Sinks below and stays there

By Susmita Dey
Published: Friday 15 May 2009

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.

"One would expect the domoic acid to rapidly leak out of the algal cells once they die. But most of it remains within the organisms as they sink through the water column. The algae have a silica cell that protects the material inside from leaking out completely. Moreover, the aggregations of the senesced diatoms are also relatively heavy which makes it easy for them to drop like stones to the seafloor," said Claudia Benitez-Nelson, associate professor, department of geological sciences, University of South Carolina, US, and one of the authors of the study.

This study, published in the March 22 issue of Nature Geoscience, for the first time provides proof of incorporation of DA into deep-sea sediments. "DA affects marine organisms, including seabirds, in the upper waters. Now deep-sea animals can also be exposed to such toxins originating from far above them," said Mary Wilcox Silver, ocean scientist, University of California, US.

Toxic blooms threaten coastal regions worldwide. "With increase in coastal population growth, inputs in coastal nutrients due to fertilizer runoff and sewage will also rise. Studies have linked increasing Pseudo-nitzschia blooms to rising nutrient inputs along the coast," said Benitez-Nelson.

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