Figure 3.

Microbial dynamics depend on the relative timescales between environmental fluctuations and microbial responses. (A) The relationship between the timescale of environmental fluctuation (T_e) and the timescale of a microbial response (T_m) determines the dynamics of the response under changing environments. Here, the environment fluctuates between two conditions (gray and white), each associated with a microbial steady state (longer or shorter dashed lines, respectively). When environmental fluctuations occur on sufficiently long timescales relative to the response (T_e > T_m), the response (i.e. the transition between the steady states characteristic of each environmental condition) completes after each shift once T_m has elapsed. When T_e ∼ T_m, the response may produce a behavior that does not stabilize. As T_e becomes smaller with respect to T_m, environmental fluctuations occur faster than the time required to stabilize at steady state, causing microorganisms to fluctuate between steady states without reaching them. Finally, environmental fluctuations can be so fast relative to the microbial response of interest (T_e < T_m) that microorganisms behave as if the environment were a single steady condition. (B) Microbial responses to environmental fluctuation span a diversity of behaviors, including changes in nutrient uptake, growth rate, gene expression and more. These responses occur on behavior-specific timescales of milliseconds to days (Shamir et al. 2016). In this schematic, we list approximate T_m for a variety of possible responses by order of magnitude, as each process realistically also spans a range. For example, different enzymes will catalyze one reaction at different rates (microseconds to tens of seconds), and bacterial generation times will vary with species and growth condition (tens of minutes to days).