Lecture 6 - Bioaccumulation

Bioavailability

Proportion of a chemical in the environment that is available for uptake by organisms

Bioaccumulation

Process where concentration of a chemical in an organism increases over time because uptake exceeds excretion

Key condition for bioaccumulation

Uptake rate is faster than excretion rate

Bioconcentration

Accumulation of a chemical in an organism from water only

Bioconcentration Factor (BCF)

Ratio of chemical concentration in an organism to concentration in surrounding water

BCF formula

Corganism / Cwater

Main exposure source for BCF

Water only

Bioaccumulation Factor (BAF)

Ratio of concentration in organism to concentration in environment including all uptake routes

Difference between BCF and BAF

BAF includes food and water; BCF includes water only

Biomagnification

Increasing concentration of a chemical across trophic levels in a food web

Why bioaccumulative substances are hazardous

They can cause chronic toxicity at low environmental concentrations

Second major hazard of bioaccumulative substances

They can biomagnify, causing high concentrations in top predators

Why media concentrations underestimate exposure

Organisms bioaccumulate chemicals beyond environmental levels

Four ways organisms are exposed to contaminants

Maternal transfer, inhalation, ingestion, dermal transport

Common feature of all exposure routes

Uptake across a biological membrane

Main exposure routes for terrestrial organisms

Food and water, air respiration, air diffusion

Main exposure routes for aquatic organisms

Gill respiration, membrane diffusion, food ingestion

Primary uptake mechanism for most organics

Passive diffusion

Driving force for uptake across membranes

Fugacity gradient between environment and organism

Fugacity capacity of organisms compared to water

Generally higher than water for most contaminants

Effect of molecular weight on uptake

Uptake decreases when molecular weight exceeds ~700–1100

Effect of molecular size and shape on uptake

Larger or bulkier molecules have reduced membrane diffusion

Effect of molecular charge on uptake

Charged molecules cross membranes less easily

Speciation importance

Controls chemical form and bioavailability

Three main processes after uptake

Metabolism, storage, elimination

Metabolism in bioaccumulation

Enzymatic breakdown and detoxification of chemicals

Main storage location for neutral organic contaminants

Lipids

Why lipid storage matters

Stored chemicals can be released when fats are metabolized

Elimination definition

Removal of parent or transformed chemical from organism

Main elimination route for hydrophilic chemicals

Passive diffusion via urine, feces, sweat, or exhalation

Growth as an elimination process

Growth dilutes chemical concentration in tissues

Role of reproduction and lactation in elimination

Chemicals can be transferred to offspring

Biotransformation

Metabolic conversion of chemicals to different forms

Effect of lipid content on BCF

Higher lipid content leads to higher BCF values

Why lipid normalization is needed

To compare bioaccumulation between organisms with different fat contents

Lipid-normalized BCF formula

BCF divided by percent lipid

Typical lipid range in fish

Approximately 0.5 to 25 percent

Lipid content of fish oil

100 percent lipid

Effect of low Kow on bioaccumulation

Less storage and more rapid excretion

Effect of high Kow on bioaccumulation

Increased storage and reduced elimination up to a point

Relationship between water solubility and bioaccumulation

Bioaccumulation decreases as water solubility increases

Relationship between Kow and bioaccumulation

Bioaccumulation increases with Kow up to a point

Why very high Kow reduces BCF and BAF

Large molecules have reduced membrane diffusion

Threshold where BCF begins to decrease

Log BCF greater than ~6

Threshold where BAF begins to decrease

Log BAF greater than ~8

Biomagnification Factor (BMF)

Ratio of concentration in organism to concentration in its food

Primary exposure source for BMF

Food only

Trophic Magnification Factor (TMF)

Average biomagnification across multiple trophic levels

Alternate name for TMF

Food Web Magnification Factor

TMF value indicating biomagnification

Greater than 1

Measurement basis for BMF and TMF

Whole-body or lipid-normalized

Why highest trophic level is used in ERA

It represents worst-case exposure

Stable isotope used to determine trophic position

Nitrogen-15 (δ15N)

What δ15N indicates

Higher values indicate higher trophic position

Role of δ13C in food web analysis

Identifies carbon sources and food web structure

Why Arctic food webs are highly sensitive

Long-lived species, high fat content, global contaminant deposition

Process causing contaminant deposition in Arctic

Global distillation

Main route of human exposure to POPs

Diet

Why pregnant women in the Arctic have higher contaminant levels

Higher dietary exposure through traditional foods

Why contaminants are high in breast milk

Lipophilic chemicals concentrate in milk fat

Social impact of contaminant fear

Reduced consumption of traditional foods

Health consequence of dietary shift in Indigenous communities

Increased obesity and diabetes rates

Canadian bioaccumulation criteria

Log Kow greater than 5 and BCF or BAF greater than 5000

Purpose of Canadian bioaccumulation criteria

Identify substances for virtual elimination

Why whole-body BCFs and BAFs are preferred

They reflect food-chain transfer risk

Why fish BCFs are emphasized

Fish better represent bioaccumulation at higher trophic levels

Role of biomagnification evidence in risk assessment

Supports bioaccumulation classification when BCF or BAF data are limited