The Effect of Nutrients and N:P Ratio on Microbial Communities: Testing the Growth Rate Hypothesis and Its Extensions in Lagunita Pond (Churince)

Abstract

The absolute and relative supplies of nutrients such as nitrogen and phosphorus can affect ecosystem properties and microbial biodiversity. More recently, the theory of biological stoichiometry has advanced connections between ecosystem ecology and cellular/molecular biology by proposing a link between biochemical features of microbial cells and their nutrient ratios. Specifically, the growth rate hypothesis (GRH) postulates that cellular stoichiometry varies according to growth rate due to increased allocation to P-rich ribosomal RNA to support rapid growth. Expanding on the GRH, it is predicted that microbes have a suite of genomic features that determine the ability to achieve rapid growth and, hence, influence biomass N:P. These genomic features include codon usage bias, number of rRNA and tRNA genes, and genome size, all of which have been individually linked to growth rate and fitness. This chapter discusses two experiments conducted at the Churince system in Lagunita pond to test the GRH. Churince is an ideal location for this because surface waters in this region have highly imbalanced N:P stoichiometry (TN:TP atomic ratio >100), where P is likely to be strongly limiting. The first experiment was a replicated in situ mesocosm experiment comparing three different nutrient treatments with varying N:P to a control treatment. The second experiment was a whole-pond perturbation, fertilizing with nutrients of N:P = 16 and using metagenomics to compare responses to replicated, internal control mesocosms. We discuss changes in microbial biomass N:P, species composition, and genomic features of the microbes in response to these perturbations of nutrient supplies and N:P.