By Brendan Borrell

The toxins produced by some algal blooms may have evolved to give predatory algae an advantage when it comes to capturing their prey, researchers say. Knowing how algae use toxins in nature could help scientists who are trying to predict when and where the devastating blooms, sometimes called "red tides," are going to strike.

Single-celled algae called dinoflagellates are one of the organisms responsible for harmful algal blooms that poison shellfish and leave fish floating belly-up. Because the toxins are energetically costly to make, biologists have long wondered whether they are more than just a way to defend algae from getting eaten or preventing competitors from moving in on their space. Although many dinoflagellates can survive through photosynthesis alone, some species are able to grow twice as fast by preying on other algae--and it is this feeding mechanism that is now thought to be aided by the production of toxins.

"People suspected that one of the roles of the toxins is to immobilize prey," says marine ecologist Diane Stoecker of the University of Maryland's Horn Point Laboratory in Cambridge, Maryland, who was not involved in the research. "This is the first paper that really shows that."

Biophysicist Jian Sheng of the University of Minnesota in Minneapolis and his colleagues studied toxic and non-toxic strains of a dinoflagellate species, found in Chesapeake Bay on the eastern US coast, called Karlodinium veneficum. In the lab, the team mixed each strain of K. veneficum with a species of algae on which it preys, and recorded the three-dimensional motions of thousands of cells using a high-speed holographic microscopy technique they described in 20071. The resulting videos contained over 4 terabytes of data, which Sheng and his colleagues crunched to compare the swimming behaviors of predator and prey species in the presence of different toxin levels.

Paralyzing poison

Toxic strains of K. veneficum immediately caused the prey to slow down by more than 50 percent, and nearly doubled the proportion of immobile algae in the water relative to non-toxic strains. After 5 hours, more than 90 percent of the prey were immobile. According to Sheng's description of the process, K. veneficum can swoop in, shoot out a "tow line" and reel in the immobile prey before swallowing it whole. The prey algae is 2-3 times the size of K. veneficum. Notably, K. veneficum also slows down in the presence of prey, which may be a means of staying within the toxic cloud to aid predation.

"The food is not usually considered in models, only the physical conditions like temperature and nutrient availability," says Sheng. The study will be published in Proceedings of the National Academy of Sciences this week.

Stoecker says that there has been circumstantial evidence that dinoflagellates use their toxins to capture prey, but notes that Sheng's work is "pretty cool" because of the detailed visualizations and quantitative data. Although some scientists have suggested that the nutrients netted by algae through fish kills are enough to justify the toxins, Stoecker believes that phenomenon is just a "side effect" of prey-capture activities.

Marine ecologist Daniel Kamykowski of North Carolina State University in Raleigh cautions that the study was conducted in a laboratory and not in the marine environment, but welcomes the findings nonetheless. "We don't know what triggers the toxin in some strains and not in others," he says, "Once that's better known--and if there is anything that can be controlled--then it may be possible to diminish the frequency of blooms in the future."