IT鈥橲 been known for some time that large colonies of algae are deadly to fish because of the toxins they produce. But now it鈥檚 been found that one of their number, Pfiesteria, has a taste for flesh.
Pfiesteria and related algae are found around the world鈥檚 oceans. When they multiply in huge numbers, many species produce toxic blooms known as red 鈥渢ides鈥� which kill millions of fish. The huge amount of toxins released can also concentrate in shellfish and other marine life, so many countries monitor seafood to safeguard human health.
But Pfiesteria poses a problem for monitoring programmes because no one has isolated the poison it produces. 鈥淚 thought it would be fun to find and identify it,鈥� says chemist Bob Gawley of the University of Miami in Coral Gables, Florida.
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They started with Pfiesteria cultures that killed fish and removed the alga cells to isolate the toxin. But the alga-free water proved harmless. Reasoning that the toxin was somehow bound up with the algae, they tried breaking open the cells and extracting the debris with solvents. Again, they found nothing.
Finally the researchers searched the Pfiesteria genome for genes encoding proteins that produce polyketide toxins, the sort of poisons linked to the toxicity of other algae (Proceedings of the National Academy of Sciences, doi: 10.1073/pnas.172221699). They found none. 鈥淎t that point, we called off the toxin search,鈥� says Gawley.
Gawley鈥檚 collaborator Wolfgang K. Vogelbein of the Virginia Institute of Marine Science in Gloucester Point and his team tried a different tack. Suspecting that some contaminating organism could be the culprit, they purified bacteria and cells in the algae tank and retested their effect on fish. None proved to be lethal.
It wasn鈥檛 until the researchers put Pfiesteria under the microscope that they found out why. The algae were pulling the skin off the fish with a cone-shaped eating appendage known as a peduncle. As a result, the team suspects, the fish die from the osmotic shock of bare flesh exposed to sea water (Nature, DOI: 10.1038/nature01008).