Quantifying nutrient colimitation of microbial growth

Abstract

Nutrient availability dictates how fast and how much microbial populations grow. Quantifying the relationship between microbial growth and nutrient concentrations makes it possible to promote, inhibit, and predict microbial activity. Microbes require many different resources, including elemental nutrients such as carbon, nitrogen, and the trace metals, as well as complex nutrients like vitamins and amino acids. In nature, many of these nutrients are scarce and their abundances co-vary. This presents the possibility of nutrient colimitation, when more than one resource limits growth simultaneously. Despite growing evidence of possible colimitation in nature, the data is often difficult to interpret and compare due to lack of quantitative definitions for limitation and colimitation. Here we introduce a set of metrics that quantify multiple nutrient limitation in microbial growth. We introduce the limitation coefficient to quantify limitation by individual resources and a colimitation index for measuring the effective number of limiting resources. These quantities demonstrate that limitation conditions are not binary or static, but rather exist on a continuum and can change over time as resource conditions change. These definitions facilitate quantitative comparisons between laboratory experiments and nutrient conditions in nature. Furthermore, they allow us to illustrate how limitation of microbial growth rate and biomass yield are related but distinct notions. To demonstrate these concepts, we measure growth ofEscherichia coliunder laboratory conditions and demonstrate that colimitation occurs in clonal populations and is readily accessible in laboratory conditions. Finally, we apply our framework to environmental data to provide intuition for what limitation conditions might exist in nature.