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BATON ROUGE -- That green slime swimming-pool owners spend thousands of dollars each year to get rid of is worth $500 a kilogram.
Microalgae, one of the most ubiquitous plants on the planet, is playing an increasingly important role in commercial seafood production, says LSU assistant professor of civil and environmental engineering Kelly Rusch. And LSU is helping produce them.
"We have to have algae to feed the rotifers that feed the commercial fish and shellfish industry," Rusch said. "There are no commercial algae providers. Facilities that need it have to produce it themselves." As much as 50 percent of the cost of raising shellfish is taken up by algae production, she said.
"The difficulty with marine aquaculture is that larval fish, when they first hatch, require some sort of live feed -- not necessarily the microalgae, but the secondary consumers, which eat the algae -- rotifers, brine shrimp, copapods. So basically, what we're trying to do here is stabilize the base of the food chain. If we cannot stabilize the base of the food chain, we can't expand above it," Rusch said.
The standard way to produce algae is in batches. The water in a large tank is purified and salted, nutrients are added to it, and then it is inoculated with one of the 20 or 30 strains of algae that are commonly used to feed fish. In one to two weeks, the algae are mature and are harvested.
"It's a simple, well-established procedure, but it's hard to control," Rusch said. "You have to have several tanks staggered so that you always have a continuous supply. Contaminants can cause the culture to collapse. It's labor intensive -- the tanks have to be scrubbed by hand after each batch."
What Rusch and her colleague, LSU civil and environmental engineering professor Ronald Malone, are doing is producing algae on a continuous basis, rather than in batches.
"Water continuously flows in, and algae continuously flow out. The process is consistent, stable, and can be automatically controlled by computers, thus minimizing labor costs. Also, we can produce more algae from the same volume of water," Rusch said.
A prototype, which is supplying algae to oyster researchers on- and off-campus, has eight large, fiberglass tanks. The first two have been "inoculated," and algae flow from them into each of the other tanks one after the other, becoming progressively more numerous and more mature in each tank. Water temperature, pH, salinity, carbon dioxide and algae concentration are monitored by a computer. The algae in the final tank are continuously harvested and pumped through a centrifuge, which strains them out. The water is then recycled.
The system can produce about $500 worth of algae a day, Rusch said, with operating costs of less than $100 a day. Production costs in various other technologies range from $100 to $1,000 a dry kilogram, Rusch said.
One advantage of Rusch and Malone's system, which they call HISTAR (Hydrostatically Integrated Serial Turbidostat Algal Reactor, if you prefer), is that only the first two tanks have to be covered. Because water flows continuously from tank to tank, contaminants are washed out without causing the culture to collapse.
Another advantage is that algae may be harvested at any stage in their life cycle. "Algae have different biochemical composition in terms of proteins, carbohydrates and lipids at different stages of their lives. Depending on what you're trying to feed the algae to, you're going to need them at different stages," Rusch said. Different species can also be mixed to obtain desired nutritional components, she said.
In commercial production, tanks will be as large as a back-yard swimming pool, holding about 20,000 gallons of water each, Malone said.
The two prototypes at LSU -- the one at LSU's Aquaculture Research Facility on Ben Hur and a refined version in the CEBA building -- were funded by national and state Sea Grant programs and have been operating for two years. Two demonstration models are being built, one to feed oyster spat in Maryland, and another to feed striped bass larvae in Massachusetts.
Feeding fish is not the only use for the algae. Pigments and thickeners can be extracted from them and the aquarium industry can use them for breeding the tiny larvae of marine tropical fish. There is also a health component to algae. One species produces beta-carotene, and another, spirulina, is sold in health-food stores.
Perhaps the most important reason for producing this humble organism was voiced by Malone.
"Algae is at the bottom of the food chain," said Malone. "If we can control the bottom of the food chain, we can control food production."
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