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article imageOp-Ed: Freeloading social algae and plants that eat other plants

By Paul Wallis     Jan 20, 2013 in Science
Sydney - Algae are plants. You could be forgiven for thinking otherwise when you see what the Golden Algae are capable of doing. This species has developed a range of toxic and non-toxic members. They also hunt prey and have an interesting social life.
Science Daily:
Driscoll (lead author William Driscoll, University of Arizona) isolated several strains of the species, Prymnesium parvum, and noticed that, some grew more quickly and do not produce any of the toxins that protect the algae against competition from other species of algae.
"When those 'cheaters' are cultured with their toxic counterparts, they can still benefit from the toxins produced by their cooperative neighbors -- they are true 'free riders,'" Driscoll explained.
This type of algae is highly invasive, toxic to fish, and has now developed a taste for freshwater environments. That’s likely to be a big problem. The algae are cooperative. One alga will attack its prey, and apparently that signals an attack by other members of the species. Sounds more like ants responding to a pheromone, doesn’t it?
Algae are mobile. They can move around with their flagella and navigate. The non-toxic type of this species/strain aren’t playing by the rules. That’s creating a certain amount of amazement. They’re also capable of breeding when the non-toxic algae don’t.
Science Daily continues:
The algae produce toxins that are deadly to fish but so far have not been shown to threaten the health of humans or cattle. Many scientists believe the toxin arose as a chemical weapon to wipe out other algae and other organisms competing for the same nutrients and sunlight on which the algae depend. The discovery of cheaters that don't bother to produce toxin, however, throws a wrench into this scenario.
One possible explanation is a genetic switch. The genes that turn on the toxin production obviously aren’t active in this revised version of the algae. Cooperating with their toxic relatives may well be a version of the numbers game.
If there are more non-toxic algae than the toxic variety, they take the weight of any predation. The predators inevitably bump into the toxic form and are wiped out. The freeloading non-toxic form may well be paying their way by acting as bait. They’re clearly part of the algal community, toxic or not, so they must produce some benefit to the wider group.
Another clue seems to be “feast or famine”. When they can get enough food from sunlight and other nutrients, they’re not so aggressive. When food is scarce, the toxic form goes hunting. The non-toxic versions can also put more effort into breeding, because they don’t hunt. Stress related genes have been linked to the toxic forms, but they’re “regulated differently” in the non-toxic forms.
Apparently the non-toxic algae can continue breeding when the others can’t, which may prove they have more energy than the other form. They’ve also taken a short cut through defence development, with their relatives taking the load for them.
It’s unclear whether the non-toxic form reacts to stress by becoming toxic. If the non-toxic members are starved, do they flip their switch and become toxic to get food? That’s quite possible. Even humans have genes which code for food shortages, and I’ve seen totally harmless plants, notably borage, which will produce a sting like a nettle when under severe stress from whitefly.
(Ironically, the human genes that code for famine, if switched on the wrong way, are also believed to play a role in diabetes.)
The researchers point out, quite reasonably, that during a big algal bloom, when all the prey and competition have been killed off already, there’s not much point in producing toxin. It’s wasted energy.
Cooperative plants aren’t really so new. Cooperative behaviours in same species of plants even happens in daisies. They modify their root growth to allow others to settle in to the neighbourhood, for example. Cooperative mobile plants with a predatory streak could be expected to do more.
The other object of this research is ultimately to control algal blooms. Some algal blooms, notably the notorious Red Tides, are extremely toxic. The work on the toxic gene regulation is valuable, in any possible context. An off switch for poisonous algae has to be a great idea. If the algae have a way of turning the genes back on when they feel like it, however, it’s likely to be a pretty demanding learning curve.
Reminds me of Brian Aldiss’ book Hothouse, where mobile vegetable predators rule an Earth covered by a gigantic banyan tree. Trust some writer to persuade nature to play along. If you see a tree carrying a knife and fork and looking a bit too interested, start running.
This opinion article was written by an independent writer. The opinions and views expressed herein are those of the author and are not necessarily intended to reflect those of
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