Fig. 3.
Motile bacteria often dominate consumption of DOM released from phycospheres. (A) Fraction of DOM released in a lysis that is consumed within the phycosphere (r < 2 mm from the diatom center), and within the duration of accumulation (10 min), by motile (red) and nonmotile (blue) bacteria, as a function of the background concentration of nonmotile bacteria, B0. (Inset) Fraction of DOM consumed in the phycosphere by both populations together. (B) Fraction of DOM consumed by motile bacteria (FMOT; color bar), calculated for a broad range of motile to nonmotile cell number ratios γ, background bacterial concentrations B0, and cell sizes aM and aNM. Red axes show FMOT for two selected values of motile cell size, aM, with nonmotile cell size fixed at aNM = 0.2 μm. Black axes shows FMOT as a function of the nonmotile consumption strength, Σ = 4π aNM D B0,[s−1], and the dimensionless consumption ratio, Γ = aM/aNM γ. Two scenarios are identified by the white symbols (discussed in the main text). (C) Fraction of DOM consumed overall by motile (orange) and nonmotile (cyan) bacteria as a function of the diffusivity D (for B0 = 107 cells per mL). The diffusivity of the effective chemoattractant in the experiments is indicated by the dashed line (D = 3 × 10−11 m2/s). Values of D for some biomolecules are indicated by arrowheads and color-matched to the population that dominates consumption of that molecule. For A, B, and D: γ = 0.1, aM = 0.5 μm, and aNM = 0.2 μm.