Lecture 14 Flashcards
(44 cards)
1
Q
what are the characteristics of mercury?
A
- naturally occurring chemical element found in the earth’s crust, including in deposits of coal
- symbol Hg and atomic number 80
- only metal that is liquid at room temperature, melting point = -38.8 degrees C
- exists in several forms: elemental (metallic mercury), inorganic mercury compounds, methylmercury and other organic compounds
2
Q
what are the characteristics of elemental mercury?
A
- liquid at room temperature, relatively high vapour
- high electrical conductivity (used in thermometers, some electrical switches, fluorescent light bulbs)
- at room temp, exposed elemental mercury can evaporate to become an invisible, odorless, toxic vapour
- if heated, it is a colourless, odorless gas
- is an element that has not reacted with another substance
- when Hg reacts with another substance, it forms a compound, such as inorganic mercury salts or methylmercury
3
Q
what are the characteristics of inorganic mercury - Hg (I) mercurous (very little), Hg (II) mercuric
A
- mercury that is combined with other elements such as chlorine, sulfur, or oxygen and subsequently weather to form inorganic salts
- occurs abundantly in the environment, primarily as the minerals cinnabar and metacinnabar, and as impurities in other materials
- inorganic mercury salts can be transported in water and occur in soil
- water soluble
4
Q
how does inorganic mercury get into the environment?
A
- dust containing mercury’s inorganic salts can can enter the air from mining deposits of ores that contain mercury
- inorganic mercury can also enter water or soil from the weathering of rocks that contain inorganic mercury salts, and from factories or water treatment facilities that release water contaminated with mercury
5
Q
how does the inorganic form Hg(II) convert to the organic form methylmercury (MeHg)?
A
Microscopic organisms can combine mercury with carbon, thus converting it from an inorganic to organic form MeHg
6
Q
what is MeHg?
A
methylmercury = the most common organic mercury found in the environment and is highly toxic
7
Q
what is the origin of the phrase ‘mad as a hatter’?
A
- during the 18th and 19th centuries, felt hat makers used mercury to stiffen hats made from beaver pelts
- a form of mercury called mercuric nitrate was used
- hatters worked in poorly ventilated rooms and inhaled Hg vapours (inorganic form)
- developed symptoms of chronic Hg poisoning, including psychosis, excitability, and tremors
8
Q
what happened in minamata bay, japan?
A
- a factory was built in 1907
- in 1925, industrial runoff contained Hg that had been used in the production of acetaldehyde, a component of plastics
- pipe from the plant drained directly into the bay resulting in high levels of MeHg exposure to aquatic biota
- plastic industry boomed after WWII, and up to 600 tons of Hg was dumped into the bay
- 1956, MeHg levels in the bay exceeded safety standards by 400-fold
- residents consumed locally caught fish and shellfish
- resulted in minamata disease
9
Q
what is minamata disease?
A
- caused by MeHg poisoning
- ‘cat dancing disease’ = symptoms included staggering, salivating, convulsions, then they would collapse
- first case was a 6 year old girl hospitalized for imbalance and delerium
- killed ~40% of the acutely ill and left thousands with varying degrees of brain damage and paralysis
10
Q
what was the minamata convention on mercury?
A
- signed by 128 countries in 2013, calling for worldwide control of the anthropogenic releases of Hg
- attention then turned to clarifying the source types and countries that are responsible for the release of Hg, as well as the options for its reduction and how the effects of the treaty might be evaluated
11
Q
what happened in grassy narrows, dryden, ontario?
A
- grassy narrows first nation’s territorial waters are situated downstream from a former pulp and paper mill which discharged 9 metric tons of Hg into the river between 1962-1975
- caused irreparable damage to aquatic, animal and human life
- several indigenous communities living along the waterways were largely impacted
12
Q
what are the sources of mercury in the environment?
A
- natural
- anthropogenic
- re-emitted sources = the remobilization of settled mercury
13
Q
what are natural sources of mercury?
A
- emits 76-300 Mg/year globally
- emissions from the oceans
- weathering of rocks
- volcanic emissions
- forest fires
14
Q
what are the anthropogenic sources of mercury?
A
- emits 2000 mg/year globally (dominant source)
- industrial metal production (copper, zinc, lead, iron, steel, gold)
- artisanal and small-scale gold mining
- coal combustion
- oil combustion
- mercury production and use
- waste incineration (municipal and medical wastes)
- cement production
15
Q
would global Hg releases be higher or lower if it was proportional to industrial production, population, or economic growth?
A
- much higher
- the lower rate of Hg growth relative to population growth in more recent decades is the result of technological progress that are replaced old fashioned, high-polluting systems with more modern processes incorporating emission control
16
Q
what is the biogeochemical cycle of mercury?
A
- chemical stability and long residence time of elemental Hg in the atmosphere is 0.5-1 years, which allows for transport on a hemispheric to global scale
- Hg released to land can be transported to rivers and streams and find its way into lakes and oceans
- atmospherically deposited Hg can cycle through the surface environment in oceans, lakes, rivers, and soils and be re-emitted to the atmosphere
- considerable mobilization and re-distribution of Hg occurs overtime
- eventually, Hg is buried in estuarine or deep ocean sediment or stable terrestrial reservoirs, but the time scale for this sequestration ranges from decades to millenia
17
Q
what is the biogeochemical cycle of mercury? (2)
A
- the majority of Hg in the atmosphere is in the form of elemental Hg (typically >95%) however Hg can be oxidized by strong oxidants such as ozone and halogens through various gas-, aerosol-, and aqueous-phase reactions
- oxidation of Hg results in the formation of reactive gaseous Hg(II) and/or particulate Hg(II), both of which have short atmospheric residence times (minutes to weeks) and are thus efficiently removed from the atmosphere via wet and dry deposition to nearby landscapes
- once deposited Hg(II) can either be sequestered in soils and aquatic sediments or reduced to Hg(0) by biotic mechanisms
- Hg(0) can then be re-emitted to the atmosphere through diffusion and air-water gas exchange, thus completing the inorganic Hg cycle
18
Q
what are the post deposition processes of Hg to the arctic ocean that are susceptible to methylation?
A
1) atmospheric deposition to ocean surface
2) sediment methylation processes
3) riverine and terrestrial sources
4) ocean water column processes
- Hg(II) can be methylated by sulfate-reducing bacteria or other microbial communities and enter benthos or diffuse into the water column as MeHg
19
Q
what are the contaminant related characteristics of mercury?
A
- persistent
- bioaccumulates (MeHg, Hg(II))
- biomagnifies (MeHg)
- toxic
- most of the Hg occurring in tissues of upper trophic level organisms is in the form of MeHg
20
Q
MeHg is efficiently:
A
bioaccumulated and biomagnified due to the high affinity of the MeHg+ cation which helps to promote retention in both muscle and fatty tissues
21
Q
MeHg production in terrestrial and marine environments:
A
- atmospheric and riverine inputs are the two most important sources of Hg to arctic marine ecosystems
- MeHg production in terrestrial environments is thought to be negligible
- Aquatic environments, especially anaerobic environments such as freshwater and
marine sediments and hypolimnetic waters are important sites of Hg(II) methylation - Hg(II) methylation is primarily carried out by microorganisms such as sulfate reducing bacteria and iron-reducing bacteria and methanogenic archaea – result is MeHg
22
Q
is MeHg hydrophobic or hydrophilic?
A
- can be either
- can interact with inorganic or organic ligands to form
- the nature of the ligand dictates whether the MeHg will be hydrophobic or hydrophilic
23
Q
MeHg toxicity is known to affect the:
A
- nervous system
- cardiovascular system
- immune system
24
Q
who is at a higher risk of MeHg exposure?
A
- indigenous people
- populations with high consumption of fish and/or marine mammals
- average human blood Hg concentrations in northern communities where diets are composed predominantly of marine mammals and fish was 40 ug/L, well above the guidelines of 8ug/L
25
can MeHg be transferred from mother to offspring?
- MeHg is actively transferred across the placenta, with concentrations becoming higher in the fetus compared with those in maternal blood, which can lead to congenital MeHg poisoning
- exposure in utero can lead to slower neurologic development resulting in lower performance on tests of language, attention, memory, and visuospatial and/or motor functions
26
what ecological factors control MeHg concentrations in biota?
- food chain length (longer food chain length tend to result in higher concentrations in top predators)
- habitat selection
- foraging behaviour
- life history
27
how do we study Hg in marine mammals?
- tissues: muscle, liver, skin, teeth, claws, hair, whiskers, fecal, baleen plates
- analytical methods eg. cold vapour atomic fluorescence spectroscopy
28
how do marine mammals intake Hg?
- placenta and lactation, with more being transferred via the placenta
- water and air through the skin and lungs
- prey, which is the most important source
29
what is the target organ for Hg?
- the liver
- it is involved in the homeostatic regulation of ingested essential elements and the detoxification of non-essential elements
30
what are Hg concentrations in mediterranean cetaceans?
- highest in the world
- often above levels that cause adverse effects
31
mercury affects multiple systems in marine mammals:
- immune system (immunotoxicity)
- genes (genotoxicity)
- kidneys (nephrotoxicity)
- liver (hepatotoxicity)
- brain (neurotoxicity)
32
mercury and neurotoxicity:
- MeHg toxicity primarily affects central nervous system
- causes loss of neurons and gliosis with neurochemical variation in cerebellum
- results in sensory and motor deficits, causing behavioural changes
- high Hg:Se molar ratio in brain may provide some protection
33
hepatotoxicity and mercury:
- highest measured Hg concentrations; primary site of Hg demethylation (detoxification)
- causes oxidative stress, inhibition of lysosomal digestive enzymes
- damages subcellular membranes and deregulates apoptopic pathways
- excessive accumulation of lipofuscin (brownish pigment left over from the breakdown and absorption of damaged blood cells) results in liver disease
34
mercury and nephrotoxicity:
- all forms of mercury are nephrotoxic; inorganic forms are most acutely toxic
- HgCl thought to induce apoptosis in renal cells
- causes acute tubular necrosis as tubular epithelial cells die
- renal function is reduced
- high levels of Se in kidney may provide some protection
35
mercury and immunotoxicity:
- MeHg and HgCl inhibit lymphocyte and phagocyte function
- affects proliferation, expression of cell activation markers, cytokine production and phagocytosis
- Hg exposure could lead to reduced host disease resistance
- Hg may act as endocrine disrupter, affecting immune and inflammatory processes
- further investigation needed
36
mercury and genotoxicity:
- limited data on Hg genotoxicity in cetaceans
- some evidence of MeHg causing DNA single-strand breaks in vitro
- further experimental testing required
37
what does selenium do to mercury?
- selenium plays a crucial role in mitigating mercury toxicity in marine mammals
- Hg-Se complex formation: Se binds with Hg to form insoluble HgSe complexes. these complexes are less toxic and less bioavailable than free Hg
38
what is the molar ratio importance of selenium?
a 1:1 molar ration of Hg:Se is often observed in marine mammal tissues. this ratio is believed to be critical for Hg detoxification
39
what is tissue-specific accumulation for selenium?
HgSe complexes tend to accumulate in liver and kidney tissues
40
what are the antioxidant properties of selenium?
Se is a component of glutathione peroxidase, an important antioxidant enzyme
41
what are MeHg-Se interactions?
- marine mammals take in MeHg with their prey, which is absorbed, entering the liver and bloodstream
- MeHg binds to cysteine (mimicking methionone) and crosses cell membranes via amino acid transporters
- MeHg is detoxified by demethylation mediated by Se-containing biomolecules
- the formation of an insoluble mercury selenide species (HgSe) has been identified in marine mammals and may offer protection against high levels of MeHg exposure
- the biotransformation of MeHg to HgSe is still a poorly understood mechanism of demethylation in marine mammals
42
what are the detoxification differences between marine and terrestrial mammals?
- terrestrial mammals are mainly exposed to inorganic Hg and detoxify the metal with metallothioneins
- marine mammals are mainly exposed to the toxic form MeHg and have evolved the ability to demethylate and sequester the metal with selenium in a nontoxic compounds, HgSe
43
what are 3 knowledge gaps of mercury?
- limited cetacean specific data: most toxicological studies are conducted on rodents or other lab animals
- lack of controlled experiments: ethical and practical constraints prevent controlled Hg exposure studies in cetaceans
- effects of Hg combination with other environmental contaminants not well studied in cetaceans
44
what is the mercury cycle?
- inorganic mercury salts can become attached to airborne particles
- rain and snow deposit these particles on land
- even after mercury gets deposited on land, it often returns to the atmosphere, as
a gas or associated with particles, and then redeposits elsewhere
- Mercury cycles between the atmosphere, land, and water, undergoing a series of complex chemical and physical transformations
