Toxic algae species is full of freeloading cheaters... and why that makes them even deadlier

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Prymnesium parvum is a single-celled, toxic algae species that wreaks havoc throughout U.S. waters. The toxin is designed to wipe out their competition for sunlight and nutrients... but for some reason, some of the algae don't bother producing toxins.

Cheating and freeloading are well-observed behaviors in more complex species like humans and chimpanzees, but it's a little bit more of a surprise when such behavior is observed in a single-celled species like this algae. University of Arizona researcher William Driscoll made the discovery, reporting that some of the algae in his lab-grown strains grew much larger than others because they didn't devote any energy to creating the toxins that are essential to the algae's species-wide defense system.

What's intriguing about this find isn't so much that some of the algae are cheating; rather, it's the fact that any algae still bother producing toxin when it's a clear individual benefit to devote all available energy to producing offspring. Natural selection should quickly wipe out the toxin-producing algae, even if it means sacrificing the toxic defense. Indeed, part of the reason "cheating" is so rare in simple species like this is that it's hard to maintain two divergent behavior patterns like this — one of these groups, either the toxin producers or the energy savers, should have won out long ago.


The reason why that didn't happen comes down to the true purpose of the toxin. While it has proven useful as an all-purpose defense system, its overriding purpose is something far more aggressive and specialized, as Driscoll explains:

"[The toxic algae] attack other cells. Using their two flagella, they swim up to the prey and latch on to it. Sometimes a struggle takes place, and more cells swim up to the scene, surround their victim and release more toxin, and then they eat it. These toxins might have evolved less as a means to keep competitors away and more like a rattlesnake venom. The algae might use it to stun or immobilize prey. When times are good and there are plenty of nutrients in the water, the algae use photosynthesis to gain energy from sunlight, but when nutrients become sparse, they attack and become toxic. That's when they start swimming around looking for prey. They are a little bit like carnivorous plants in that way — like a Venus fly trap."


Basically, the toxic algae are crucial to wiping out species that might be competition, but they also tend to get rid of prey as well. That's when the "cheaters" take over, as Driscoll explains:

"During a bloom you have killed off all the prey or a huge amount of it, so why produce toxins and go looking for something that isn't there? It might be better to just keep growing and not even try to bother to keep looking for prey because it's gone. We are ultimately interested in disrupting the competitive abilities of these bloom-forming populations. While this research is just scratching the surface, understanding how natural selection may work over the course of a bloom can provide a deeper understanding of the traits that are most important to the success of this species."


Indeed, these new findings may prove crucial in dealing with the increasingly common algae blooms. While the sudden fishkills haven't had a direct impact on the human and livestock populations back on dry land, their growing prevalence isn't good for the long-term environmental balance, and left unchecked these algae could well start to pose a real threat to our food supply.

Evolution via the University of Arizona. Top image of algae beds by mmmavocado on Flickr. Side image of Prymnesium parvum by William Driscoll.