Biology 3602A/B Lecture Notes - Lecture 37: Paralytic Shellfish Poisoning, Amnesic Shellfish Poisoning, Algal Bloom

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5 Aug 2016
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These algal blooms happen from unnatural enrichment of macronutrients in aquatic
environments
They cause such incidents called “red tide” b/c of red chlorophyll from red algae
Some algae are green
Left: satellite photo with enhanced colour, showing chlorophyll off the south coast of the
UK
Macronutrients important for algal growth: phosphates, nitrates
How can these be harmful?
In Florida, red tide affected manatees  algal blooms causing physiological effects on
manatees, going into seizures – some kind of neurological effect
They can have effects on APs
One really important algae responsible for harmful algal bloom is called Alexandrium
fundyense (which is from the Bay of Fundy)
Make up phytoplankton, and are dinoflagellates
Dinoflagellates can put out a toxin called saxitoxin, which can cause a disease called
paralytic shellfish poisoning
Saxitoxin blocks voltage-gated sodium channels, responsible for depolarizing a
membrane once it reaches threshold
When reached membrane pot’l threshold, causes voltage-gated Na channels to open
and then you get an AP – unless there’s saxitoxin in there
Saxitoxin will block the sodium channel so even if it opens up sodium won't flow and
APs won’t occur
You become paralyzed afterwards
If you take a collective nerve from a mammal and expose it to diff concs of saxitoxin and
look at AP (top right fig):
No STX: if stim’d, full AP occurs
Bit of STX: knocks down some of the axons in the nerve so much smaller compound AP
– only some of the neurons can fire
More STX: block all Na channels so AP cannot travel down nerve, rendering nerve
useless
Dinoflagellates like Karenia, when in overproduction, produces brevetoxin, and instead
of blocking the voltage-gated Na channels, it permanently activates them so voltage-
gated Na channels which should be closed when reaching membrane potential will
always be open
All Na will leak into cell and cell will die from having too much sodium
This will become toxic to the neurons and thus you get neurotoxic shellfish poisoning
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How do animals like manatees and humans get these diseases? We don’t eat a lot of
dinoflagellates/algae, but sometimes we eat clams
Clams are filter-feeders, and it burrows underneath the mud, sends siphons up to
surface, draws in water, and filters out a lot of phytoplankton – so they eat a lot of this
algae and store toxins within their tissues
A molecule of those toxins can last up to 2 years in the tissues
Clams / bivalve molluscs are a lot less sensitive to the toxin than other animals
The voltage-gated Na channel has mutated in these animals and leads to less
sensitivity to saxitoxin in clams, so less risk of shellfish poisoning to the clams
This graph sums up the results of this study: on x axis is conc of saxitoxin (STX) and y
axis is % reduction in compound AP amplitude; open circles are frog neurons and the
closed circles are clam neurons
Higher conc: greater reduction in AP in frog, and starts at really low concs, but you have
to get to really high concs of saxitoxin before you knock down the APs in the clam
There are 1 or 2 specific point mutations in the homologous gene that is responsible for
this
Clams can eat the algae and not get sick
But if something eats the clam, toxins stored in the clams can get into the other animal
and cause neurotoxic poisoning
These poisons are pretty stable under heating, so cooking the clams into a chowder
won’t help; they’re also chemically stable (stable at low pHs)
So if stable in animal not much you can do
So you get higher incidence of these harmful algal blooms when there’s eutrophication
(when there’s a lot of human input in areas where algae grow)
In winter time, you can put a lot of nitrates and phosphates in there, but it’s too cold and
not enough sunlight for algal bloom to occur; but in summer, lots of sunlight and
nutrients so algae will bloom
In east coast of Canada: don’t harvest/eat clams in months that don’t have an r in it
(May, June, July, Aug)
Algae and how they can be affected
Global climate change: maybe we can mitigate [make less severe] global climate
change if we fertilize the ocean; if we fertilize the ocean, that will stim algae growth and
draw CO2 out of environment and maybe provide more food for fish
One of the micronutrients that people thought were very important for algal growth was
iron; growth of algae might be lim’d by iron under some circumstances
Project: dump iron fertilizer into ocean
Results: satellite view of chlorophyll level; a lot of high chlorophyll level close to the
coast b/c water is warmer, shallower, and more nutrients there
A couple days after dumping the iron, a lot of high chlorophyll levels going out into the
Pacific
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