Figure 3.

Inner (blue) and outer (red) boundaries of the region in which cells reliably perceive gradients. Dashed line shows v = 30 µm s⁻¹, maximum swimming speeds of E. coli [10]; dashed-dotted line shows v = 66 µm s⁻¹, typical cruising speed of Vibrio coralliilyticus; dotted line shows v = 96 µm s⁻¹, maximum speed of V. coralliilyticus after initiating chemokinesis [17]. Other parameters as in figure 2. Solid grey curve is the outer boundary, r_c, of the region within which cells can resolve absolute concentration. Solid black curve is the radius at which the SNR is maximized for a static profile (green curve in figure 2). Inset shows relative sizes of the region where cells can detect gradients (r_i < r < r_o, blue region), and the region where cells can resolve absolute concentration (r < r_c, grey region inward) at t = 90 s (v = 66 µm s⁻¹). (Online version in colour.)