Simulation of bacterial attraction and adhesion to falling particles in an aquatic environment

Chemosensory movement of bacteria toward the cloud of leaked material around planktonic algae has been suggested as an important aspect of bacterial nutrition in aquatic ecosystems. Inability of bacteria to keep pace with falling algae has been suggested as inhibiting such interactions. The interaction between bacteria and falling particles, including microalgae, is explored with a simulation model of bacterial chemotaxis that incorporates low Reynolds number fluid flow and resulting substrate spatial distributions. Results show that although bacteria cannot maintain a position relative to a falling particle unless the particle is extremely large, such as a marine snow aggregate, chemosensory response does allow them to stay in the high substrate environment of the leaky particle longer than if they did not have it. The ecological significance for bacterial metabolism depends on several factors, including the frequency of contact between algae and bacteria and the relative concentrations of substrate at the cell surface and in the background. Optimal enhancement should occur in eutrophic conditions of large algal cells present in high abundances. Presently available values for the ratio of substrate concentration next to the algal cell relative to that in solution are not large enough to suggest that the microzones around algal cells provide significant enhancement of bacterial nutrition. Chemotaxis should enhance interactions with large marine snow aggregates, both by allowing bacteria to stay for significant periods around them and by enhancing the rates at which bacteria attach to the aggregate surfaces. This enhancement is greater for surfaces in which there is a lower probability of sticking.