Ratios of carbon, nitrogen and phosphorus in Pseudomonas fluorescens as a model for bacterial element ratios and nutrient regeneration

Abstract

Bacteria play a significant role in the nutrient dynamics of planktonic systems, yet their nutrient demands are poorly known. Here, we characterize the element ratios of a common freshwater bacterium relative to growth rate and nutrient availability. Pseudomonas fluorescens was grown in chemostats at dilution rates of 0.03, 0.06, and 0.09 h-1. Phosphorus (P) was supplied at approximately 5 μM and nitrogen (N) was supplied at varying concentrations to establish resource molar-N·P ratios (SN P) ranging between 5.1 and 117:1. Carbon was supplied in excess. Chemostats were sampled at steady state, the element composition of cells (Q) was determined, and element ratios (QX Y) were calculated. QC N was 7.6.1 and varied little with respect to growth rate or the element composition of the medium. In contrast, QN P was variable and positively related to the SN,P until an upper limit of ∼21:1 was attained at a SN P of 33:1. QC P mimicked QN P and was constrained by the fixed QC N. P. fluorescens appeared to have a high capacity to accumulate P and the data suggested that the organism may be capable of incorporating P above that required to meet metabolic demand ('luxury uptake'). Comparison of these data with previously published data revealed that the bacteria have a much greater potential to adjust their QN P when the N:P ratio of resources supporting growth is low (N scarce), than when the ratio is high (N abundant). The C:N:P ratio of bacteria was found to vary between 52:8:1 when N was scarce relative to P (N:P < 40:1), to as high as 163:25:1 when N was abundant relative to P (N:P > 40:1).