Cooperators Can Coexist With Cheaters, as Long as There Is Room to Grow

Feb. 1, 2013 — Microbes exhibit bewildering diversity even in relatively tight living quarters. But when a population is a mix of cooperators, microbes that share resources, and cheaters, those that selfishly take yet give nothing back, the natural outcome is perpetual war. A new model by a team of researchers from Princeton University in New Jersey and Ben-Gurion University in Israel reveals that even with never-ending battles, the exploiter and the exploited can survive, but only if they have room to expand and grow.


 

The researchers present their findings at the 57th Annual Meeting of the Biophysical Society (BPS), held Feb. 2-6, 2013, in Philadelphia, Pa.

“In a fixed population, cells that share can’t live together with cells that only take,” said David Bruce Borenstein, a researcher at Princeton. “But if the population repeatedly expands and contracts then such ‘cooperators’ and ‘cheaters’ can coexist.”

Our world and our bodies play host to a vast array of microbes. On our teeth alone, there are approximately a thousand different kinds of bacteria, all living in very close quarters. This is amazing, the researchers observe, because many of those species share resources with nearby neighbors, who might not be so cooperative or even related [1].

At the scale of cells, individuals cooperate mainly by exporting resources into the environment and letting them float away. “This is a deceptively complex process in which cells interact at long ranges, but compete only with nearby individuals,” explained Borenstein. “Our models predict that, even when this exploitation prevents any possibility of peaceful coexistence, the exploiter and the exploited can survive across generations in what is basically a perpetual war.” The researchers speculate that similar competition might occur between cancer cells and normal tissue.

Borenstein and his colleagues made their conclusions based on a computer model that considered two types of cells, cooperators and cheaters, and laid them out on a grid. Cooperators were given the ability, not uncommon in nature, to make a resource that speeds up growth in both kinds of cells. Producing this resource slowed down the growth of cooperators, because they have to divert some energy to resource production. This resource then spread out from the cooperator by diffusion, so that the cells closest to a producer have the greatest resource access. The model revealed that the producers tended to cluster, meaning that being a producer gave you greater access to resources. It also meant that even though cheaters are avoiding the cost of production, they pay for it with reduced resource access.

Within these basic constraints it was found that when the two populations must compete directly for survival, no coexistence is possible. “One type always wins out,” observed Borenstein. However, when the two populations can grow into empty space, the researchers found a strange and paradoxical interaction: cheaters may be outcompeting cooperators locally, even as cooperators grow better overall. These complex interactions may play an important role in the maintenance of diverse microbial communities, like those seen in the mouth.

“To our astonishment, we found that while cheaters can exploit cooperators, cooperators can isolate cheaters, just from cooperation and growth,” concludes Borenstein. “As a result, the community can persist in a sort of perpetual race from which a winner need not emerge.”

[1] J. M. ten Cate. “Biofilms, a new approach to the microbiology of dental plaque.” Odontolgy 2006(94):1-9.

 

Story Source:

The above story is reprinted from materials provided byBiophysical Society, via Newswise.

Note: Materials may be edited for content and length. For further information, please contact the source cited above.


Biophysical Society (2013, February 1). Cooperators can coexist with cheaters, as long as there is room to grow. ScienceDaily. Retrieved February 3, 2013, from http://www.sciencedaily.com/releases/2013/02/130201095947.htm

Note: If no author is given, the source is cited instead.

 
Advertisements

They Hunt, They Kill, They Cheat: Single-Celled Algae Shed Light On Social Lives of Microbes

Jan. 18, 2013 — Cheating is a behavior not limited to humans, animals and plants. Even microscopically small, single-celled algae do it, a team of University of Arizona researchers has discovered.

Shimmering golden under a microscope, Prymnesium algae are seen swarming and devouring a green alga. A human hair would cover most of the swarm. (Credit: William Driscoll)

 

Humans do it, chimpanzees do it, cuckoos do it — cheating to score a free ride is a well-documented behavior by many animals, even plants. But microscopically small, single-celled algae? Yes, they do it too, biologists with the University of Arizona’s department of ecology and evolutionary biology have discovered.

“There are cheaters out there that we didn’t know of,” said William Driscoll, lead author of a research report on the topic who studied an environmentally devastating toxic alga that is invading U.S. waters as part of his doctoral research in the lab of Jeremiah Hackett, an assistant professor of ecology and evolutionary biology.

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 neighbors — they are true ‘free riders,'” Driscoll explained.

The study, published in 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 behavior 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 favor 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 immobilize 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 behavior 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.”

Unraveling the molecular mechanisms behind all this chemical warfare, cheating behavior and maximizing 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 neighbors. 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.”

 

Story Source:

The above story is reprinted from materials provided byUniversity of Arizona, via EurekAlert!, a service of AAAS.

Note: Materials may be edited for content and length. For further information, please contact the source cited above.


Journal Reference:

  1. William W. Driscoll, Noelle J. Espinosa, Omar T. Eldakar, Jeremiah D. Hackett. ALLELOPATHY AS AN EMERGENT, EXPLOITABLE PUBLIC GOOD IN THE BLOOM-FORMING MICROALGAPRYMNESIUM PARVUMEvolution, 2013; DOI: 10.1111/evo.12030
University of Arizona (2013, January 18). They hunt, they kill, they cheat: Single-celled algae shed light on social lives of microbes.ScienceDaily. Retrieved January 27, 2013, from http://www.sciencedaily.com/releases/2013/01/130118172335.htm