Bacterial populations whose growth depends on the cooperative production of public goods are usually threatened by the rise of cheaters that do not contribute but just consume the common resource. Minimizing cheater invasions appears then as a necessary mechanism to maintain these populations. However, that invasions result instead in the persistence of cooperation is a prospect that has yet remained largely unexplored. Here, we show that the demographic collapse induced by cheaters in the population can actually contribute to the rescue of cooperation, in a clear illustration of how ecology and evolution can influence each other. The effect is made possible by the interplay between spatial constraints and the essentiality of the shared resource. We validate this result by carefully combining theory and experiments, with the engineering of a synthetic bacterial community in which the public compound allows survival to a lethal stress. The characterization of the experimental system identifies additional factors that can matter, like the impact of the lag phase on the tolerance to stress, or the appearance of spontaneous mutants. Our work explains the unanticipated dynamics that eco-evolutionary feedbacks can generate in microbial communities, feedbacks that reveal fundamental for the adaptive change of ecosystems at all scales.

Genome-scale genetic interaction networks are progressively contributing to map the molecular circuitry that determines cellular behavior. To what extent this mapping changes in response to different environmental or genetic conditions is, however, largely unknown. Here, we assembled a genetic network using an in silico model of metabolism in yeast to explicitly ask how separate genetic backgrounds alter network structure. Backgrounds defined by single deletions of metabolically active enzymes induce strong rewiring when the deletion corresponds to a catabolic gene, evidencing a broad redistribution of fluxes to alternative pathways. We also show how change is more pronounced in interactions linking genes in distinct functional modules and in those connections that present weak epistasis. These patterns reflect overall the distributed robustness of catabolism. In a second class of genetic backgrounds, in which a number of neutral mutations accumulate, we dominantly observe modifications in the negative interactions that together with an increase in the number of essential genes indicate a global reduction in buffering. Notably, neutral trajectories that originate considerable changes in the wild-type network comprise mutations that diminished the environmental plasticity of the corresponding metabolism, what emphasizes a mechanistic integration of genetic and environmental buffering. More generally, our work demonstrates how the specific mechanistic causes of robustness influence the architecture of multiconditional genetic interaction maps.

29How are public goods 1-4 maintained in bacterial cooperative populations? The presence of 30 these compounds is usually threatened by the rise of cheaters that do not contribute but 31 just exploit the common resource 5,6 . Minimizing cheater invasions appears then as a 32 necessary maintenance mechanism 7,8 . However, that invasions can instead add to the 33 persistence of cooperation is a prospect that has yet remained largely unexplored 6 . Here, 34we show that the detrimental consequences of cheaters can actually preserve public goods, 35at the cost of recurrent collapses and revivals of the population. The result is made 36 possible by the interplay between spatial constraints and the essentiality of the shared 37 resource. We validate this counter-intuitive effect by carefully combining theory and 38 experiment, with the engineering of an explicit synthetic community in which the public 39compound allows survival to a bactericidal stress. Notably, the characterization of the 40 experimental system identifies additional factors that can matter, like the impact of the lag 41 phase on the tolerance to stress, or the appearance of spontaneous mutants. Our work 42 emphasizes the unanticipated consequences of the eco-evolutionary feedbacks that emerge 43 in microbial communities relying on essential public goods to function, feedbacks that 44 reveal fundamental for the adaptive change of ecosystems at all scales. 45 46 47 48 49 50 51 52 53 54 55 3 Main text 56 57The threat of cheaters represents at a microbial scale a well-known public good 58 (PG) dilemma, known as the "tragedy of the commons" 9 , and can fundamentally interfere 59 with the sustainability of microbial communities. The necessity of recognizing the 60 consequences of social dilemmas in microorganisms thus becomes essential, given their 61 impact in many aspects of life on Earth, and also its particular relevance to humans in 62 matters of health (microbiome) 10 , and industry (bioremediation, biofuels, etc) 11 . We 63 considered specifically a scenario in which a community is organized as a dynamical 64 metapopulation (i.e., the community is transiently separated into groups) 12 , and the action 65 of a PG is essential for its survival. Spatial structure is a well-known universal mechanism 66to promote cooperation 13 , which frequently emerges in bacterial populations, for instance, 67due to the restricted range of microbial interactions 14,15 . However, it is much less 68 understood how the presence of structure affects the maintenance of cooperation when 69 combined with explicit population dynamics (earlier work usually assumed constant 70 population and only examined evolutionary dynamics) 16 . The change in population size 71associated to the essentiality of the PG can indeed bring about complex eco-evolutionary 72 feedbacks 17-20 , in which both population density and frequency of "cooperators" influence 73 each other. The connection between these feedbacks and spatial structure remains thus an 74open problem that has started to be addres...

Pyoverdin is a water-soluble metal-chelator synthesized by members of the genus Pseudomonas and used for the acquisition of insoluble ferric iron. Although freely diffusible in aqueous environments, preferential dissemination of pyoverdin among adjacent cells, fine-tuning of intracellular siderophore concentrations, and fitness advantages to pyoverdin-producing versus nonproducing cells, indicate control of location and release. Here, using time-lapse fluorescence microscopy to track single cells in growing microcolonies of Pseudomonas fluorescens SBW25, we show accumulation of pyoverdin at cell poles. Accumulation is induced by arrest of cell division, is achieved by cross-feeding in pyoverdin-nonproducing mutants, is independent of cell shape, and is reversible. Furthermore, it occurs in multi-species communities. Analysis of the performance of pyoverdin-producing and nonproducing cells under conditions promoting polar localization shows an advantage to accumulation on resumption of growth after stress. While the genetic basis of polarization remains unclear, evaluation of deletion mutants of pyoverdin transporters (opmQ, fpvA) establishes non-involvement of these candidate loci. Examination of pyoverdin polar accumulation in a model community and in a range of laboratory and natural species of Pseudomonas, including P. aeruginosa PAO1 and P. putida KT2440, confirms that the phenotype is characteristic of Pseudomonas.