Fluctuating conditions favor cooperation
Simulations performed by LMU biophysicists, in which both environmental and demographic fluctuations were modeled for the first time, indicate that cooperative interactions favor the survival of bacterial populations in variable habitats.
Like other biological communities, bacterial collectives engage in social interactions that promote their survival. However, not all the members of any given population necessarily benefit from cooperative actions. For example, cells that produce and secrete compounds that stimulate population growth often grow at lower rates than non-producers because the synthesis of such agents requires the expenditure of energy. In spite of such fitness costs, cooperating bacteria that synthesize and share such ‘public goods’ are found in many bacterial populations – a finding which appears to be at odds with the Darwinian principle of the survival of the fittest. The LMU physicists Professor Erwin Frey and Karl Wienand, in collaboration with Professor Mauro Mobilia (University of Leeds), have now looked more closely at this phenomenon. Their study, which was based on mathematical simulations, demonstrates that cooperators have a greater chance of surviving than non-cooperators if the “quality of life” provided by their environment is subject to strong fluctuations. The new findings appear in the journal Physical Review Letters.
As a general rule, the environments in which bacteria are found seldom remain stable for long: Temperatures and nutrient availability are subject to more or less drastic variations, and living conditions tend to be highly unpredictable. In addition, demographic fluctuations must be taken into account, as random variations caused by individual cell division and death events can have a highly significant effect on the evolution of bacterial populations – and may even lead to the extinction of strains that are otherwise comparatively well adapted. Fluctuations in the environment and the population affect growth, but also each other. For the effects of demographic fluctuations depend on the size of the population, which is in turn influenced by the environment. “Despite their mutual dependence, these effects have usually been considered separately,” says Karl Wienand, the first author of the new study. “We have now analyzed their combined effects on the survival of a bacterial strain that exhibits cooperative behavior, which specifically reduces the relative growth rates of the cooperating cells.”
The researchers simulated the growth dynamics of a well mixed colony made up of two bacterial strains, one of which displayed cooperative behavior while the other did not. The two strains competed for resources, whose level varied at random between abundance and scarcity. The simulations indicated that, the cooperative strain is significantly more likely to survive in a fluctuating environment than it is when conditions remain constant. If the concentration of nutrient resources is varied, colony size shrinks when times are hard and cell numbers can fall to very low levels. Under such conditions, demographic fluctuations have a greater impact, and in small populations the non-cooperating strain is more likely to become extinct – and in the absence of competition the slower growth rate of the cooperating strain becomes irrelevant and the cooperators prosper.
In this model system, the authors were able to show that the magnitude of the impact of variable environmental conditions on the survival of the cooperators depends on the frequency of the fluctuations: If conditions change relatively rarely, the overall population size varies in accordance with the level of resources available. If, on the other hand, such switches from surfeit to dearth and back again often occur, the population size settles down to an intermediate value. “The population is then smaller than it would be under optimal conditions, but not necessarily so small that demographic fluctuations have a positive effect on the numbers of cooperators,” says Frey. “We have constructed a mathematical model which captures these interactions very well, and can help to give us a better understanding of the complex relationships between bacteria and their environments.”
Physical Review Letters 2017