Green tide

Natural History, May, 2005 by Stephan Reebs

250,000,000 B.C. -- Earth's most devastating mass extinction of the past half-billion years swept across our planet about 250 million years ago, bringing the Permian period to an end. Some estimates put the casualty rate at more than 90 percent of marine species and 70 percent of land species. So, who did the deed?

One prime suspect has been extreme "euxinia" near the surface of the ancient Tethys Sea. In euxinic seas, possibly because of shifting currents, normally bottom-dwelling sulfide moves up into the "photic zone," where oxygen-dependent photosynthesis takes place. The water soon becomes full of hydrogen sulfide and depleted of oxygen.

Sulfide is toxic to most organisms, but green sulfur bacteria thrive on it. So if photic-zone euxinia was widespread around the time of the Late Permian extinction, you'd expect to see signs of green sulfur bacteria in the rocks. Sure enough, Kliti Grice, a geochemist at the Curtin University of Technology in Perth, Australia, and her colleagues discovered that Late Permian sediments from both western Australia and southern China hold an abundance of the unique molecular fossils derived from the bacteria's odd biochemistry.

OK, euxinia may have killed off many residents of the sea. But how did it affect residents of the land? Grice's team suggests that the lapping of sulfide-laden waters onto continental shelves might have given rise to plumes of hydrogen sulfide gas that wafted across the landscape and poisoned the terrestrial species in its path. (Science 307:706-709, 2005)--S.R.