Researchers at the Romberg Tiburon Center for Environmental Studies are uncovering what’s behind harmful toxins in the ocean that sicken thousands of sea mammals and birds every year.
The lead scientist at San Francisco State University’s research center in Tiburon found that nitrogen entering the ocean — whether through natural processes or pollution — boosts the growth and toxicity of a group of phytoplankton that in turn can cause the potent toxin called domoic acid.
“Regardless of its source, nitrogen has a powerful impact on the growth of phytoplankton that are the foundation of the marine food web,” said William Cochlan, senior research scientist at Romberg. “Scientists and regulators need to be aware of the implications of both natural and pollutant sources of nitrogen entering the sea.”
When the phytoplankton produces domoic acid in algae, the nerve toxin is absorbed by anchovies, mussels and small fish, like sardines. Those animals, in turn, are eaten by larger sea animals in great numbers, and they become sick and die.
Each year the Marine Mammal Center in the Marin Headlands treats dozens of sea lions sickened by the domoic acid. In 2012, 14 percent of the 235 sea lions brought in for treatment suffered from domoic acid poisoning.
It often affects the brain, causing seizures and sometimes impairs navigational abilities.
“We have tracked sea lions that swim out to sea and they drown because they can’t haul out (on land),” said Jim Oswald, spokesman for the mammal center. “Conversely, some end up on roads and in cornfields. This is not always because of domoic acid, but it can be the cause.”
While cases involving humans are rare, domoic acid poisoning can cause disorientation and memory loss that can become permanent. In 1987, more than 100 people in Prince Edward Island, Canada, became ill and suffered neurological problems after eating cultured blue mussels contaminated with domoic acid. Three people died.
Biologists believe domoic acid poisoning was also the cause of a massive bird poisoning in Santa Cruz in 1961, which resulted in birds acting as if they were intoxicated. The event was part of the inspiration for Alfred Hitchcock’s movie “The Birds.”
The Romberg team found nitrogen can occur naturally in marine waters dbecause of coastal upwelling, which draws cool, nutrient-rich water containing nitrates from deeper depths into sunlit surface waters. Pollution, including agricultural runoff containing fertilizer and effluent from sewage plants, is also responsible for adding nitrogen, including ammonium and urea, to ocean waters, but in most regions these types of nitrogen occur at relatively low concentrations.
In laboratory studies, Cochlan and former graduate student Maureen Auro found that natural and pollution-caused nitrogen equally support the growth of the harmful algae cause the production of the domoic acid. But in all cases the natural form of nitrogen caused the most toxic cells.
Scientists already know toxicity increases when the cells grow slower, but in previous studies the slowing of cellular growth was due to the limitation of vital nutrients, such as silicate.
Cochlan’s latest study, however, found that the toxicity of these small phytoplankton is affected by the type of nitrogen they consume. He found that under low light levels — leading to slow growth — phytoplankton cells that were fed on naturally occurring nitrate were more toxic than cells that were fed on either urea or ammonium-caused by pollution.
“The findings may shed light on why these microorganisms produce a potent neurotoxin,” Cochlan said.
The study was published in the February issue of the Journal of Phycology.
Contact Mark Prado via email at mprado@marinij.com
