The invaders belong to the invisible universe of algae, dinoflagellates and other one-celled organisms that form the base of the food chain in the world's oceans. A minute fraction are known to produce neurotoxins and other poisons that can harm higher animal forms. Some of the outbreaks, such as the pfiesteria attacks in North Carolina and Maryland, have killed large numbers of fish and prompted the closure of waterways. As recently as last week, an apparently unknown species of toxic algae was suspected in the deaths of thousands of reef-dwelling tropical fish off the coast of southeast Florida. Many of the fish were covered with sores similar to those caused by pfiesteria.
Some microbes are a direct threat to people.
At least five kinds of seafood-borne illnesses are known to be caused by toxic algae, two of which are potentially fatal to humans. One of them, amnesic shellfish poisoning, can cause neurological symptoms such as disorientation and memory loss. Other toxic outbreaks have decimated wildlife populations. This year, the toxic algae attack that destroyed scores of dolphins in Mexico's Sea of Cortez also killed a sea lion and at least four whales. While scientists cite strong circumstantial evidence that links human pollution to the recent outbreaks, they acknowledge that a cause has not yet been established.