Evolution of organismal stoichiometry in a long-term experiment with Escherichia coli

Abstract

Organismal stoichiometry refers to the relative proportion of chemical elements in the biomass of organisms, and it can have important effects on ecological interactions from population to ecosystem scales. Although stoichiometry has been studied extensively from an ecological perspective, much less is known about the rates and directions of evolutionary changes in elemental composition. We measured carbon, nitrogen and phosphorus content of 12 Escherichia coli populations that evolved under controlled carbon-limited, serial-transfer conditions for 50 000 generations. The bacteria evolved higher relative nitrogen and phosphorus content, consistent with selection for increased use of the more abundant elements. Total carbon assimilated also increased, indicating more efficient use of the limiting element. We also measured stoichiometry in one population repeatedly through time. Stoichiometry changed more rapidly in early generations than later on, similar to the trajectory seen for competitive fitness. Altogether, our study shows that stoichiometry evolved over long time periods, and that it did so in a predictable direction, given the carbon-limited environment.

Keywords: Escherichia coli; carbon limitation; experimental evolution; stoichiometry.

Figure 1.

Molar C : N and C : P ratios of the ancestral (filled diamonds) and 50 000-generation evolved (open squares) clones from the LTEE with E. coli. The evolved clone at the far upper right is from the only population that evolved the ability to consume citrate; this clone was excluded from the statistical analyses owing to the much higher carbon availability that it experienced.

Figure 2.

Changes in carbon and biomass. (a) The per cent carbon in cellular biomass does not differ significantly between the ancestral and evolved clones (p = 0.888). (b) Total dry biomass is significantly higher in the evolved clones than in the ancestral clones (p < 0.001). (c) The total carbon retained in biomass is also significantly higher in the evolved clones (p < 0.001). All data shown are means with 95% confidence intervals.

Figure 3.

Trajectories for elemental ratios and biomass in population Ara-1 between 0 and 60 000 generations. The (a) C : N, (b) C : P and (c) N : P ratios all declined over time, while the (d) dry biomass per volume increased.