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Golden algae: They hunt, They kill, They cheat

pub­lished 23 Jan­u­ary 2013 | mod­i­fied 23 Jan­u­ary 2013
Golden algaeHumans do it, chim­panzees do it, cuck­oos do it — cheat­ing to score a free ride is a well-​documented behav­iour by many ani­mals, even plants. But micro­scop­i­cally small, single-​celled algae? Yes, they do it too, biol­o­gists with the Uni­ver­sity of Ari­zona’sdepart­ment of ecol­ogy and evo­lu­tion­ary biol­ogy have dis­cov­ered in research on an envi­ron­men­tally dev­as­tat­ing toxic alga that is invad­ing U.S. Waters.

There are cheaters out there that we didn’t know of
William Driscoll, lead author »

As part of his doc­toral research Driscoll 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 neigh­bours — they are true ‘free rid­ers,’” Driscoll explained.

The study,pub­lished online in the Jan­u­ary 11 issue of the jour­nal Evo­lu­tion, adds to the emerg­ing view that microbes often have active social lives. Future research into the social side of toxic algae could open up new approaches to con­trol or coun­ter­act toxic algal blooms, which can pose seri­ous threats to human health and wipe out local fish­eries, for exam­ple.

Prym­ne­sium belong to a group of algae known as golden algae, so named for their acces­sory pig­ments, which give the cells a golden sheen. This toxic species lives mostly in oceans and only recently has invaded fresh­wa­ter envi­ron­ments. Its dis­tant rel­a­tives include the equally micro­scopic diatoms, which make up a large part of phy­to­plank­ton, and giant kelp. 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.

“We are try­ing to under­stand the eco­log­i­cal side in these algae,” Driscoll said. “If you’re a sin­gle cell, regard­less of whether you make a toxin or not, you’re just drift­ing through the water, and every­thing is drift­ing with you,” Driscoll explained. “Pro­duc­ing tox­ins only makes sense if the entire pop­u­la­tion does it. Any given indi­vid­ual cell won’t get any ben­e­fit from the chem­i­cals it makes because they imme­di­ately dif­fuse away. It’s a bit like school­ing behav­iour in fish: A sin­gle fish can’t con­fuse a preda­tor; you need every­one else do the same thing.”

For that rea­son, he explained, the cheaters should have an imme­di­ate advan­tage over their “hon­est” peers because they can invest the energy and resources they save into mak­ing more off­spring. “The­ory tells us coop­er­a­tion should break down in these cir­cum­stances. If you are secret­ing a toxin and it’s ben­e­fi­cial to your species, then every­body gets access to that ben­e­fit. In a well-​mixed pop­u­la­tion where there is no group struc­ture, nat­ural selec­tion should favour self­ish­ness, and the cheaters should take over.”

But for some rea­son, they don’t. An alter­na­tive expla­na­tion for tox­i­c­ity becomes clear when toxic cells are observed along­side their com­pe­ti­tion under a micro­scope. “They attack other cells,” he said. “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 immo­bilise prey.”

Driscoll and his co-​workers iso­lated the toxic and the non-​toxic strains side by side from the same water sam­ple, taken from a late bloom as the bloom started to crash. “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,” Driscoll said. “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.”

The group observed that as soon as nutri­ents become scarce, the toxic pop­u­la­tion ceases to grow, but the cheaters keep mul­ti­ply­ing.

Driscoll and his team think the cheat­ing behav­iour could be an adap­ta­tion to the algae bloom life style. “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.”

Driscoll said the research illus­trates how lit­tle is known about the ecol­ogy of microbes:

We’re just start­ing to under­stand what the mech­a­nisms are that main­tain coop­er­a­tion in microbes. The the­ory is heav­ily slanted toward mul­ti­cel­lu­lar organ­isms. Only recently have peo­ple started to think about microbes cooperating.

To bet­ter under­stand the genes and bio­chem­i­cal path­ways that con­trol how the algae make their tox­ins, the group in Hackett’s lab is inves­ti­gat­ing which genes are active in the toxic com­pared to the non-​toxic strains. “We are find­ing a num­ber of stress-​related genes are reg­u­lated dif­fer­ently in the cheaters,” Driscoll said. “A lot of the other genes have not been stud­ied before, espe­cially those most likely involved in toxin pro­duc­tion. The prob­lem is that noth­ing close to these algae has had its genome sequenced, so they’re pretty mys­te­ri­ous. Many of the genes we have sequenced are novel, so under­stand­ing their func­tion is a big part of the chal­lenge.”

Unrav­el­ling the mol­e­c­u­lar mech­a­nisms behind all this chem­i­cal war­fare, cheat­ing behav­iour and max­imis­ing growth could poten­tially lead to new appli­ca­tions, the researchers spec­u­late, albeit cau­tiously. Driscoll explained the cheat­ing trait might be an Achilles heel that could be exploited to curb algal blooms. “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.”

In addi­tion, the cheaters’ ten­dency to keep grow­ing when their toxic peers no longer can is in some ways rem­i­nis­cent of can­cer cells. Accord­ing to Driscoll, one way to think about can­cer is that can­cer­ous cells have an imme­di­ate advan­tage over their non-​cancerous, well-​behaved neigh­bours. But this advan­tage, if unchecked, is very short­sighted because it will inter­fere with the basic func­tion­ing of the mul­ti­cel­lu­lar organ­ism of which they are all a part.

“What we may be see­ing in our algae is a — far less extreme — ver­sion of a sim­i­lar story, because a short-​term advan­tage to not pro­duc­ing tox­ins may inter­fere with the long-​term com­pet­i­tive abil­ity of the pop­u­la­tion.”

(Source: UA News media release, 17.01.2013)
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