Diatoms: Our Friend, Our Foe

by EBJ

When most people think of photosynthesis they think of lush, green plants and trees, but that is only part of the story. One fifth of all photosynthesis on Earth is performed by a type of oceanic plankton called diatoms (1). Diatoms are unicellular brown algae that are unique in that they form their cell walls out of silica. Most of the carbon they fix feeds directly into the oceanic food chain; the remaining fraction is sequestered when it falls to the deep sea floor (1). Despite the vital role diatoms play in the global carbon cycle, a few species can cause life-threatening illness.

A diatom bloom off Vancouver Island (2)

Seafood causes one fifth of cases of food-borne illness in the United States, and half of these are due to toxins produced by algae (3). Some chemicals, such as these algal toxins, accumulate in the tissues of their predators, and become even more concentrated in the next predators in the chain. By the time it gets to our plate, seafood can have enough toxin to cause serious illness, and even death. The elderly and those with poor renal function are especially vulnerable to these toxins (3).

There are five illnesses caused by harmful algal blooms. One of these, amnesic shellfish poisoning, is caused by the diatom toxin domoic acid. There has only been one major human outbreak of ASP to date. In the fall of 1987, over 100 people on Prince Edward Island in Canada were affected, including three fatalities, after the ingestion of blue mussells infected with Pseudonitzschia multiseries (3, 4). They suffered from gastrointestinal and neurological symptoms including seizures and permanent short-term memory loss. Domoic acid excites glutamate receptors in the brain, causing neurons to load with Ca²⁺ and die, causing lesions mostly in the hippocampus, which is responsible for memory (4). It is under investigation as to whether long-term, low-level exposure is dangerous (5). Although domoic acid has been used in Japan for years to treat nematode and roundworm infections without adverse effect, the amount used is much less than that ingested by consumption of contaminated seafood (4).

In addition to the human outbreak in Canada, fish kills and several well documented incidents of widespread animal death have been observed. Cormorants and brown pelicans in Monteray Bay in California were poisoned with domoic acid in 1991 (6). They were found to have been feeding on anchovies contaminated with domoic acid from P. australis (7). Later that year, domoic acid was detected in clams and crabs along the entire West coast of the U.S. (6). In 1996, hundreds of brown pelicans were again affected, this time in Cabo San Lucas in Mexico (7). In the summer of 1998, a population of sealions was affected in the same area as the 1991 seabird outbreak in California. Four hundred sealions died, and many more showed neurological symptoms. Again, anchovies infected with P. australis were shown to be responsible. During the outbreak, blue mussels, like the ones that caused the outbreak in Canada a decade earlier, were shown not to be infected. This shows that simply monitoring mussel populations for contamination is not sufficient (8). Chemical screening of mussels before sale for consumption has prevented further incidents (3), but what about other seafood?

Clearly, various organisms can be affected by this toxin. Anchovies were the vector in two major incidents. They filter feed on plankton, and chemicals which are not metabolized or excreted accumulate in their flesh, and concentrate even more further up the food chain. During the 1991 outbreak, domoic acid was detected not only in the stomach contents, but also in the flesh of anchovies studied (6), suggesting that domoic acid may bioaccumulate in this manner. Anchovies are heavily preyed upon by not only seabirds and sealions, but dolphins, and salmon (6). It is possible that salmon stocks could get contaminated, though it is also possible that the salmon, as the secondary predator, would be poisoned itself before it would ever be caught for food. Theoretically anything that eats plankton, or that eats something that eats plankton, could be contaminated during a bloom and cause illness.

Instead of relying solely on mussels for detection, DNA probes have been used to monitor diatom blooms before they become a problem (6). Another approach may be to predict, or even prevent, blooms before they happen. Pseudonitzschia have been shown to produce more domoic acid when water pH is high, when starved for silicate, phosphate, or iron (9), and when grown with certain bacteria (10), but these conditions are difficult to control. Factors that influence diatom blooms that we do have control over incude local or regional eutrophication of coastal waters, dispersal of species by human travel and shipping, and the many effects of global warming (3).

Global warming changes weather and ocean current patterns, a slow rise in sea level and drop in pH. All of these have direct effects on blooms of algae, including diatoms. Both harmful diatoms and those that feed the ocean’s life are affected. In addition to continual monitoring of diatom activity and screening for seafood contamination, further research is crucial for us to understand how we influence both the diatoms toxic to us and those that drive life in our oceans.

References

1. Armbrust, E. V. “The Life of Diatoms in the World's Oceans.” Nature 459 (2009):185- 192.

2. marchettilab.web.unc.edu/research-for-non-scientists/

3. Van Dolah, F. M. "Marine Algal Toxins: Origins, Health Effects, and their Increased     Occurrence." Environmental Health Perspectives 108(2000):133-141.

4. Perl, T. M., et al. "An Outbreak of Toxic Encephalopathy Caused by Eating Mussels   Contaminated with Domoic Acid." The New England Journal of Medicine 322.25 (1990):1775-1780.

5. Andjelkovic, M., et al. “Exposure to domoic acid through shellfish consumption in Belgium.” Environ Int. 49 (2012):115-119.

6. Fritz, L, et al. "An Outbreak of Domoic Acid Poisoning Attributed to the Pennate Diatom Pseudonitzschia australis." Journal of Phycology 28.4 (1992):439-442.

7. Lefebvre, K.A., et al. "Detection of Domoic Acid in Northern Anchovies and California Sea Lions Associated with an Unusual Mortality Event." Natural Toxins 7.3 (1999):85-92.

8. Scholin, C.A., et al. "Mortality of Sea Lions Along the Central California Coast Linked to a Toxic Diatom Bloom." Nature 403.6765 (2000):80-84.

9. Lundholm, N., P.J. Hansen, and Y. Kotaki. "Effect of pH on Growth and Domoic Acid Production by Potentially Toxic Diatoms of the Genera Pseudonitzschia and Nitzschia." Marine Ecology Progress Series 273(2004):1-15.

10. Kotaki, Yuichi. "Production of Domoic Acid by Diverse Species of Pennate    Diatoms." Fisheries Science 68(2002):525-528.