Growth rate and resource imbalance interactively control biomass stoichiometry and elemental quotas of aquatic bacteria.

Emily A Whitaker,James B Cotner,Casey M Godwin

doi:10.1002/ecy.1705

Abstract

The effects of resource stoichiometry and growth rate on the elemental composition of biomass have been examined in a wide variety of organisms, but the interaction among these effects is often overlooked. To determine how growth rate and resource imbalance affect bacterial carbon (C): nitrogen (N): phosphorus (P) stoichiometry and elemental content, we cultured two strains of aquatic heterotrophic bacteria in chemostats at a range of dilution rates and P supply levels (C:P of 100:1 to 10,000:1). When growing below 50% of their maximum growth rate, P availability and dilution rate had strong interactive effects on biomass C:N:P, elemental quotas, cell size, respiration rate, and growth efficiency. In contrast, at faster growth rates, biomass stoichiometry was strongly homeostatic in both strains (C:N:P of 70:13:1 and 73:14:1) and elemental quotas of C, N, and P were tightly coupled (but not constant). Respiration and cell size increased with both growth rate and P limitation, and P limitation induced C accumulation and excess respiration. These results show that bacterial biomass stoichiometry is relatively constrained when all resources are abundant and growth rates are high, but at low growth rates resource imbalance is relatively more important than growth rate in controlling bacterial biomass composition.

Highlights

The interaction between organisms and their resources is a central concept at every level of ecological organization, yet our characterization of these relationships is often oversimplified. it is widely understood that resource availability causes changes in the growth rate (Monod 1949) and biomass composition of species (Droop 1974, Bracken et al 2015), the interdependence of these effects receives little attention
We show that relative growth rate strongly modified the relationship between resource stoichiometry and bacterial biomass composition
The physiology of the bacteria and their response to P availability was markedly different at low versus high μ:μ max, with the most extreme biomass stoichiometry observed at the lowest growth rates

Summary

Introduction

It is widely understood that resource availability causes changes in the growth rate (Monod 1949) and biomass composition of species (Droop 1974, Bracken et al 2015), the interdependence of these effects receives little attention. Bacteria are key biogeochemical reactors in all of Earth’s ecosystems (Falkowski et al 2008), so their elemental composition and growth rates determine how they affect the cycles of energy, carbon, and nutrients within ecosystems. Because the stoichiometry of a bacterium’s resources is not necessarily the same as the stoichiometry required for biomass and metabolism, bacteria are likely to experience stoichiometric imbalance and low relative growth rates in many ecosystems. Understanding the interactions among resource availability, growth rate, and biomass stoichiometry is key to understanding how biogeochemical cycles will respond to imbalance in the availability of organic carbon and inorganic nutrients (Elser et al.2009, Peñuelas et al 2012)

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Conclusion