Simulating Bacterial Clustering around Phytoplankton Cells in a Turbulent Ocean

The hypothesis that bacteria can cluster around phytoplankton cells in the turbulent mixed layer was tested with a model that simulates bacterial chemotaxis toward a neutrally buoyant phytoplankton cell exuding dissolved organic C. The model results indicate that bacteria can attain population densities orders of magnitude above background levels in microzones occupying <0.1% of the fluid volume surrounding each phytoplankton cell. The simulation results indicate that at turbulence intensities expected in the upper mixed layer of the ocean (shear rates of approximately 0.15 s-1) bacteria initially approach phytoplankton through 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 rates, up to 20% of the chemotactic bacteria population could be clustered around exuding phytoplankton cells, even though individual bacteria stay in a cluster less than a minute. For these conditions the time-averaged exudate exposure of the bacterial population could be 10 times higher than that of a nonchemotactic population. Exudate exposures in unsteady shearing were found to equal or exceed the corresponding steady shear values. Although unsteady bursts of turbulent mixing in the oceanic surface layer should disperse clusters, intervening calm periods are long enough to allow clusters to reform. The model indicates that bacterial clustering is unlikely to have a significant effect on phytoplankton nutrient uptake or on the fate of bacterial secondary production in the microbial food web.