Testing the black queen hypothesis in communities of soil-living bacteria

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Sequencing the genomes of bacteria from environmental samples commonly reveals that the corresponding strains lack essential biosynthetic genes. However, how do bacteria, which have lost the ability to produce key metabolites such as amino acids, vitamins, or even nucleotides, survive in environments that are well known to contain very low concentrations of these compounds? The recently proposed black queen hypothesis explains the emergence and persistence of these mutants by adaptive benefits auxotrophic bacteria gain by utilizing metabolites that are produced by other bacterial cells. If true, this process should facilitate the formation of multiply auxotrophic genotypes and the establishment of intercellular metabolic networks, in which the survival of individual cells rests upon the metabolites that are produced by other community members. To test this idea, a previously established collection of 7,000 bacteria will be used that have been isolated from 27 different soil microbial communities. These strains have been genotypically (16SrRNA) and phenotypically (metabolic auxotrophies, motility) well characterized. Taking advantage of this valuable resource, the main goal of this project is to experimentally test the black queen hypothesis in communities of soil-dwelling bacteria. Specifically, we aim at identifying the ecological and physiological factors that govern the establishment and functioning of metabolic cross-feeding interactions within communities of auxotrophic bacterial genotypes. In addition, the evolutionary consequences resulting from this process will be unravelled. By combining carefully controlled coculture experiments in soil microcosms with microscopic and chemical analyses of the interacting consortia, theoretical predictions made by the black queen hypothesis can be empirically verified. The results of this work will provide fundamental new insights into the forces that shape the structure and functioning of natural microbial communities.