The hypothesis that bacteria can cluster around phytoplankton cells in
the turbulent mixed layer was tested with a model that simulates bact
erial chemotaxis toward a neutrally buoyant phytoplankton cell exuding
dissolved organic C. The model results indicate that bacteria can att
ain population densities orders of magnitude above background levels i
n microzones occupying <0.1% of the fluid volume surrounding each phyt
oplankton cell. The simulation results indicate that at turbulence int
ensities expected in the upper mixed layer of the ocean (shear rates o
f approximately 0.15 s-1) bacteria initially approach phytoplankton th
rough random swimming and relative fluid motions. Chemotactic response
serves to prolong a bacteria's stay near the phytoplankter before it
is carried away by random swimming and fluid motions. At these shear r
ates, up to 20% of the chemotactic bacteria population could be cluste
red around exuding phytoplankton cells, even though individual bacteri
a stay in a cluster less than a minute. For these conditions the time-
averaged exudate exposure of the bacterial population could be 10 time
s higher than that of a nonchemotactic population. Exudate exposures i
n unsteady shearing were found to equal or exceed the corresponding st
eady shear values. Although unsteady bursts of turbulent mixing in the
oceanic surface layer should disperse clusters, intervening calm peri
ods are long enough to allow clusters to reform. The model indicates t
hat bacterial clustering is unlikely to have a significant effect on p
hytoplankton nutrient uptake or on the fate of bacterial secondary pro
duction in the microbial food web.