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Poisoning the waters

Natural History, Nov, 2003 by Stephan Reebs

Algae are a diverse crew. They range from single cells less than one ten-thousandth of an inch across to gigantic organisms hundreds of feet long. They're also the mainstay of the marine food chain, but that doesn't mean they passively accept their status as food. Certain microalgae actively emit toxins, and recent investigations show the toxins may have an offensive as well as a defensive role. Marine biologists Alf Skovgaard of the Institute of Marine Sciences in Barcelona, Spain, and Per Juel Hansen of the University of Copenhagen now have hard evidence that toxins from the microalga Prymnesium parvum may ward off competitors or even help the algae procure lunch.

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Marine biologists have long believed that algal toxins repel would-be predators, such as fish and crustaceans. And it is well known that when algae proliferate, their toxic blooms can wipe out a region's aquaculture or close down its seafood restaurants. P. parvum--a notorious source of toxic blooms worldwide--photosynthesizes like a plant, but it also has animal ambitions: the creature ingests prey, sidling up to other microorganisms and engulfing them. But that process works well only with prey that aren't mobile: single-celled algae, after all, have no limbs or mouthparts to catch and hold their next meal. P. parvum's way around that limitation is to secrete chemical stun guns.

Skovgaard and Hansen have shown that the higher the concentration of P. parvum secretions, the more the motile microorganisms become immobilized and the more R parvum move in to feed on them. And besides helping to provide meals, toxins could be disabling competitors and predators. In fact, the multifaceted function of toxins could contribute to the alga's periodic, and destructive, population explosions. ("Food uptake in the harmful alga Prymnesium parvum mediated by excreted toxins," Limnology and Oceanography 48:1161-66, May 2003)

Poisoning the waters

Natural History, Nov, 2003 by Stephan Reebs

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