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What is Paralytic shellfish poisoning and explain in detail?
Paralytic shellfish poisoning (PSP) is one of the four recognized syndromes of shellfish poisoning, which share some common features and are primarily associated with bivalve mollusks (such as mussels, clams, oysters and scallops). These shellfish are filter feeders and accumulate neurotoxins, chiefly saxitoxin, produced by microscopic algae, such as dinoflagellates, diatoms, and cyanobacteria. Dinoflagellates of the genus Alexandrium are the most numerous and widespread saxitoxin producers and are responsible for PSP blooms in subarctic, temperate, and tropical locations. The majority of toxic blooms have been caused by the morphospecies Alexandrium catenella, Alexandrium tamarense, Gonyaulax catenella and Alexandrium fundyense, which together comprise the A. tamarense species complex. In Asia, PSP is mostly associated with the occurrence of the species Pyrodinium bahamense.Some pufferfish, including the chamaeleon puffer, also contain saxitoxin, making their consumption hazardous.

PSP and cyanobacteria
Saxitoxin can be produced in both eukaryotic dinoflagellates as well as prokaryotic cyanobacteria (usually referred to as blue-green algae). The biosynthesis pathway of this toxin within cyanobacteria is well defined, while the pathway within dinoflagellates is mostly unknown.
Within cyanobacteria the saxitoxin pathway is complex with many steps, enzymes and chemical reactions. Using radioisotope tracing experiments scientists have determined how the initial reagent of the pathway, L-arginine (via acetyl-CoA), turns into this toxin. L-arginine goes through a rare reaction known as Claisen condensation forming four intermediates before turning into the final product of saxitoxin.Cyanobacteria is general responsible for the accumulation of PSP toxins, mainly being saxitoxin, within freshwater marine life. Studies have been completed showing that the Australian freshwater mussel Alathyria condola are highly susceptible to neurotoxin accumulation. This specific shellfish has shown upwards of 80 ug of neurotoxin accumulation for every 100g of the mussel after a 2-3 day of exposure to the cyanobacteria A. circinalis , which is significant enough to cause health risks for humans.

Pathophysiology
The toxins responsible for most shellfish poisonings are water insoluble,  heat and acid-stable, and ordinary cooking methods do not eliminate the toxins. The principal toxin responsible for PSP is saxitoxin. Some shellfish can store this toxin for several weeks after a harmful algal bloom passes, but others, such as butter clams, are known to store the toxin for up to two years. Additional toxins are found, such as neosaxitoxin and gonyautoxins I to IV. All of them act primarily on the nervous system.
PSP and its associated toxin can cause paralysis in extreme cases by targeting the sodium ion channels. Saxitoxin is able to bind to the receptor site of a cellular membrane near the sodium ion channel, blocking the entrance and not allowing potassium or sodium to pass into the cell. This blockage restricts the transmission of signals between neurons potentially resulting in paralysis.PSP can be fatal in extreme cases, particularly in immunocompromised individuals