Metabolic interactions between cells affect microbial community compositions and hence their function in ecosystems. It is well-known that under competition for the exchanged metabolite, concentration gradients constrain the distances over which interactions can occur. However, interaction distances are typically quantified in two-dimensional systems or without accounting for competition or other metabolite-removal, conditions which may not very often match natural ecosystems. We here analyze the impact of cell-to-cell distance on unidirectional cross-feeding in a three-dimensional aqueous system with competition for the exchanged metabolite. Effective interaction distances were computed with a reaction-diffusion model and experimentally verified by growing a synthetic consortium of 1 µm-sized metabolite producer, receiver, and competitor cells in different spatial structures. We show that receivers cannot interact with producers located on average 15 µm away from them, as product concentration gradients flatten close to producer cells. We developed an aggregation protocol and varied the receiver cells’ product affinity, to show that within producer–receiver aggregates even low-affinity receiver cells could interact with producers. These results show that competition or other metabolite-removal of a public good in a three-dimensional system reduces metabolic interaction distances to the low µm-range, highlighting the importance of concentration gradients as physical constraint for cellular interactions.

细胞之间的代谢相互作用影响微生物群落的组成，从而影响它们在生态系统中的功能。众所周知，在交换代谢物的竞争下，浓度梯度限制了相互作用发生的距离。然而，相互作用距离通常在二维系统中进行量化，或者不考虑竞争或其他代谢物去除，这些条件通常可能与自然生态系统不匹配。我们在这里分析了细胞间距离对三维水系统中单向交叉喂养的影响以及对交换代谢物的竞争。有效相互作用距离是用反应扩散模型计算的，并通过在不同空间结构中培养 1 µm 大小的代谢物生产者、接收者和竞争者细胞的合成联合体进行实验验证。我们表明，接收器无法与距离它们平均 15 µm 的生产者相互作用，因为靠近生产者细胞的产物浓度梯度变平。我们开发了一种聚合方案并改变了接收细胞的产物亲和力，以表明在生产者-接收者聚集体中，即使是低亲和力的接收细胞也可以与生产者相互作用。这些结果表明，三维系统中公共物品的竞争或其他代谢物去除将代谢相互作用距离降低至低微米范围，突出了浓度梯度作为细胞相互作用的物理约束的重要性。