Figure 3 Surface Uptake as a Function of Local Velocity and Organismal Surface Transport Show full caption −3–10−4 cm/s)], organismal uptake is more sensitive to a difference in velocity (U) than to differences in the active transport rate (k) at the surface of the organism. Surface uptake rate (per unit of surface area per unit of concentration outside the diffusive boundary layer [DBL], in cm/s) as a function of local flow velocity (U) and a surface/membrane transport parameter (k). At lower velocities, diffusion across the DBL is limiting, and any change in local velocity impacts the diffusive boundary layer controlling the amount of uptake. Higher velocities remove diffusive limits to uptake, and organismal transport (k) limits uptake. Values of k determined for a prokaryotic mat “PM,” frog skin “FS,” and seagrass “SG” are shown (see Figure S2 for determination). Horizontal contours of uptake (lower right) indicate uptake controlled exclusively by the transport parameter k. Vertical contours of uptake indicate that velocity completely controls uptake. In the region of interest likely to characterize flow within the rangeomorph communities [dashed box: U ≤ O(1 cm/s) and k ∼ O(10–10cm/s)], organismal uptake is more sensitive to a difference in velocity (U) than to differences in the active transport rate (k) at the surface of the organism.

Figure 4 Uptake as a Function of Height Off the Bottom in the Rangeomorph Community Show full caption (A) Increase in velocity with height for the canopy flow produced by the community modeled (red) and a boundary layer flow (blue) generated in the absence of a dense community, as in Figure 2 C. (B) Increase in uptake (per unit of surface area per unit of concentration, in cm/s) with height above the bottom, for k = 1 × 10−3 cm/s. (C) Vertical gradients of potential uptake in the flow. This indicates the adaptive growth advantage of increased height. There is a much greater adaptive impetus to grow to larger size in a canopy flow due to the stronger velocity gradients at the top of the community (see also Figures S3 and S4 ). The ratio of the two models (black line) is also plotted. This ratio serves to show that, with the exception of a thin region near the bed, the modification of flow produced by the community is responsible for a dramatic upward growth advantage that is not present in an unencumbered flow (see also Figure S4 ).

Uptake by the community is controlled by two factors: diffusion across the DBL near the organismal surface, which is strongly impacted by the local velocity, and transport across the biological surface. To quantify active transport, we employ a surface transport parameter k with units of velocity (see Supplemental Experimental Procedures ). The vertical velocity profile through this community reveals that growth off the seafloor exposed rangeomorphs to increasing flow in an otherwise low-velocity setting ( Figure 2 C). In this low-velocity regime, exposure to higher flow overcomes limits imposed by the submillimeter DBL at the organismal surface ( Figure 3 ), dramatically increasing the potential for metabolite uptake. In the parameter space likely to be operative in the community (based on known surface transport rates at comparable biological surfaces and the relevant local velocities within the community of order 1 cm/s or less), changes in flow velocity will contribute more substantially to change in uptake than will comparable changes in the kinetics at the organism’s surface. Furthermore, if one assumes a particular surface transport rate (k) for the rangeomorph organisms, the uptake rate at any given height in the community can be determined ( Figure 4 B) on the basis of the velocity profile in the community ( Figure 4 A). The advantage of upward growth, as determined by the increase in uptake of a slightly taller organism (or growing tip), is then determined by the slope (derivative) of the uptake curve relative to height ( Figure 4 C). This exercise illustrates the dramatic advantage of upward growth in the community. This effect is substantial over a wide range of heights ( Figure 4 C), encompassing the growing tips throughout much of the community ( Figure 2 A). The ratio of uptake gradients between the canopy flow in the community and the boundary layer flow generated in the absence of a dense community ( Figure 4 C) shows that the community provides an upward impetus to growth at all heights above approximately 1 cm. Thus, upward growth of the community appears to be in significant part a response to the structure of the flow generated by the dense community itself.