by Matthew Schwartz
A new danger is threatening the economic stability of the west coast of the United States and has the potential to cause a public health crisis. A massive harmful algal bloom has accumulated across most of the west coast and may be the largest toxic algal bloom ever recorded . The bloom is a threat because it is releasing a toxin which is poisonous to both humans and marine animals. This threat must be managed carefully in order to protect humans and our vital ocean ecosystems.
At the base of ocean food chains are a variety of single-celled organisms called phytoplankton. Like plants, phytoplankton use photosynthesis and carbon dioxide to produce organic compounds and in turn provide food to sustain animal life in the oceans. One major group of algae, called diatoms, are one of the most common types of phytoplankton. They are notable for having cell walls made of silicon dioxide, the main component of sand and glass, and for their ability to exist as large colonies of filaments. Most diatoms are essential for healthy ocean ecosystems, but some species produce toxins which are a threat to both aquatic animals and humans.
A Tiny Toxic Threat
Diatoms of the genus Pseudo-nitzschia (Figure 1) produce a toxin called domoic acid. Domoic acid is able to cross the blood-brain barrier and cause permanent damage to nerve cells in mammals and birds. Domoic acid poisoning may result in short-term memory loss, seizures, vomiting, nausea, cardiac arrhythmia, and coma. At high enough levels, domoic acid poisoning may even be fatal. The first human domoic acid poisoning outbreak occurred when many people ate contaminated mussels from an algal bloom off the coast of Prince Edward Island in Canada in 1987 . It resulted in three deaths, illness in over 100 individuals, and permanent short-term memory loss in several survivors. Since domoic acid is not harmful to shellfish and fish, the toxin accumulates in the these animals when they consume the algae. These fish and shellfish then become unsafe for human consumption. Unfortunately, domoic acid is a highly stable compound, so even cooking or freezing affected fish and shellfish will not reduce their toxicity . Currently, there is no known treatment for domoic acid poisoning, so the effects will continue until the toxin is cleared from the body and in the most serious cases may be permanent .
How Domoic Acid Affects the Body
Domoic acid mimics glutamate, an important neurotransmitter that is secreted by neurons to signal to other neurons. Glutamate is typically excitatory, meaning that it sends activating signals between neurons. Domoic acid binds very strongly to the same receptors that detect glutamate, which results in neurons exposed to domoic acid becoming over-activated to the point of degeneration. Domoic acid is particularly harmful to the hippocampus and the amygdala, regions of the brain required for proper memory formation. Over-excitation of these neurons results in an increased flow of calcium into them, which at such uncontrolled levels causes irreversible neuronal degeneration .
Recent work in mice also suggests that domoic acid may be harmful to human kidneys at levels 100 times lower than those thought to be toxic to the brain. Our kidneys function as a filtration system for the body and must clear any domoic acid exposure, which accounts for why the kidneys are particularly susceptible to exposure. Like our brains, our kidneys also contain glutamate receptors. The accumulation of domoic acid in our kidneys results in damage to both the tubular cells of the kidneys, which are required for filtration, and to the vascular cells, which supply the kidneys with blood .
A Bloom of Unprecedented Size
The full extent of the algal bloom is still unknown, but it seems to stretch across much of the west coast of North America, from southern California to Alaska (Figure 2). Vera Trainer, the manager of the Marine Biotoxin Program at the National Oceanic and Atmospheric Administration’s (NOAA) Northwest Fisheries Science Center, has called the algal bloom the “longest lasting, highest toxicity, and densest bloom… ever seen .” The algal bloom was first detected in early May in Monterey Bay, but it is uncommon for such algal blooms to persist for more than a few weeks or to extend across such a large distance. This bloom is also atypical for occurring so early in the season; large algal blooms are more common in the fall than the spring. A research vessel from NOAA is currently investigating the full extent of the outbreak by collecting and testing algae and water samples for the presence of domoic acid, measuring the water temperature, and sampling fish which feed on algae from San Diego to Vancouver Island [1, 7].
The cause of such a large-scale harmful algal bloom is unfortunately still not entirely understood. Hopefully, the NOAA investigation will help scientists understand the cause and mitigate the effects of this outbreak. Scientists think the unprecedented scope of this algal bloom is at least in part because of the region of unusually warm water observed off the west coast since 2013. Warmer water temperatures prevent the oceans from mixing, and both increased temperatures and stagnant water can contribute to increased algal growth [6, 8, 9].
The economic effects of the algal bloom are already being felt across much of the coast. Commercial and recreational shellfish harvesting is currently banned (Figure 3) on much of the west coast and closures have even extended into the Puget Sound . Collecting razor clams, which are particularly susceptible to accumulation of domoic acid, has been banned in all of Washington and Oregon, costing coastal Washington communities as much as $9 million according to the NOAA . Commercial collection of Dungeness crabs has also been severely impacted. The commercial crabbing industry in Washington is worth approximately $84 million annually, and although the full impact of the closure is still unknown, currently 38-miles of prime coastline near the Columbia River are off limits to all crab harvesting .
Fortunately, the outbreak will not necessarily result in an impact on human health. Although there is no known treatment for domoic acid poisoning, careful monitoring of fisheries and their domoic acid levels should prevent any commercial harvesting of unsafe fish and shellfish. Recreational harvesting for personal use, however, is very dangerous and is currently largely forbidden across most of the coast . Of course, marine mammals and seabirds do not obey fishery closures. Harm to marine mammals and seabirds could have a highly disruptive impact on marine ecosystems. Deaths of whales, seals, birds, and fish have all been reported across the west coast. In addition to the poisonous effect of domoic acid on animals, as algae decomposes it also uses up all the oxygen available in the water—creating regional dead zones in the ocean which are unlivable .
The west coast, however, is not the only region under threat from harmful algal blooms this season. Unusually large blooms have also been seen this summer in Lake Erie, the Chesapeake Bay, the Baltic Sea, and along the Irish coast [12-15]. Even Boston’s own Jamaica Pond was closed to recreational activities including swimming, boating, and fishing for part of the summer due to a harmful algal bloom . If global climate change continues to cause rising ocean temperatures, then harmful algal blooms like the one off the west coast of North America may soon be all too common [1, 9].
Matthew Schwartz is a PhD candidate in the Biological and Biomedical Sciences program and the Department of Genetics at Harvard Medical School.
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