The Romberg Report
Printed in The ARK, October 7, 1998
Life Has Evolved & Thrived Where Least Expected
Although the sulfide that poisons humans is usually produced by industrial processes, sulfide occurs naturally in many marine and aquatic environments. Some can be quite exotic, such as the spectacular hydrothermal vents on the ocean floor that spew out tons of sulfides as scalding water gushes up from volcanic activity miles below. Much more familiar sulfide habitats are in mud flats and marshes along the California coast, including some of the marshes along the Corte Madera shoreline and mud flats in Richardson Bay. (At these locations, the layer of sulfide is only a few inches thick and lies beneath the surface. You’d have to dig up quite a bit of it for it to be toxic to a human.)
These toxic sites can contain enough plant, animal and bacterial material to constitute a nutritional bonanza. Any animal that could manage to tolerate sulfide would be richly rewarded – an opportunity evolution rarely passes up. Exactly how these animals can thrive in environments "poisoned" by sulfide is the focus of intense investigation in the lab of Alissa Arp, director of the Romberg Tiburon Center. Alissa, several of her graduate students and I try to understand not only how and why these animals thrive in a sulfide-rich habitat, but also whether their adaptations can be used to understand the effects of sulfide on humans.
The Curse of the Expected
Until recently, biologists didn’t realize that life in high sulfide environments was even possible. Biology students investigating the diversity of animals living in mud flats and marshes were told that their studies should exclude the black, smelly mud that contained sulfide. They were told that the sulfide would contaminate their samples and kill any animals they collected and hoped to study. So when they began sifting through the sand and mud, they unintentionally guaranteed that they could never discover the animals living in sulfide.
All of this changed in 1977 when geologists studying deep-sea hydrothermal vents discovered vast communities of enormous worms, clams and crabs living immediately next to boiling hot water jetting from the sea floor. And if that weren’t amazing enough, the hot water contained seemingly lethal concentrations of sulfide. A graduate student at U.C. Santa Barbara at the time, Alissa Arp was a member of the first major expedition to study these amazing animals. "Many of us on that expedition were filled with disbelief at what we were seeing," she recalls. She and other biologists soon unraveled an amazing tale.
Many of the giant animals at the hydrothermal vents had no mouth or gut, and instead had massive colonies of bacteria living in specialized sacs in their bodies. The animals delivered sulfide to their bacteria using specialized hemoglobin, and the bacteria then used this sulfide for energy. In exchange for this generosity, the bacteria manufactured sugar molecules needed by their animal hosts. This relationship between the hydrothermal vent animals and their bacteria is a classic example of symbiosis.
Life Just Below the Surface
Excited by these new discoveries, and with a new appreciation for the abilities of animals to tolerate seemingly hostile conditions, biologists soon returned back to the mud flats and marshes, this time focusing exclusively on the thin layer of sulfide just below the surface. To their surprise, tiny bacteria-filled animals were present in huge numbers. Over the past 20 years, scientists have discovered that many of these animals have symbiotic relationships with bacteria, much like that in hydrothermal vents.
Biologists have recently discovered specialized animals in a number of other environments with high sulfide concentrations, such as in the mud of mangrove swamps, the effluent of paper pulp mills in Washington state, around Los Angeles sewer outlets off the coast of southern California and even the carcasses of dead whales on the sea floor. In fact, in almost every location on Earth where sulfide exists, communities of animals have been found that depend on it.
At the Romberg Center, we have been studying a variety of these environments. Over the past decade we have been primarily investigating animals from nearby mud flats, but we have now begun looking at animals from hydrothermal seeps in the Gulf of Mexico, from marshes in the San Francisco Bay, from the deep canyons of Monterey Bay and from hydrothermal vents off the coast of Mexico and Canada.
We just returned from a research expedition to hydrothermal vents in the north Pacific with an international team led by Charles Fisher at Pennsylvania State University. Diving to a depth of 6,600 feet in the three-person research submarine Alvin, I was able to observe and study large communities of animals completely dependent on sulfide. The density and variety of these animals was astounding. At every location where sulfide was venting from the sea floor, a colony of animals was there to exploit it.
We feel that this is a very exciting time in our research. Alissa Arp says, "We now recognize that we are studying a specialized group of animals that may exist in similar habitats across the Earth. This emerging understanding confirms the wisdom – in research as in life – of expecting the unexpected."
Editor's note: This is one of a series of articles written for The Ark by scientists at the Romberg Tiburon Center for Environmental Studies, San Francisco State University. David Julian, Ph.D., is an assistant research scientist at the center.
(The Ark is the weekly community newspaper of Tiburon, California)