Biologists Find Clams Deriving Energy From Chemicals in Sewage

Scientists studying half-dollar-size clams that live by an ocean sewage outfall near Los Angeles said Thursday that the small creatures are the first animals known to be capable of generating energy from inorganic compounds.

Until now, they said, the ability to transform inorganic molecules and compounds into life-sustaining energy was thought to be limited to bacteria. Thus the finding raises new possibilities about the basic workings of life, they said.

“This was very much of a surprise . . . but now we think it’s a more common phenomenon than just this animal,” said George Somero, professor of marine biology at the Scripps Institution of Oceanography here.

Somero and colleague Mark Powell, a Scripps graduate student at the time of the studies, reported their findings in this week’s Science magazine.

The implications of their discovery could be significant for ecologists and biochemists, Somero said.

Need for Food Chain

All other known animals depend--directly or indirectly--on a food chain that begins with photosynthesis in plants, according to Powell, who is now a post-doctoral researcher at the Salk Institute here. It is by eating and breaking down compounds such as sugar, fats, starches and proteins that animals obtain the energy and carbon necessary for their existence.

But the small orange clams living by a sewage outfall in Santa Monica Bay were found to have the ability to break down inorganic sulfide--derived from the decomposition of sewage--and transform it into energy.

“We knew bacteria could do that . . . but we were very, very surprised that an animal could carry out the first step in the oxidation of sulfide,” Powell said.

Equally surprising to the scientists was that the clams, Solemya reidi, were breaking down sulfide that was as toxic as cyanide.

But instead of having a poisonous effect, the clams were able not only to protect themselves, but also to gain a source of energy.

Somero said he believes a similar system may exist in humans, but on a much smaller scale that wouldn’t allow humans to overcome the toxicity levels of the sulfide.

The clams, which have no digestive tract, also rely on energy from bacteria that live in the clams’ gills, the scientists said. The bacteria further break down sulfur compounds in a second-step process that provides the clams with life-sustaining carbon.

The new finding raises the possibility that other animals living in or near sulfide-rich environments, like mud flats, mangrove swamps, deep-sea vents and sewage outfalls, may have a similar ability to metabolize inorganic compounds.

The scientists said they and others want to study more closely the biology of worms and other clams in places such as Bodega Bay in Northern California, Morro Bay and the Pacific Coast of Mexico. Somero said he and a group of French scientists are embarking on an expedition next May to hydro-thermal vents on the Pacific Ocean floor in search of large clams and tube worms to see if these animals also are metabolizing sulfide.

The scientists began studying the Santa Monica Bay clams about four years ago. It wasn’t until earlier this year, however, that Somero and Powell made their discovery of the clams’ ability to break down sulfide.