R TECON¹, D OR<sup>1

1</sup>Department of Environmental Systems Science, ETH Zürich, 8092 Zürich, Switzerland

In natural environments, microorganisms often live in rich multispecies assemblages (e.g., biofilms), where resource utilization is constantly optimized and could yield stable spatial patterns. In unsaturated soils where aqueous habitats are highly fragmented, the complex nutrient diffusion fields promote microbial self-organization when motion is possible. The principles and conditions that control microbial self-organization remain understudied. Here, we examine how carbon source utilization, surface attachment and water film configuration shape the spatial organization of a two-member bacterial consortium. In our experimental system, two fluorescently-tagged mutant strains of the soil bacterium Pseudomonas putida were forced to cooperate to degrade and utilize toluene, resulting in mutualistic trophic interactions. Replacing toluene by benzoate, a carbon source degraded by both strains, changed the interaction from cooperation to exploitation competition. On agar surfaces, we showed that mutualism was expressed by distinct intermixing patterns, whereas competition promoted genotypic segregation. Characteristics of these patterns were quantified using image analyses. Results show that the ratio of cooperators in the mutualistic consortium converged to nearly 1:1 on solid surfaces and in liquid cultures regardless of the initial ratio. The consortium productivity, as measured by cell growth, was affected by the initial ratio on surfaces but not in liquid cultures. The consortium was grown on a porous surface model that mimics unsaturated soils to evaluate the role of hydration conditions on consortium spatial self-assembly. The study suggests that trophic dependencies among multispecies surface-attached microbial communities result in specific spatial patterns for optimal ecological functioning.