Movie S1.

Seawater bacteria transiently clustering around natural phytoplankton cells. The clustering response is driven by chemotaxis toward the chemical gradients (DOM) surrounding the phytoplankton cells. Cell swimming trajectories for this case are shown in Fig. 1. Dark-field microscopy movies were acquired at three time points (0, 6, and 9 min) at 15 frames per s and playback is real time. (Scale bar, 50 μm.)