Algal neurotoxin enters deep-sea food web, kills whales

while crab cakes and marinated clams make for delightful cuisine, shellfish poisoning is a health risk. Shellfish feed on algae that produce harmful toxins. Domoic acid (DA) is one such neurotoxin, dangerously high levels of which are produced during algal blooms of the species, Pseudo-nitzschia. The toxin was earlier believed to dissolve in the waters, thus reducing risk of toxicity. A study revealed it sinks and enters the deep-sea food web via benthic feeders (benthic zone is the lowest level of a waterbody). This addresses the issue of previously unexplained high levels of DA in whales and sharks.

Consumption of DA-contaminated food leads to gastrointestinal illness, disorientation, seizures and death. In 1991, DA was detected in razor clams for the first time off the coast of Washington, US.The neurotoxin has since been responsible for the deaths of over 400 California sea lions in 1998 and left a trail of 80 sick pelicans off the Southern California coast in early 2006.

A team led by Emily Sekula-Wood from the University of South Carolina, US, deployed sediment traps up to a depth of 800m off the coast of southern California. The experiment was carried out from November 2004 to July 2006. The sediment samples were analyzed for the presence of DA. The results were correlated with surface bloom samples taken at a depth of 0-1m from surface sites corresponding to trap locations. The team found concentrations of the neurotoxin in surface waters and those measured in the sediment traps to be proportional. The US regulatory limit for DA in shellfish is 20,000 nanogrammes (ng) per gramme tissue. The concentrations obtained were eight times that limit ranging from 20 ng to 1,63,000 ng DA per gramme dry sediment weight.