This article was researched and written by a student at Mount Holyoke College participating in the Encyclopedia of Earth's (EoE) Student Science Communication Project. The project encourages students in undergraduate and graduate programs to write about timely scientific issues under close faculty guidance. All articles have been reviewed by internal EoE editors, and by independent experts on each topic.

Algal Blooms and Harmful Algal Blooms (HABs)

Algal blooms were once considered solely a natural phenomenon in coastal ecosystems. But, in recent years, the frequency and severity of algal blooms have increased dramatically, due primarily to anthropogenic activities that create agricultural and sewage runoff resulting in nutrient loading in coastal ecosystems. This runoff and nutrient loading stimulates the growth of many algal species. Algal blooms that have harmful implications to the surrounding environment and to human activities are known as harmful algal blooms (HABs).

HABs can impair the general health of aquatic ecosystems. Usually, the degradation is due to HABs' high respiration rates at night, which subsequently deplete oxygen in the water column that is needed by other organisms. Often, this can result in the death of these organisms. Additionally, some harmful algae produce chemical poisons (toxins) that cause many adverse human impacts and economic effects. These can include health threats to humans, usually through consumption of contaminated seafood, and inhalation of toxins. Also, HABs impact coastal fisheries, and tourism. According to the National Oceanic and Atmospheric Administration, on average, HABs cause around $85 million in damages per year in the United States (U.S.) alone. There are many different species of algae that produce harmful toxins, but one of the most potentially destructive is Pfiesteria.


caption A few forms of Pfiesteria. (Source: Wikipedia)

Pfiesteria, referred to by the popular media as “the cell from hell,” is a dinoflagellate—and, unlike most dinoflagellates, has a complex life cycle, consisting of about 20 different stages.

Pfiesteria can become toxic in several of these stages, and the toxin production is enhanced when Pfiesteria detects excreta and secreta from live finfish. Under the right environmental conditions, when Pfiesteria is exposed to a large number of finfish (for example, a school of fish), the dinoflagellate increases toxin production. When finfish are not present—or when only a few fish are present—Pfiesteria takes a nontoxic form or becomes dormant. When live fish are absent, Pfiesteria forms have very little toxin production and are “prey generalists,” consuming a wide array of other organisms. Although Pfiesteria is a colorless, animal-like dinoflagellate, it can gain chloroplasts by ingesting phytoplankton; and it at times retains the “kleptochloroplasts” (that is, "stolen" chloroplasts) for short periods to supplement its nutrition through photosynthesis.

caption Pfiesteria outbreaks occur usually in coastal estuaries impacted heavily by anthropogenic activities. (Source: U.S. EPA)

Pfiesteria occurs all over the world, but the only toxic blooms that have been reported have occurred along the eastern coast of the United States, specifically in estuaries of North Carolina—where well over a billion fish died in toxic outbreaks during the 1990s—and within the Chesapeake Bay area of Maryland which lost about 50,000 fish in toxic outbreaks. Outbreaks tend to be limited to slowly moving estuarine waters, and are confined mainly to areas that already have conditions of poor water quality due to nutrient over-enrichment from anthropogenic activities. Also, Pfiesteria blooms are limited by temperature, as they occur only during warm months—mainly May through October along the east coast of the U.S.

It has taken, on average, 15-25 years to identify the toxins of other toxic dinoflagellates. This is a challenging process because the extremely small amounts of highly potent substances must be separated out from the many other compounds that occur in estuarine waters, then purified, and finally, characterized chemically. Pfiesteria requires specific culture conditions to stimulate toxin production, and it must be maintained in expensive biohazard III containment systems, making the toxins difficult to study. Also, the toxins it produces are very unstable and remain active only for hours, making them difficult to purify rapidly enough to enable their analysis. In 2007, however, after only nine years, the toxins were identified as metal-based, highly reactive oxygen species.

Fish Kills

caption Lesions caused by Pfiesteria. (Source: USGS)

Pfiesteria blooms are most well known for the extensive fish kills that result from outbreaks in estuaries. The presence of live fish stimulates Pfiesteria to release powerful toxins that stun the fish and begin to destroy the outer epidermis (skin), resulting in damage and, sometimes, sores. Once the fish is dead, Pfiesteria reduces its toxin production and feeds on the fish remains either as a swimming stage called a zoospore or as other forms. When conditions change and live fish are removed from the environment, Pfiesteria assumes nontoxic stages or becomes dormant. Also, Pfiesteria has been shown to adversely affect or kill various shellfish species and other aquatic life.

As nutrient-enriched waters become more prevalent due to poor management of human and animal wastes, temperature, and other environmental conditions, the frequency of fish kills has increased, including those related to Pfiesteria.

Human Health

Pfiesteria can have an effect on human health. These effects were first experienced by—and exhibited in—scientists working in close quarters with Pfiesteria. The most prevalent symptom is a learning impairment that manifests as short-term memory loss (similar to Alzheimer’s disease). Rats exposed to Pfiesteria showed significant learning impairments; but further studies need to be done in order to determine what this means for humans. Other symptoms include nausea, headaches, difficulty breathing, blurred vision, and the development of lesions. In one extreme case, a researcher working with Pfiesteria experienced a reduction in reading level equivalent to that of a seven-year-old for three months. Most cases involving health impacts resulting from Pfiesteria exposure are not this extreme and have occurred in such people who work very close to contaminated waters as fishermen and scientists surveying fish kills related to the blooms.

Climate, Weather and Pfiesteria

There seems to be a significant correlation between major storm activity and toxic Pfiesteria outbreaks. The last toxic bloom of Pfiesteria occurred in 1999 in North Carolina when four major hurricanes struck the coastline. Before the mid-1990s, when most Pfiesteria outbreaks occurred, the North Carolina coast experienced approximately 30 hurricane-free years. Since the 1999 hurricane season, there have been sparse, nontoxic populations of Pfiesteria detected in estuaries where outbreaks had been most abundant. This suggests that it will take decades of hurricane-free years for the residual populations to rebuild. Hopefully during the interim, scientists can learn enough about Pfiesteria toxins to enable the development of improved methods for detecting the toxins—and for mitigating or preventing their effects on human health—during future blooms and Pfiesteria-related fish kills.


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Hasselgren, E. (2014). Pfiesteria. Retrieved from


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