Optimal management of nutrient reserves in microorganisms under time-varying environmental conditions.

•We present a stoichiometrically consistent macro-chemical model of microbial growth and metabolism that accounts for variable stores as well as adaptive allocation of building blocks to various types of catalytic machinery.•The model posits feedback loops linking expression of assimilatory machinery to reserve density; the precise form of the “regulatory law” at the heart of such a loop expresses how the cell manages internal stores.•We demonstrate how this “regulatory law” can be recovered from experimental data using several empirical data sets.•We find that stores should be expected to be negligibly small in stable growth-sustaining environments, but prominent in environments characterised by marked fluctuations on time scales commensurate with the inherent dynamic time scale of the organismal system.