☣️Toxicology Unit 8 – Environmental & Ecotoxicology

Key Concepts & Definitions

• Toxicology studies the adverse effects of chemical, physical, or biological agents on living organisms and the ecosystem
• Environmental toxicology focuses on the impact of pollutants on the environment and ecosystem health
• Ecotoxicology investigates the effects of toxicants on populations, communities, and ecosystems
• Toxicants are substances that can cause adverse effects on living organisms at certain doses or concentrations
• Pollutants are substances introduced into the environment that have undesirable effects on the ecosystem (pesticides, heavy metals, oil spills)
  • Can be classified as primary pollutants directly emitted from a source or secondary pollutants formed through chemical reactions in the environment
• Dose-response relationship describes the change in effect on an organism caused by differing levels of exposure to a toxicant
• Biomarkers are measurable indicators of exposure to a toxicant or its effects on an organism (enzyme levels, DNA damage)

Environmental Toxins & Pollutants

• Heavy metals (lead, mercury, cadmium) can accumulate in the environment and cause toxicity to organisms
  • Lead exposure can lead to neurological damage, developmental issues, and reproductive problems
  • Mercury can cause neurological disorders, especially in developing fetuses exposed through maternal consumption of contaminated fish
• Pesticides (insecticides, herbicides, fungicides) can have unintended effects on non-target species and disrupt ecosystem balance
  • DDT, a once widely used insecticide, caused eggshell thinning in birds leading to population declines
• Polychlorinated biphenyls (PCBs) are persistent organic pollutants that can bioaccumulate in the food chain
• Endocrine-disrupting chemicals (EDCs) interfere with hormone systems and can cause developmental, reproductive, and neurological issues (bisphenol A, phthalates)
• Oil spills can have devastating effects on marine ecosystems, coating organisms and disrupting food webs
• Microplastics, tiny plastic particles, can accumulate in the environment and be ingested by organisms causing physical and chemical harm

Toxicity Testing Methods

• In vitro tests use cell cultures or isolated tissues to assess the effects of toxicants on specific biological processes
  • Ames test uses bacteria to detect mutagenic compounds by measuring the rate of mutations
• In vivo tests involve whole living organisms to evaluate the effects of toxicants on survival, growth, reproduction, and behavior
  • Fish embryo toxicity (FET) test assesses the effects of chemicals on the early life stages of fish
• Acute toxicity tests determine the short-term effects of a toxicant, usually over a period of hours to days (LC50, LD50)
• Chronic toxicity tests evaluate the long-term effects of a toxicant, often spanning a significant portion of an organism's life cycle
• Mesocosm studies simulate natural ecosystems under controlled conditions to assess the impact of toxicants on communities and ecosystem processes
• Ecological risk assessment integrates toxicity data with environmental exposure and ecological effects to evaluate the potential risks of toxicants to ecosystems

Bioaccumulation & Biomagnification

• Bioaccumulation occurs when an organism absorbs a substance at a rate faster than it is lost, leading to an increase in concentration over time
  • Depends on factors such as the chemical's lipophilicity, metabolism, and excretion rates
• Biomagnification is the increasing concentration of a substance in the tissues of organisms at successively higher levels in a food chain
  • Persistent organic pollutants (POPs) like PCBs and DDT are prone to biomagnification due to their stability and lipophilicity
• Trophic transfer is the movement of a substance from one trophic level to another in a food chain through consumption
• Bioconcentration factor (BCF) is the ratio of a substance's concentration in an organism to its concentration in the surrounding environment
• Bioaccumulation and biomagnification can lead to high levels of toxicants in top predators, posing health risks to these species and humans who consume them
  • Mercury in predatory fish like tuna and sharks can reach levels unsafe for human consumption

Ecosystem Effects & Food Chain Impacts

• Toxicants can alter species composition and diversity in ecosystems by selectively impacting sensitive species
  • Pesticides can reduce insect populations, affecting pollination and food sources for insectivorous species
• Trophic cascades occur when changes in one trophic level indirectly affect other levels in the food chain
  • Decline in predatory bird populations due to DDT led to increases in small mammal populations and changes in plant communities
• Ecosystem services, such as nutrient cycling and water purification, can be impaired by the presence of toxicants
• Sublethal effects of toxicants on organisms (reduced growth, impaired reproduction) can have population-level consequences
• Habitat degradation caused by pollutants can lead to reduced biodiversity and altered ecosystem functioning
  • Oil spills can damage coastal habitats, impacting multiple species and their interactions
• Food web disruption can occur when keystone species are affected by toxicants, leading to cascading effects on the entire ecosystem

Risk Assessment & Management

• Ecological risk assessment evaluates the likelihood and consequences of adverse effects on ecosystems due to toxicant exposure
  • Problem formulation identifies the stressors, receptors, and endpoints of concern
  • Exposure assessment determines the concentration and duration of toxicant exposure to organisms
  • Effects assessment characterizes the relationship between toxicant exposure and ecological effects
  • Risk characterization integrates exposure and effects data to estimate the likelihood and magnitude of adverse effects
• Hazard quotient (HQ) compares the environmental concentration of a toxicant to a reference value indicative of ecological risk
• Ecological risk management involves decision-making and actions to reduce or mitigate the risks identified in the assessment
  • Establishing environmental quality standards and regulations for toxicant releases
  • Implementing best management practices (BMPs) to minimize the use and release of toxicants (integrated pest management, erosion control)
• Adaptive management is an iterative approach that incorporates monitoring, evaluation, and adjustment of risk management strategies based on new information
• Stakeholder involvement is crucial in risk assessment and management to incorporate diverse perspectives and ensure effective implementation of strategies

Case Studies & Real-World Applications

• Deepwater Horizon oil spill (2010) in the Gulf of Mexico caused widespread ecological damage to marine and coastal ecosystems
  • Impacted multiple species, from plankton to marine mammals, through direct exposure and food web disruption
  • Long-term effects on ecosystem health and recovery are still being studied
• Minamata disease in Japan resulted from methylmercury pollution in Minamata Bay due to industrial wastewater discharge
  • Biomagnification of methylmercury in the food chain led to severe neurological disorders in humans who consumed contaminated fish
• Dichlorodiphenyltrichloroethane (DDT) use as a pesticide in the mid-20th century caused significant declines in bird populations
  • Biomagnification of DDT in the food chain led to eggshell thinning and reproductive failure in predatory birds like bald eagles and peregrine falcons
  • DDT ban in many countries allowed for the recovery of affected bird populations
• Eutrophication of aquatic ecosystems due to nutrient pollution (nitrogen, phosphorus) from agricultural runoff and sewage discharge
  • Excess nutrients stimulate algal blooms, which can lead to hypoxia, fish kills, and changes in species composition
• Neonicotinoid insecticides have been linked to declines in pollinator populations, particularly honey bees
  • Sublethal effects on bee behavior and colony health can have cascading impacts on pollination services and agricultural productivity

Current Research & Future Directions

• Developing alternative testing methods that reduce the use of animals and increase the efficiency and predictive power of toxicity assessments
  • High-throughput screening (HTS) using automated in vitro assays to rapidly test large numbers of chemicals
  • In silico methods, such as quantitative structure-activity relationship (QSAR) models, predict toxicity based on chemical structure
• Investigating the effects of chemical mixtures and multiple stressors on ecosystem health
  • Evaluating the cumulative and interactive effects of toxicants, climate change, habitat loss, and invasive species
• Incorporating ecological complexity and resilience into risk assessment and management frameworks
  • Considering the role of species interactions, functional redundancy, and ecosystem services in assessing and mitigating ecological risks
• Advancing the use of omics technologies (genomics, proteomics, metabolomics) in ecotoxicology to better understand the mechanisms of toxicity and identify biomarkers of exposure and effect
• Developing sustainable and green chemistry approaches to minimize the production and release of toxic substances into the environment
  • Designing safer chemicals and processes that reduce the use of hazardous materials and generate less waste
• Promoting interdisciplinary collaboration among toxicologists, ecologists, chemists, and policymakers to address the complex challenges of environmental toxicology and develop effective solutions for ecosystem health and sustainability