Lake algae dine on bacteria.

Sunlight has long been considered the major, if not sole, energy source for algae as for most other plants. But now scientists report that some common species of lake algae graze on bacteria as well. This finding adds a kink and another link to the aquatic food chain.

"It's the same sort of ideas as the Venus's-flytrap," says David F. Bird of McGill University in Montreal. A clean, clear lake is low in nutrients, as is the boggy environment where most carnivorous plants grow. To supplement their photosynthetic production, the plants ingest oher organisms. In the dim depths of the lake, algae obtain at least half their total carbon from ingested bacteria, rather than from photosynthesis, Bird and his colleague Jacob Kalff report.

In Lac Cromwell in Quebec, four species of the alga genus Dinobryon were found to be major consumers of bacteria. To demonstrate this appetite, the scientists mixed the algae with fluorescent latex beads just the size of bacteria. The algae ingested the beads, which are easy for the scientists to track, only a little less readily than they ingested bacteria.

Under their natural conditions, the algae each consume an average of 36 bacteria every hour, the scientists calculate. "This is equivalent to an individual Dinobryon removing bacteria [daily] from a volume equal to 1,500,000 times its cell volume and ingesting almost 30 percent of its weight in bacteria per day," Bird and Kalff say in the Jan. 31 SCIENCE. Algae of two species of another genus, Uroglena, also ingest bacteria, but at only about one-sixth that rate.

How do the relatively immobile Dinobryon scour the water for bacteria? Individual algal cells share a branching fibrous casing called a lorica. They use their flagella, which extend through the top of the lorica, to force water inside the casing. With its membrane, a cell then engulfs a passing bacterium. Using electron micrographs, the scientists have viewed bacteria entrapped in food vacuoles within algal cells.

"Our results show that [algal ingestion of bacteria] is quantitatively important in nature," Bird and Kalff say. Because large numbers of Dinobryon populate eastern North American lakes, averaging 150,000 to 650,000 cells per liter, they remove more bacteria from the water than do the crustacean, rotifer and ciliate communities combined -- the animals long known to eat bacteria. Bird says the algal grazing rates are similar to those measured for marine microflagellates, abundant nonphotosynthetic microorganisms recently recognized as important in consuming algae.

"The algae and microflagellates provide a way of returning plant nutrients captured by bacteria back into plants and animals," Bird says. A conventional food chain would have plants manufacturing food photosynthetically; animals consuming both plants and bacteria; and bacteria living off both plants and animals. But the bacteria-eating algae and microflagellates, which are eaten by microscopic animals, some of which also eat bacteria, may be considered a new link in the food chain, and they introduce a new king.