A major theme driving research in biology is the relationship between form and function. In particular, a longstanding goal has been to understand how the evolution of multicellularity conferred fitness advantages. Here we show that biofilms of the bacterium Pseudomonas aeruginosa produce structures that maximize cellular reproduction. Specifically, we develop a mathematical model of resource availability and metabolic response within colony features. This analysis accurately predicts the measured distribution of two types of electron acceptors: oxygen, which is available from the atmosphere, and phenazines, redox-active antibiotics produced by the bacterium. Using this model, we demonstrate that the geometry of colony structures is optimal with respect to growth efficiency. Because our model is based on resource dynamics, we also can anticipate shifts in feature geometry based on changes to the availability of electron acceptors, including variations in the external availability of oxygen and genetic manipulation that renders the cells incapable of phenazine production.A desire to understand the relationship between form and function motivates many lines of inquiry in biology, including the study of multicellular morphologies and the evolution of these features. Cells grow and survive in populations in many types of natural systems. These multicellular populations include bacterial biofilms, such as Pseudomonas aeruginosa microcolonies in the lungs of cystic fibrosis patients (1) and cyanobacterial colonies and mats in marshes, lakes, and oceans (2, 3), and complex eukaryotic macroorganisms, such as plants and animals. In many of these contexts, enhanced survival arises from advantages associated with multicellularity at multiple scales. Recent work demonstrated that simple cooperation within aqueous microbial biofilms allows groups of genetically similar cells to form tall mushroom-like structures that reach beyond local depletion zones into areas of fresh resources (4-12). Other work has shown that basic colonial growth (e.g., that of budding yeast) and the evolution of complex multicellular life are accompanied by enhanced growth efficiency and several energetic advantages (13-15).The specific organization of and relationship between cells living within a population are important characteristics that determine whether organisms benefit from the multicellular lifestyle. For example, in mammals the metabolic rate of individual cells is regulated by the size of the whole organism, an effect that confers greater efficiency (15). Cells have dramatically elevated metabolic rates when living as individuals in culture compared with when they survive and grow as members of a multicellular organism (15). This regulation is considered a natural consequence of resource supply within hierarchical vascular networks (15-17) and highlights the importance of morphology and structure in dictating cellular physiology for multicellular systems. Variations in gross morphology also may provide information about how different spec...