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Algae and Human Disease

Because sunlight is unavailable inside the human body, photosynthetic algae so not cause infectious disease. One alga, Prototheca, is unique in that it lacks chlorophyll and therefore the capacity to photosynthesize and is pathogen for humans. It causes the disease protothecosis, found principally among people living in the tropics. Prototheca is believed to be a variant of the green alga Chlorella that has adapted to a heterotrophic existence. Similarities in morphology mode of reproduction and several chemical features suggest that Prototheca arose from a Chlorella that lost its chloroplasts; Prototheca can be isolated from soil, water, sewage and the human gastrointestinal tract. Protothecosis, however, is rare occurring either in individuals with impaired immune systems or in populations with poor nutrition or poor sanitation. The most common manifestation is the formation of skin ulcer.
A more common problem associated with algae is paralytic shellfish poisoning, an occasionally fatal condition that follows consumption of shellfish contaminated with toxic algal products. Gonyaulax, certain species of Gymnodinium, and other dinoflagellates produce a potent nerve toxin that causes paralysis in mammals and humans. Shellfish that consume the algae are not affected by their toxin, which becomes concentrated in their tissues. Persons who then eat these shellfish may contract paralytic shellfish poisoning.
The danger of paralytic shellfish poisoning increases during seasons when conditions are favorable for algal multiplication. During these times the toxic dinofalgellates can increase to concentrations of 50,000 cells per milliliter of water, their pigments imparting a characteristic red color to the waters they inhabit. This phenomenon, known as red tide, can be correlated with a great abundance of neurotoxin in the shellfish of the region.
Paralytic shellfish poisoning is not always associated with red tide, since algal populations too small to change the water color are still dangerous. Ingestion of 0.5mg of toxin can be fatal to humans. This amount of toxin is easily acquired in a single meal of shellfish, even after freezing or routine cooking. There is no antidote for the poison; treatment is symptomatic and is usually begun by inducing the patient to vomit in an attempt to reduce to a non-lethal level the amount of toxin-contaminated food remaining in the victim. If treatment is not successful, death usually occurs within 3 to 12 hours.
It is important in disease presentation that coastal waters and shellfish be strictly monitored to identify the areas where shellfish are most likely to be contaminated. The disease is most commonly associated with shellfish harvested from the coastal waters of the Pacific Northwest and Alaska as well as those of New England and Florida. Red tide is also responsible for killing large populations of fish that either eat  the phytoplankton’s or are poisoned by absorbing the toxin released into the water (such as the fish killed by the “phantom” dinoflagelate discussed in this chapter’s introduction). Thousands of fish may also die when oxygen levels in the water are depleted during the night (when algae are consuming oxygen rather than producing it).
In recent years, toxic blooms have increased in frequency and new types of toxins, including some produced by diatoms, have joined the list of dangerous chemicals in the sea.
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