Hundreds of meters beneath the surface of the Gulf of Mexico there is a lake.
"This brine pool is so dense," says biologist Chuck Fisher, "that when you come down onto it in a submarine, you bump. You float. Little ripples spread out. It's a very surreal experience."
Fisher is an expert on symbiosis, the state in which two life forms are fully intertwined. What draws him down to the briny pool are the mussel beds on its edges — mussels that have methane-eating bacteria packing their gills "like eggs in an Easter basket."
Another mussel, at a deeper site, has two symbionts: one that lives off methane, the other off sulfides.
Then there are the tube worms — "no mouth, no gut, no anus, essentially just a bag packed with bacteria and a plume" — which he finds both at cold seeps beneath the Gulf of Mexico, spots where methane- and sulfide-rich fluids escape from the Gulf's bed of salt and sediment and oil-bearing shales, and at the deep-sea hydrothermal vents, where the continental plates are spreading apart in the middle of the Pacific Ocean.
At hot or cold sites, the animals "have one thing in common," says Fisher, who spends some 5 to 10 percent of his time on the sea. "They exist at the interface." The chemistry, over the width of a few centimeters, can go "from unbelievably toxic to nontoxic concentrations," the temperature from 2 degrees C to 140 degrees C.
Yet, "You can get astoundingly productive communities," Fisher says. With their symbiotic bacteria, these common-looking mussels and not-so-common tube worms "are very efficient at converting chemical energy into biomass — in terms of biomass, these communities are as high as any aquatic environment on the face of the earth."
Fisher and his graduate students are developing tools and techniques for studying these cooperative creatures of the deep: for monitoring their environments (sampling on the level of milliliters of water, since conditions vary so wildly); assessing their growth rates (the tube worms at hydrothermal vents have been called "the fastest growing invertebrates on the planet," yet those at the cold seeps grow only half a centimeter per year and may live to be 100 years old); exploring their physiology (the blood of a tube worm, says Fisher, "is a very special fluid. It binds both oxygen and sulfur"); and determining the benefits of the symbiotic partnership to both the bacteria and the host (the tube worms, for their part, says Fisher, "provide their symbionts with an environment which free-living bacteria can only regard with envy").
The hope is to "put hard numbers on" the ecological importance of these underwater communities. The mussels, for instance, with their methane-eating symbionts, "can consume methane at very high rates," Fisher notes. Which makes them noteworthy not only as "important primary producers for the deep sea fauna," but as another link in the chain of the global carbon cycle. For, says Fisher, "Our ultimate goal is to model the carbon flux and flow in these deep sea ecosystems."
Charles R. Fisher Jr., Ph.D., is assistant professor of biology in the Eberly College of Science, 219 Mueller Building, University Park, PA 16802; 814-865-3365. His research is supported by awards from the National Science Foundation, including a Presidential Young Investigator award, and by the National Oceanic and Atmospheric Administration (NOAA) and the Mineral Management Service (MMS).
Graduate students currently working on these projects include Steve MacKenzie, Kathleen Scott, Megan Streams, and Istvan Ircuyo; recent master's degree graduates Erica Nix and Mike Simpkins completed theses on this work; current undergraduate researchers include Becky Shipe, Belinda Bell, Shoban Dave, and Emily Smith.