For these tiny organisms it is often advantageous to divide the labor of certain metabolic processes rather than performing all biochemical functions autonomously. Bacteria that engage in this cooperative exchange of nutrients can save a significant amount of energy.
Indeed, in a previous study, the researchers could already demonstrate that this division-of-metabolic-labor can positively affect bacterial growth. In the new study, they addressed the question how such cooperative interactions can persist if non-cooperating bacteria consume amino acids without providing nutrients in return. The evolutionary disadvantage that results for cooperative cells could lead to a collapse of the cross-feeding interaction.
To experimentally verify this possibility, the scientists have monitored co-cultures of cooperating and non-cooperating bacteria. For this, they genetically engineered “cooperators” of two bacterial species that released increased amounts of certain amino acids into their environment. “As a matter of fact, non-cooperators grew better than cooperators in a well-mixed liquid medium, because under these conditions, they had an unrestricted access to the amino acids in the medium. Their growth, however, was considerably reduced when placed on a two-dimensional surface,” said Kost, summarizing the results of the experiments. A more detailed analysis revealed that non-cooperating bacteria could only exist at the very fringe of colonies consisting of cooperating bacteria. More.
The problem is, as I noted elsewhere, something must be doing the thinking in or for the bacteria that know enough to pay attention to whether the benefits are reciprocated.
When theorists used to claim that natural selection somehow evolves it all, acting on random mutation (Darwinism), they probably didn’t have anything this complex in mind. (But just watch some of them claim they did, and either double down on dogmatism or add elaborate, improbable extensions to their theory.)
Something is missing in our understanding of what happens.
See also: What can we hope to learn about animal minds?
Researchers ask, Was early animal evolution co-operative? The new thesis suggests that the typical Ediacaran animals created an environment around them that enabled the more mobile creatures to evolve.
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Here’s the abstract:
Metabolic cross-feeding interactions are ubiquitous in natural microbial communities. However, it remains generally unclear whether the production and exchange of metabolites incurs fitness costs to the producing cells and if so, which ecological mechanisms can facilitate a cooperative exchange of metabolites among unrelated individuals. We hypothesized that positive assortment within structured environments can maintain mutualistic cross-feeding. To test this, we engineered Acinetobacter baylyi and Escherichia coli to reciprocally exchange essential amino acids. Interspecific coculture experiments confirmed that non-cooperating types were selectively favoured in spatially unstructured (liquid culture), yet disfavoured in spatially structured environments (agar plates). Both an individual-based model and experiments with engineered genotypes indicated that a segregation of cross-feeders and non-cooperating auxotrophs stabilized cooperative cross-feeding in spatially structured environments. Chemical imaging confirmed that auxotrophs were spatially excluded from cooperative benefits. Together, these results demonstrate that cooperative cross-feeding between different bacterial species is favoured in structured environments such as bacterial biofilms, suggesting this type of interactions might be common in natural bacterial communities. (paywall) – Samay Pande, Filip Kaftan, Stefan Lang, Aleš Svatoš, Sebastian Germerodt, Christian Kost. Privatization of cooperative benefits stabilizes mutualistic cross-feeding interactions in spatially structured environments. The ISME Journal, 2015; DOI: 10.1038/ismej.2015.212