Figure 1. Effect of swimming algae on particle diffusivity.

(a) Semi-log plot of the normalized density profiles of 1 μm-PS colloids along the gravity direction for different cell concentrations. Full lines are best exponential fits to the data. Curves have been shifted apart along the y axis for clarity. Colour bar: CR concentration N_c in units of 10⁶ cells ml⁻¹. Error bars represent the s.d. (b) Gravitational length l_g,eff−l₀ as a function of N_c extracted from the fits in a. The solid orange line is the best linear fit to the data, l_g,eff−l₀=((62.8±5)N_c)μm (N_c in units of 10⁶ cells ml⁻¹). Vertical error bars represent the uncertainty on the fits in a. Horizontal error bars are the standard deviations of cell concentrations. (c) Effective microparticle diffusivity (D_eff−D₀) as a function of N_c for the three experiments. Diffusivities are rescaled by the ratio of the average CR speeds , where j stands for sedimentation (S), spreading (CS) or tracking (HS) experiments. Solid lines are best linear fits to the data. Orange squares: sedimentation experiment (slope α_S=1.71±0.14(μm² s⁻¹)/(10⁶ cells ml⁻¹); all other values in the same units); green triangles: spreading experiment (α_CS=1.62±0.14); blue circles: Hele–Shaw experiment (α_HS=1.67±0.13). Orange circles and dashed line: direct tracking in the sedimentation experiment (α_T=0.074±0.014). Black dashed line: fit to the experimental diffusivity obtained by direct tracking in ref. 26 (α_L=0.041). See Supplementary Fig. 5 for a close-up on the low-concentration values. Vertical error bars represent the uncertainty on the fits to obtain the effective diffusivities. Horizontal error bars are the s.d. of the cell concentrations.