Metals as pollutants Flashcards
What group of naturally occurring elements most frequently cause toxicity from environmental exposures?
Heavy metals
What are some examples of heavy metals?
Mercury Lead Chromium Iron Geranium Cobalt Silver Cadmium Copper Molybdenum
What are some examples of lighter elements?
Aluminium
Arsenic
Selenium
What is the definition of background concentrations?
Concentrations of elements that occur in the environment in situations that have not been significantly influenced by anthropogenic emissions or unusual natural exposures.
What is toxicity?
The study of effects of poisonous substances on living organisms, including the way in which they gain entry into the organism.
What 2 factors is a toxic effect related to?
- the exposure or dose related to:
- available conc of poison in the environment
- period of exposure - the susceptibility of an organism to a specific poison being considered. “tolerance”
When can progressive bioaccumulation over time occur until?
Until a toxic dose or threshold concentration is exceeded
Long term exposure to a small concentration of some elements may cause a toxic effect
What is toxicity also affected by?
- the chemical form of the toxic element
- the chemical environment
- difference in susceptibility among individuals, populations and species
What are some anthropogenic sources of toxic elements?
- Agricultural/industrial processes
- inorganic pesticides
- sewage sludge
- irrigation systems - Metal mining and processing
- mining waste
- Fal estuary - Automobile emissions
- leaded petrol antiknock compound
What is the role of metals in marine animals?
- at least 20 essential elements and another 6 necessary for healthy growth
- several of the essential ones are metals e.g. oxygen transport - metallo-enzymes and enzyme cofactors where metals are required in electron transport processes
What is the classification of essential elements?
Major:
- bulk elements: C, H, N, O, S
- major ions: Na, K, Ca, Mg, P, Cl
Minor:
- essential: Co, Cu, Fe, Mn, I
- desirable: As, Cr, Ni, Sn, V, F
What is the effect of metals on cellular metabolism?
Memorise figure from lecture!
- several metals like Cu and Zn are beneficial at low concentrations
- intracellular concentrations of essential trace metals are controlled by homeostatic mechanisms
What are the toxicity tests?
- Lethal Dose (LD50); acute toxic effects are quantified by controlled experiments to determine the dose causing the immediate death of 50% of the organism exposed
- Median lethal time (LT50); time for death of 50% of a sample - depends on the concentration
- Median lethal concentration (LC50); e.g. 4 day LC50; short term tests exposure of aquatic organism to toxin for constant period of time
- Median Effect Concentration (EC50); e.g. 100 day EC50; reduction in growth response
How do toxic metals interfere with cellular metabolism?
- toxic metals displace essential metals in enzymes to form stronger complexes and thus interfere with cellular metabolism
- general binding to cellular components disrupts cell activity e.g. damage to cell membrane
- binding of metals may be specific e.g. Pb with nucleic acids
- organometallic compounds may be particularly toxic e.g. methyl mercury and tributyl tin
What are some forms of detoxification?
- excretion in waste products
- production of sulphur (-SH) containing low mol. wt. protein compounds or ‘metallothioneins’
- in plants similar compounds called ‘phytochelatins’
- other cellular components that isolate metals e.g. granule formation
When do metals become toxic (in terms of detoxification)?
When storage and other detoxification processes are exceeded.
What are factors affecting toxicity?
- speciation of metals; dissolved ionic form most toxic; particulate forms important in filter feeders
- increased temperatures and low salinities
- compounded or synergistic effects of multiple pollutants
- condition of organism or life stage
What are some detectable effects of metal pollution on marine organisms\?
- biochem response e.g. metallothionein production
- reduced growth rates
- morphological changes e.g. imposex in dog whelk
- reproduction and recruitment
- behaviour effects
- changes in community structure
Metals are conservative pollutants. What does this mean?
They’re not subject to bacterial attack or degradation and are permanent additions to the marine environment.
How does bioaccumulation of heavy metals occur?
Bioaccumulation occurs through metals binding to cell membranes and intracellular uptake.
It occurs within organisms if they’re unable to excrete the metal.
What is biomagnification or bioamplification?
When toxic metals are transferred through the food chain by feeding. If predators cannot excrete the metals, they in turn acquire a larger body burden of the metal.
What are the natural sources and uses of the toxic metal mercury?
Natural sources: weathering of mercury-bearing rocks and volcanic and geothermal vents. Mercury in seawater as hydroxide or chloride. Methyl mercury accumulates in sediments.
Uses: chlor-alkali industry, use of mercury electrodes, switches, slimicides in lumber and pulp industries, seed dressings (fungicide)
What is the maximum tolerable consumption of mercury in food according to the WHO?
0.03mg per week
What form is tin highly toxic in? what is it used in? what can it cause?
its organic form as TBT (Tributyl Tin oxide).
used as antifouling paint.
Leaches into sea in harbours and estuaries, causing deformed shells in oysters and imposex in common dog whelk
