Extended Data Fig. 5: 1A01 has a dispersal lifestyle.

a) Chitin particles pre-colonized with cells (green cells) were isolated at various time (τ) from an exponentially increasing chitin culture, and resuspended in the same volume of fresh media without chitin particles. The amount of particle-associated cells was estimated using the planktonic OD at the time of particle isolation ρ_b(τ) and the constant planktonic fraction ρ_b: ρ_total obtained in Fig. 1, that is, \({\rho }_{s}(\tau )=(\frac{{\rho }_{total}}{{\rho }_{b}}-1){\rho }_{b}(\tau )\). This is taken as the initial cell density \({\rho }_{s}^{init}\) of the resuspended culture. For each resuspended culture, the planktonic OD (Δρ_b, giving the density of blue cells) was measured at regular time intervals Δt. The increase in planktonic OD is expected to be linear in time with a rate k_d for some time after resuspension. b) Inset: the planktonic OD for one such resuspensions increases linearly starting from a background reading ρ₀, due to the turbidity caused by small chitin particles. The dashed line is the line of best fit to the data with a slope \({k}_{d}{\rho }_{s}^{init}\). Main: Traces of the planktonic OD (Δρ_b) were normalized to the initial amount of cells on the particles (\({\rho }_{s}^{init}\)) and plotted as a function of time. The normalized traces collapse on top of one another. The slope of the line of best fit (dashed line) allows an estimate of the detachment rate, with k_d ≈ 0.18 ± 0.02h⁻¹, with the error given by the 95% confidence interval of the fit. c) Similarly to Panel a, pre-colonized chitin particles from a 10mL exponentially increasing chitin culture were separated and re-suspended into the same volume of fresh media without chitin particles. After a time interval of Δt = 2h, the planktonic OD increased by an amount Δρ_b as a result of cell detachment and with the rate observed Panel b. At this point, the particles were sedimented, and resuspended in the same volume of fresh media without chitin particles. This process, which effectively removed all the planktonic cells and thereby prevented reattachment (effectively setting the attachment rate k_a ≈ 0), was repeated 8 times, for a total of 16 hours. d) Traces of the planktonic OD as a function of time for successive resuspension cycles. From an initial OD measurement ρ₀, which is the background OD due to the residual turbidity of small chitin particles, we observed an increase in the planktonic OD, Δρ_b, after the 2h time cycle. The background OD exhibits a time dependence ρ₀(t) due to the successive removal of small particles in the fractionation process. The data shows that in the absence of planktonic cells, the particle-associated cells alone were able to sustain replication and shedding of new planktonic cells in the duration of our experiments, demonstrating that the planktonic culture is not necessary for the replication of cells on the particles. e) From the traces in Panel b, we plot the planktonic increase relative to the background OD as a function of time. The normalization by the dynamic background OD allows to adjust for the loss of small particles and hence of surface-associated cells from the culture during the sedimentation process. The dashed line is obtained by fitting an exponential model to the data, with the growth exponent being 0.065 h⁻¹. Since the cell detachment rate is proportional to the number of cells on particles, the relative increase in planktonic OD, Δρ_b(t)/ρ₀(t), is thus a proxy for the increase in cell density on the particles. We see that this relative increase in planktonic OD is exponential, and that its rate matches the exponential rate measured in an undisturbed chitin culture, λ ≈ 0.06 h⁻¹ (Fig. 1). Since the rate of exponential increase in the planktonic population is hardly affected by the removal of planktonic cells from the chitin culture, we conclude from the data that the attachment rate \({k}_{a}\ll \lambda {\rho }_{s}/{\rho }_{b}=0.02{h}^{-1}\) is thus negligible compared to other rates in our culture.

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