The consumption of shark cartilage supplements and shark fin soup has been linked to the development of Alzheimer's disease and ALS (Lou Gehrig disease), due to biomagnification of a toxin produced by algae.
In a study conducted at the University of Miami, Florida, researchers discovered high levels of an algae derived neurotoxin in the fins of all seven species of sharks they examined. The toxin, beta-N-methylamino-l-alanine (BMAA), is produced by an alga that is ubiquitous on the earth and in the oceans - the tiny unicellular cyanobacteria, or blue/green algae.
Normally these algae are not a problem, but because they are present everywhere from the ocean floor to the surface the toxin from their cells can build up through the food chain and accumulate in significant amounts in the tissues of the apex predators, such as sharks. BMAA binds with collagen proteins in the shark fin cartilage.
Since some sharks have wide ranges or migrate, it is unpredictable which sharks will contain potent levels of the toxin, and the researchers found high amounts of BMAA in sharks where there had not been recent cyanobacteria blooms. They conclude:
Since BMAA has been linked to neurodegenerative diseases, these results may have important relevance to human health. We suggest that consumption of shark fins may increase the risk for human exposure to the cyanobacterial neurotoxin BMAA.
It is also possible that consumption of shark fins could lead to Parkinson's disease.
According to another study high levels of this toxin are correlated with Parkinsonian-dementia complex as well as Alzheimer's and ALS. The Chamorro people of Guam consider flying foxes (which are fruit bats) to be a delicacy, and BMAA is concentrated in the tissues of these bats through a form of cyanobacteria that is associated with seeds in that area. They write:
Although BMAA as a putative cause of ALS/PDC was initially disputed, this hypothesis has recently regained attention when it was discovered that BMAA is biomagnified within the Guam ecosystem and is found in the brain tissues of Chamorros who died of ALS/PDC, but not in patients who died of causes unrelated to neurodegenerative disease.
Once the toxin has been ingested it can bind with proteins in the body where it accumulates and is later broken down, explaining why these neurological diseases often appear years after consumption.
The consequences of cyanobacterial toxins on human health, water-based industries, recreation, and wildlife are of increasing concern as eutrophication and rising global temperatures trigger increases in the geographical extent, population densities, and duration of cyanobacterial blooms in fresh, brackish, and marine waters. Human poisonings from cyanobacterial blooms can be serious; 150 persons who drank cyanobacteria-contaminated water in Australia were hospitalized, and >50 kidney dialysis patients at a Brazilian clinic who were exposed to microcystins died.The algae itself is essential and the toxin appears to be the result of how the algae accomplishes its own life processes - processes which have allowed this prolific algae to oxygenate Earth's atmosphere and eventually create conditions that support human life.