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

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 mu M and nitrogen (N) was supplied at varying concentrations to establish resource molar-N:P ratios (S-N:P) ranging between 5:1 and 117:1. Carbon was supplied in excess. Chemostats were sampled at steady stale, the element composition of cells (Q) was determined, and element ratios (Q(X:Y)) were calculated. Q(C:N) was 7.6:1 and varied little with respect to growth rate or the element composition of the medium. In contrast, Q(N:P) was variable and positively related to the S-N:P until an upper limit of similar to 21:1 was attained al a S-N:P Of 33:1. Q(C:P) mimicked Q(N:P) and was constrained by the fixed Q(C: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 Q(N: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).