Overall biomass yield on multiple nutrient sources

Microorganisms utilize nutrients primarily to generate biomass and replicate. When a single nutrient source is available, the produced biomass increases linearly with the initial amount of the available nutrient. This linear trend can be predicted to high accuracy by "black box models" that consider growth as a single chemical reaction with nutrients as substrates and biomass as a product. Since natural environments typically feature multiple nutrients, we here quantify the effect of co-utilization of multiple nutrients on bacterial biomass production. First, we demonstrate a mutual effect between the metabolism of different nutrient sources where the ability to utilize one is affected by the other. Second, we show that for some nutrient combinations, the produced biomass is no longer linear to the initial amount of nutrients. These observations cannot be explained by the traditional "black box models", presumably because the metabolism of one nutrient affects another, which is not accounted for by these models. To capture these observations, we extent "black box models" to include catabolism, anabolism, and biosynthesis of biomass precursors and phenomenologically add a mutual effect between the metabolism of the nutrient sources. The expanded model qualitatively recaptures the experimental observations and, unexpectedly, predicts that the produced biomass is not only dependent on the combination of nutrient sources but also on their relative initial amounts. We validate this prediction experimentally by demonstrating how measurement of the produced biomass can be used to determine how each nutrient effects the metabolic processes of another.