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 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.