Allied army and was called a "magic bullet" that, second to the atomic bomb, turned the outcome of the war toward the Allies. DDT prior to this time was a little-known compound with little purpose (Jaram and Ballschmiter, 2012).

DDT arose from coal tar in the late 1800s via the search for dyes and then for a compound that could protect the dyed material from moths. It was rediscovered by Paul Muller, a Swedish chemist, in the 1930s. As noted in Jaram and Ballschmiter (2012), Muller in his Nobel Prize acceptance speech said that the...