Element ratios and growth dynamics of bacteria in an oligotrophic Canadian shield lake

Basic bacterial physiology predicts that the element composition of cells changes as a function of generation time. Rapidly growing cells are characterized by low C:P and N:P ratios compared to ratios of slowly growing cells. Working from this premise, we (1) determined the element composition of newly synthesized bacteria collected from an oligotrophic Canadian shield lake and (2) compared the changes in element composition of bacteria to their temporal changes in abundance. The element composition of new biomass was determined from the change in bacterial element composition following nutrient enrichment and incubation with nutrients derived from seston concentrated from a eutrophic lake. Bacterial C:P and C:N ratios generally decreased during incubation but N:P ratios changed only slightly. When averaged, new bacterial biomass had a C:P ratio (atomic) of 44.4, N:P of 8.5, and C:N of 5.5 (C:N:P 44:9:1). The element composition of the in situ bacterial assemblage of an oligotrophic lake was compared to the temporal dynamics of the assemblage. Cell N:P and C:P ratios were low when cell abundance was increasing compared to the ratios obtained following a rapid decline in cell abundance. The data indicate that bacteria may alter their cellular element composition, and that the growth dynamics of bacterial assemblages may be reflected by the element ratios of their biomass.