The evolution of microbial cooperation under chaotic environmental flows
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2023-07-07
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Universidade Estadual Paulista (Unesp)
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Comunidades microbianas exibem uma variedade de comportamentos sociais que podem melhorar o desempenho de múltiplas tarefas. Um desses comportamentos é a cooperação, que podemos definir amplamente como qualquer ação em que o executor assume um custo para gerar um benefício que é compartilhado com o restante da população. Como os subprodutos do comportamento cooperativo podem ser compartilhados com indivíduos que não contribuem para esse comportamento, os indivíduos cooperadores estão propensos à exploração por cepas ``trapaceiras''. Um exemplo paradigmático de cooperação no mundo microbiano é a síntese de bens públicos, exoprodutos produzidos a um custo, que proporcionam benefício a células nas proximidades do produtor, independentemente de estas contribuírem ou não para a sua produção. De acordo com os modelos simples, os cooperadores são sempre extintos devido ao surgimento de mutações trapaceiras. No entanto, a cooperação ainda é um comportamento onipresente na natureza, o que motivou um desenvolvimento de pesquisa na área com o objetivo de desvendar os mecanismos capazes de tornar o comportamento cooperativo evolutivamente estável. Espécies cooperativas podem desenvolver múltiplas características que funcionam como contramedidas diretas contra a sua exploração, como policiamento ou punição destes trapaceiros. Outras características podem fornecer benefícios indiretos aos cooperadores, como a agregação espacial. Neste trabalho, estamos interessados em saber se -- e até que ponto -- o ambiente físico de uma comunidade microbiana pode ou não fornecer um mecanismo que facilite a sobrevivência dos cooperadores. As populações microbianas são frequentemente encontradas em habitats aquosos, onde os fluxos ambientais podem misturar os organismos para gerar estruturas espaciais muito diferentes daquelas formadas na ausência de fluxos externos. Combinamos simulações numéricas e aproximações analíticas para investigar o efeito de fluxos ambientais caóticos na dinâmica de um sistema de duas cepas microbianas, uma que produz bens públicos e outra que não. Nossas simulações numéricas mostram que a mistura caótica dos organismos pode levar à coexistência estável das duas cepas, um resultado não previsto pela aproximação de campo médio que derivamos para nosso modelo de duas cepas. Para melhor entender como a coexistência das cepas se torna estável, comparamos o cenário de fluxo caótico com um em que os organismos se difundem muito rapidamente. Comparando esses dois cenários, observamos que fluxos caóticos rápidos permitem a coexistência não por criar estruturas espaciais que separam cooperadores de trapaceiros, mas por enfraquecer as correlações espaciais criadas por eventos de reprodução. A quebra das correlações espaciais aproxima o sistema de um cenário com distribuição homogênea de indivíduos. No entanto, devido ao alcance finito das interações e à natureza estocástica do sistema, a vizinhança de cada produtor é diferente e muda constantemente devido ao fluxo externo. Concluímos que a combinação de mistura caótica, estocasticidade demográfica e interações de alcance finito permite a coexistência, pois a vizinhança de cada cooperador alterna continuamente entre os estados de presença e ausência de trapaceiros. Por fim, propomos direções para investigações futuras para melhor entender a coexistência no sistema estudado.
Microbial communities display a range of social behaviors that can enhance the performance of multiple tasks. One of these behaviors is cooperation, which we can broadly define as any action in which the executor assumes a cost to generate a benefit that is shared with the rest of the population. Because the byproducts of the cooperative behavior might be shared with individuals that do not contribute to this behavior, cooperating individuals are prone to exploitation by defecting strains. A paradigmatic example of cooperation in the microbial world is the synthesis of public goods (PG), exoproducts that are costly to produce and provide a benefit to cells in the vicinity of the producer, regardless of whether they contribute to its production or not. According to basic models, cooperators are always driven to extinction due to the emergence of cheating mutations. However, cooperation is still a ubiquitous behavior in nature, which motivated the development of a large body of research aiming to uncover the mechanisms capable of making cooperative behavior evolutionarily stable. Cooperating species can evolve multiple traits that work as direct countermeasures against exploitation by defectors, such as policing or punishment of defectors. Other traits can provide indirect benefits to cooperators, such as spatial aggregation. In this work, we are interested in whether and to what extent the physical environment of a microbial community can or cannot provide a mechanism that facilitates the survival of cooperators. Microbial populations are often found in aqueous habitats, where environmental flows can mix the organisms to generate spatial structures very different from the ones formed in the absence of external flows. We combine intensive numerical simulations and analytical approximations to investigate the effect of chaotic environmental flows on the dynamics of a system of two microbial strains, one that produces public goods and another that does not. Our numerical simulations show that fast chaotic mixing can lead to stable coexistence of the two strains, a result that was not predicted by the mean-field approximation we derived for our two-strain model. To better understand how strain coexistence becomes stable, we compare the vi chaotic-flow scenario to one in which organisms diffuse very fast. Comparing these two scenarios, we find that fast chaotic flows allow for coexistence not by creating spatial structures that segregate cooperators from defectors, but by breaking the spatial correlations created by reproduction events. The break of spatial correlations brings the system close to a well-mixed scenario. However, because of the finite range of interactions and the stochastic nature of the system, the neighborhood of each producer is different, and constantly changing due to the external flow. We conclude that the combination of chaotic mixing, demographic stochasticity, and finite-range interactions allows for coexistence as the neighborhood of each cooperator continuously switches between states of presence and absence of defectors. Finally, we propose routes for future investigation to better understand the coexistence in the studied system
Microbial communities display a range of social behaviors that can enhance the performance of multiple tasks. One of these behaviors is cooperation, which we can broadly define as any action in which the executor assumes a cost to generate a benefit that is shared with the rest of the population. Because the byproducts of the cooperative behavior might be shared with individuals that do not contribute to this behavior, cooperating individuals are prone to exploitation by defecting strains. A paradigmatic example of cooperation in the microbial world is the synthesis of public goods (PG), exoproducts that are costly to produce and provide a benefit to cells in the vicinity of the producer, regardless of whether they contribute to its production or not. According to basic models, cooperators are always driven to extinction due to the emergence of cheating mutations. However, cooperation is still a ubiquitous behavior in nature, which motivated the development of a large body of research aiming to uncover the mechanisms capable of making cooperative behavior evolutionarily stable. Cooperating species can evolve multiple traits that work as direct countermeasures against exploitation by defectors, such as policing or punishment of defectors. Other traits can provide indirect benefits to cooperators, such as spatial aggregation. In this work, we are interested in whether and to what extent the physical environment of a microbial community can or cannot provide a mechanism that facilitates the survival of cooperators. Microbial populations are often found in aqueous habitats, where environmental flows can mix the organisms to generate spatial structures very different from the ones formed in the absence of external flows. We combine intensive numerical simulations and analytical approximations to investigate the effect of chaotic environmental flows on the dynamics of a system of two microbial strains, one that produces public goods and another that does not. Our numerical simulations show that fast chaotic mixing can lead to stable coexistence of the two strains, a result that was not predicted by the mean-field approximation we derived for our two-strain model. To better understand how strain coexistence becomes stable, we compare the vi chaotic-flow scenario to one in which organisms diffuse very fast. Comparing these two scenarios, we find that fast chaotic flows allow for coexistence not by creating spatial structures that segregate cooperators from defectors, but by breaking the spatial correlations created by reproduction events. The break of spatial correlations brings the system close to a well-mixed scenario. However, because of the finite range of interactions and the stochastic nature of the system, the neighborhood of each producer is different, and constantly changing due to the external flow. We conclude that the combination of chaotic mixing, demographic stochasticity, and finite-range interactions allows for coexistence as the neighborhood of each cooperator continuously switches between states of presence and absence of defectors. Finally, we propose routes for future investigation to better understand the coexistence in the studied system
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Ecologia microbiana, Turbulência, Dinâmica populacional