Several studies have noted a disparity between the stoichiometric regulation of bacterial assemblages and populations. In response to phosphorus availability, assemblages of bacteria often exhibit greater flexibility in their biomass carbon (C) to phosphorus (P) ratios (C:P) than axenic populations, some of which are homeostatic. We hypothesized that assemblages are inherently non-homeostatic as the result of resource-driven shifts in dominance between more homeostatic strains at low resource C:P ratios and highly flexible strains when P is scarce relative to C. We enriched 6 assemblages of heterotrophic bacteria from 4 lakes in Minnesota, USA, using7 chemostats with varying supply C:P ratios and measured the bacterial biomass C:P ratios. The initial assemblage cultures exhibited non-homeostasis in biomass C:P across treatments, but there was no significant effect of lake trophic state on the strength of homeostasis. This suggests that the non-homeostatic physiology was abundant in each of the lakes. Using the initial assemblage chemostat cultures as a species sorting filter, we subsequently cultured the high-P and low-P selected fractions at varying supply C:P ratios and found that all of the low-P selected fractions were non-homeostatic, and that the high-P selected fractions were strongly homeostatic in 3 of the lakes. These results demonstrate that multiple stoichiometric regulation strategies were present among in situ bacterial communities and confirm that high P availability can select for homeostatic bacterial strains. The divergence in stoichiometric regulation strategies was coupled with changes in the assemblage composition, which highlights the role of ecological selection in the stoichiometric homeostasis of assemblages.
- Heterotrophic bacteria