Deep-sea Habitats and Inhabitants Astound Scientists on WSG-funded Cruise
By Colleen Craig
This past summer’s newspaper headlines told the tale: a glass-sponge reef had been located, 30 miles off the coast of Grays Harbor, Washington.
Reef-building glass sponges were thought to be extinct until Canadian scientists discovered them off the coast of British Columbia in the late 1980s. With funding from Washington Sea Grant, UW oceanographer Paul Johnson expanded our understanding of glass-sponge reefs the range and conditions under which their unusual builders can operate.
The Washington reefs are thousands of feet long and 15 feet tall, growing at depths of 650 feet below the ocean’s surface. They represent an oasis of life in an otherwise barren stretch of seafloor, providing protection for zooplankton, rockfish, squat lobsters and other species. Unlike the British Columbia reefs, which exist in relatively protected waters, the Washington reefs grow in the open ocean, meaning they can withstand large ocean swells associated with winter storms. This suggests that glass-sponge reefs may exist in other regions of the Pacific continental shelf.
In June 2007, Johnson spearheaded the cruise that located Washington’s reefs. The UW professor of marine geology and geophysics has spent 30 years studying deep-sea geological phenomena. He was inspired to look for glass sponges in Washington after joining a Canadian expedition to study an 8000-year-old reef in the Georgia Strait.
“I’ve spent a lot of time looking at exotic animals around hydrothermal vents, but this was really impressive,” says Johnson of the biologically dynamic, 20-meter tall Georgia Strait reef.
After obtaining circumstantial evidence that the Washington continental shelf hosted its own reef, he received funds from Washington Sea Grant to support the research, plus six days of ship time aboard the RV Thompson from the UW School of Oceanography.
“I’m grateful to Washington Sea Grant for its support,” says Johnson. “Research done at sea is really expensive but, fortunately, Sea Grant recognized the potential of the project.”
A Sponge Made of Glass
Glass sponges are among the most primitive of animals and may date to the Pre-Cambrian era. Although individual sponges can grow up to 11/2 feet tall, they are basically single-celled organisms supported by a skeleton of tiny glass shards, called “spicules.” The organisms precipitate the spicules from silica that is dissolved in sea water. Most species of glass sponges have unconnected spicules, so their supporting structures fall apart when they die. The spicules of reef-building sponge species are fused together, says Jonathan Rose, research assistant in biology at University of Victoria, British Columbia, and a member of the June cruise. “When they die, the glass skeleton remains intact,” allowing future generations of sponges to build on the skeletons of the dead.
The glass-sponge skeletons “look like a pile of very thin barbed wire, and they don’t dissolve in seawater,” says Johnson. “They’ll last for 100 million years.” There are fossilized glass-sponge reefs running through Europe that are nearly 200 million years old.
Although individual glass sponges have been found at various depths in every ocean, the reef-building sponges were thought to have been extinct for 100 million years — until they were discovered in the Georgia and Hecate straits in British Columbia. Scientists believed that the evolution of diatoms — single-celled algae that build their cell walls from silica — drove the reef-builders to extinction, due to competition for silica. Apparently, the sponges simply adapted by living deeper in the ocean, out of reach of the sun and the photosynthetic, silicahungry diatoms.
Meet the Methanotrophs
Glass sponges remain fixed in place during their 100- to 200-year life spans. They feed by filtering bacteria from seawater. In the case of the Washington reefs, the bacteria may be living on methane, or natural gas, that the crew discovered as it seeped out of the ocean floor near the reefs. Although an organism that lives on natural gas may seem improbable, so-called “methanotrophic” (“methane-eating”) bacteria are very common.
“Methane is a tremendous energy source for biology,” says Johnson. “Whenever you find methane seeps in the ocean, you find massive populations of bacteria.” The bacteria extract energy from methane in much the same way a human cell extracts energy from sugar. Such bacterial mats usually support sea-life communities dominated by tubeworms and clams, says Rose. The Washington reef could represent a new kind of undersea community.
Methane is a simple compound of carbon and hydrogen and is produced from ancient organic matter buried deep in the Earth’s crust. While undersea methane seeps are not unusual, “our sponges are sitting right on top of one, which is unusual,” says Johnson. He believes that this ancient methane could be supporting an entire food chain off the Washington coast, with the methanotrophic bacteria nourishing the sponges and other creatures in the vicinity.
“That’s my fantasy anyway,” Johnson says. He has an idea of how to test the reality. Carbon atoms come in three forms, called isotopes. The ratio of carbon isotopes in the methane near the Washington reef is very distinct, and Johnson predicts that the cells of organisms depending on the methane seep will carry the same ratio.
“We don’t have the right samples yet to prove this island of life gets its nutrients using chemosynthesis from fluid from below the seafloor, rather than photosynthesis from the sea surface,” says Johnson. “I’m working hard to get another cruise to go out there.”
University of Victoria researchers recently found evidence of a methane seep near the Hecate Strait glass sponge reef. However, a connection to methanotrophic bacteria has not been established. There is no indication of a seep near the Georgia Strait reef, but this community is probably supported by nutrients from the Fraser River, says Rose.
Don’t Trawl on Me
The Washington coast supports an extremely active fishery, possibly attributed in part to the existence of glass-sponge reefs. The adults of species such as rockfish don’t spend much time in the reefs, but the reefs do provide protection for juveniles and pregnant females, says Johnson. He also predicts that such marine nurseries could be found in many locations along the northern Pacific Rim, from California to Alaska and across the Pacific Ocean to the coast of Russia. “This is an enormous fishing area,” says Johnson, “and these are all very similar environments.”
Contact David G. Gordon, Science Writer for Washington Sea Grant, for further information.