Picky protozoa may sense poison in prey.

When a person bites into horseradish, it bites back--producing a stinging sensation that deters many people from taking a second mouthful.

Activities like the chomping action of teeth destroy barriers inside a plant cell that keep certain enzymes and substrates separate. When mixed, these molecules produce chemicals that can protect the plant from being eaten by would-be predators.

Now, researchers report that single-celled organisms may defend themselves in a similar way.

Many types of marine algae produce a substrate called dimethlysulfoniopropionate (DMSP), which is broken down chemically into dimethyl sulfide (DMS) and acrylate by an enzyme called DMSP lyase. This DMS plays a major role in the global sulfur cycle, cloud formation, and possibly climate control. Acrylate is known to be poisonous to microorganisms.

Both DMSP and DMSP lyase reside within algal cells, but the chemical reaction doesn't take place unless a cell is injured--by a hungry predator, for example. Researchers can monitor the amount of algae ingested by a predator in the lab by measuring the amount of DMS that diffuses out of it.

Gordon V. Wolfe, a microbial ecologist at Oregon State University in Corvallis, and his colleagues at the University of Bremen in Germany have found that some protozoa which are sensitive to large amounts of DMSP lyase can survive on algae that make only small amounts of the enzyme. Even protozoa that eat high-enzyme-producing algae prefer the low producers, they report in the June 26 Nature. The researchers suggest that the potential for creating acrylate somehow deters predators.

"The idea that DMSP lyase is a grazing deterrent is really interesting," says Diane Stoecker, a biological oceanographer at the University of Maryland's Horn Point Environmental Laboratory in Cambridge, Md. "DMSP is produced in response to extreme environmental conditions, such as high salt concentrations, but a lot of algae that aren't exposed to extreme conditions produce it too. Until now, there hasn't been a good explanation."

When Wolfe's research team fed a protozoan, Oxyrrhis marina, a mixture of low- and high-DMSP lyase-producing algae, they detected little DMS for 24 hours. Then the amounts rose. Wolfe interprets this to mean that the protozoa consumed the low-enzyme producers before attacking the others.

"Oxyrrhis is the Tyrannosaurus rex of marine protozoa," says Wolfe. "It's voracious. It'll eat almost anything. But even this nonfussy eater prefers food A over food B."

Wolfe cautions that he has not proved that the DMSP lyase reaction underlies the protozoan's food choice. The different algae strains are members of the same species, but they could differ in other ways besides lyase activity. The results do provide a solution to a previously troubling problem: Acrylate is toxic only at extremely high concentrations. Such concentrations, however, could be reached if algae were confined to the small digestive compartments within protozoa.

There are still "major gaps in our understanding," Wolfe said. He wonders how predators know in advance which strains of algae to avoid. The correct choice benefits the high-enzyme-producing algae because, unlike multicellular plants, which can sacrifice small amounts of tissue for the sake of the whole organism, single-celled creatures do not have any cells to spare.

The recent work also contributes to a growing appreciation that protozoa are more sophisticated than previously realized. "Unicellular organisms [like protozoa] are usually thought of as being simple, especially in terms of their behavior," says Peter G. Verity, an ecologist at the Skidaway Institute of Oceanography in Savannah, Ga. "But unicellular does not necessarily equate with simplicity in terms of form or function."

"A lot of people are building computer models of marine microbial food webs," says Wolfe. "They assume that if a prey is present, a predator will eat it. The models need to be made more sophisticated."