32 33 Microorganisms are typically found as complex microbial communities that 34 altogether govern global biogeochemical cycles. Microbes have developed highly 35 regulated metabolic capabilities to efficiently use available substrates including 36 preferential substrate usage that can result in diauxic shifts. This and other 37 metabolic behaviors have been discovered in studies of microbes in mono-38 culture when grown on low-complexity (e.g. two-component) mixtures of 39 substrates, however, little is known about how species partition environmental 40 substrates through substrate competition in more complex substrate mixtures. 41 Here we use exometabolomic profiling to examine the time-varying substrate 42 depletion from a mixture of 19 amino acids and glucose by two Pseudomonads 43 and one Bacillus species isolated from ground water. We examine if the first 44 substrates depleted result in maximal growth rate, or relate to growth medium or 45 biomass composition and find surprisingly few correlations. Patterns of substrate 46 depletion are modeled, and these models are used to examine if substrate usage 47preferences and substrate depletion kinetics of three microbial isolates can be 48used to predict the metabolism of the pooled isolates in co-culture. We find that 49 most of the substrates fit the model predictions, indicating that the microbes are 50 not altering their behaviors for these substrates in the presence of competitors. 51Glucose and histidine were depleted more slowly than predicted, while proline, 52glycine, glutamate, lysine, and arginine were all consumed significantly faster; 53 these compounds highlight substrates that could be involved in species-species 54 interactions within the consortium. 55 56Introduction 57 58Microbial communities drive elemental cycling 1 , such as the carbon cycle 2 and 59 the nitrogen cycle 3 . We are also learning that they are critical for the health of 60 their host plant 4 or animal 5 . In both cases the net microbial metabolic 61 processes are of particular interest for predicting nutrient cycles 6,7 and 62 harnessing microbes to improve host health 8 .The environments in which 63 microbial communities live often contain complex mixtures of substrates, and 64understanding how microbial communities partition these substrates is central to 65predicting community metabolism and developing interventions that alter 66 community structure and/or metabolic activities. 67 68Exometabolomics, also known as metabolic footprinting, is a powerful platform 69 for studying how microbes and their consortia modify substrate pools, as analysis 70is only of the extracellular metabolites 9 . With the development of 71 exometabolomics pipelines, the metabolic connections between microbes have 72 begun to be studied at a large scale and have allowed for a more comprehensive 73 approach to monitoring the dynamic transformations of relatively complex 74 mixtures of substrates 9 . Some key examples include optimizing multiple steps of 75 lignocellulose degradation 10,11 , understan...