EXPERIMENTAL SETUP
In our experimental microcosms (Fig. 1C), enhanced motility of the
diatom Navicula arenaria var. rostellata (seeMaterials and Methods for detailed descriptions on its basic
information, Fig. 1E is a picture of electron microscope image of the
studied species) was stimulated by exposing cells to low concentrations
of dissolved silicic acid (dSi, 15 mg/L). Sample is enclosed in a sealed
chamber which consists of a silicone well, and two cover slips. The well
has a diameter of 0.8 cm and a height of 0.1 cm (Fig. 1C). We used a
tracing technique to quantify the movement pattern of individual diatom
cells (Fig. 1D and 2A). We subsequently developed a simple theoretical
model that can well capture the spatial trajectories as well as the
statistical properties of their foraging movement. During the
experiments, cells were placed in a coverslip chamber in dSi depleted
culture medium (about 15 \(\text{cells}/mm^{2}\)). The movement patterns
of the diatom cells were recorded by a Ti-E Nikon phase contrast
microscope with high temporal-spatial resolutions (see Materials
and Methods ).