Searching for biological keys to unlock bountiful ocean harvests
The latest techniques of biotechnology are being applied to increase the ocean's bounty. In the past, the prodigious natural productivity of Earth's oceans fueled the belief that the sea could feed the planet's teeming billions. But in recent years marine scientists have realized that mankind's ocean harvests of fish, shellfish, seaweed, kelp, and more - which contribute roughly 10 percent of the world's food production - are now at or near maximum yields.
Thus, ''to increase this harvest we need to turn to intensive approaches such as mariculture,'' explains Daniel E. Morse, professor of Molecular Genetics and Biochemistry at the University of California, Santa Barbara (UCSB), who covered the marine biotechnology waterfront at the annual meeting of the Council for the Advancement of Science Writing.
Traditional mariculture, however, tends to be extremely labor- and capital-intensive. That is largely why intensive cultivation of marine plants and animals adds only 10 percent to the world's ocean harvest. In particular, this holds true for the United States, which currently imports 60 percent of its fishery products, adding $2.5 billion to its annual trade deficit.
Enter genetic engineering and biotechnology. Dr. Morse and a number of other marine biologists are turning to these techniques to overcome the major obstacles that stand in the way of aquaculture:
* Shellfish. One longstanding problem with the cultivation of shellfish has been reproduction. At UCSB Dr. Morse and his colleagues have found that spawning of abalone, clams, oysters, and mussels, among other shellfish, is controlled by chemical signals in the environment. The chemical turns out to be prostaglandin, also a human hormone. And they have discovered that adding trace amounts of the common chemical hydrogen peroxide stimulates the shellfish to produce prostaglandin and to spawn.
A second major problem with shellfish culture has been mortality of the larvae once they have spawned. These tiny creatures float for several weeks among the microscopic ocean plants called plankton. Then, responding to a mysterious call, they settle to the bottom and begin their transformation into adults.
According to Dr. Morse, the Santa Barbara scientists reasoned that this mysterious call must be another chemical signal. Without any idea what it might be, they asked divers to bring back the rocks where they found the smallest abalone. These rocks were covered with a red algae, and when this algae was added to tanks containing abalone larvae, they settled to the bottom and began their metamorphosis. Isolating the chemical produced by the algae, they discovered it was a close chemical cousin of a substance found in the human brain, the neurotransmitter gamma-aminobutyric acid (GABA).
Oyster larvae, on the other hand, respond to the presence of a previously unknown bacteria, researchers at the University of Maryland have discovered. And UM scientist Ron Weiner has determined that the chemical signal involved was related to another neurotransmitter, dihydroxy phenylanaline (DOPA).
A final problem with shellfish culture is slow growth. Abalone, for instance, are only 1 to 2 inches in length after a year. The Santa Barbara researchers have found that abalone growth rates can be stimulated by a growth hormone or by insulin. And they are currently in the process of cloning the genes that code for these substances. They hope to add these to the abalone's genetic makeup to create faster growing varieties.
* Seaweed culture. Currently, the worldwide seaweed industry grosses over $2 billion per year. Besides being a food source in some parts of the world, seaweed is also a source of food additives and special chemicals. Compounds extracted from it are used as food extenders and thickeners, and in pharmaceuticals and medical research and diagnosis. World seaweed supply is currently running well below the rising demand.
Of the 20 species cultivated worldwide, about half are ''improved'' cultivars created using tissue culture techniques. The Chinese have started large seaweed farms based entirely on biologically engineered seaweed strains.
In the US, development and commercialization of an improved strain of the seaweed used to wrap sushi has allowed the US to begin exporting this variety to Japan.
* Sulfur wastes and aquaculture. A major environmental problem is disposal of large amounts of sulfur wastes. Recent discoveries from the deep ocean raise the possibility that sulfur wastes might be put to good use in mariculture.
Several years ago the exploration ship Alvin discovered hot springs in the deep ocean which support an astounding community of life, including sea worms that look like big lipsticks, giant clams, and crabs. The entire community is fed by bacteria which, in turn, live by consuming sulfides, research by James Childress of UCSB and Charles Berg of Woods Hole have revealed.
This finding raises the possibility that an aquaculture system could be set up that would consume sulfur wastes. Not only would this help dispose of sulfur, but it would also reduce the most costly aspect of an aquacultural system: aerating the water to grow the plankton required to feed the fish.
* Artificial gills. The deep sea communities discovered by Alvin have another peculiarity. In order to breathe at such great depths, where the concentrations of oxygen in the water are very low, creatures have evolved a very special form of hemoglobin in their blood. This has been characterized by C. and J. Bonaventura of Duke University. And they have combined this special substance with a form of polyurethane to create what they call Hemosponge: the potential basis for an artificial gill.
Hemosponge absorbs oxygen from the water very efficiently and releases it when electrically stimulated. This has allowed the Bonaventuras to envision a life support system for a diver that fits into a backpack and includes a small diesel engine for propulsion. They are currently in the breadboard stage of development.
A life support system for people is a complex problem, however. A person has complex requirements besides oxygen. An easier problem would be use of such a gill for remotely piloted undersea vehicles. These have both commercial and military application. The artificial gill would only use 20 percent of the energy produced by a diesel engine and would allow such a vehicle to operate underwater for as long as its fuel supply lasts.