Active control of surface immobilization in the foraging behavior of Paramecium

Key results

1. ShownIn bacterized medium the population splits into fast swimmers and cells stopped on both top and bottom surfaces while feeding; individual cells stop for a few minutes and then leave, so the set of immobile positions turns over within minutes, resembling animal foraging.

   Whole-drop tracking (n = 55, 2 h): immobile fraction exceeded 50% within 20 min, then settled near 25%; swimming speeds bimodal (near 0 and near 500 µm/s); immobile positions fully replaced over 5 min; stops lasted 193 ± 271 s (mean ± s.d.), roughly exponentially distributed. Stopped cells showed ciliary currents carrying bacteria and moving food vacuoles. · Foraging behavior of Paramecium; Figure legends

2. ShownBacteria act as a trigger for immobilization, not as an attachment substrate: in bacteria-filtered medium almost all cells keep swimming, while washing cells into a sterile Na+-free calcium-potassium solution produces similar transient stops on glass or plastic, which added Na+ prevents.

   Filtered medium (n = 38): almost all cells kept swimming, vs progressive stopping and clustering in bacterized medium (n = 24). Washed into 1 mM CaCl2 + 4 mM KCl, most stationary (n = 18) and many logarithmic (n = 19) cells stopped, on both top and bottom glass surfaces, and also on a Petri dish; most swam again after 6 h. Adding 11.5 mM Na+ abolished this (n = 50 vs 50). · Foraging behavior of Paramecium; Surface immobilization in inorganic solutions; Figure legends

3. ShownSurface immobilization requires entry of extracellular calcium: partial calcium chelation and the channel blocker ruthenium red both suppress it, and ruthenium red also slows population growth.

   0.37 mM EGTA cut maximal immobilization to 18% (n = 51; p = 3·10^-8 vs control, Welch's t-test), whereas 0.32 mM EGTA (n = 44) and control (n = 56) reached nearly 50%; 0.4 mM EGTA killed all cells. 10 µM ruthenium red almost completely blocked immobilization (n = 30 vs 38 control), and cell counts rose about 11% over 3 h vs 34% in control. · Surface immobilization requires a calcium influx; Figure legends

4. SuggestedCells do not anchor with trichocysts; instead an immobilized cell drives a flow that is strong along the oral groove and anterior end but weak dorsally, which a Stokeslet model reproduces when locomotor ciliary force is below 0.1 of oral ciliary force.

   A trichless mutant (no trichocysts) still attached transiently. PIV with 1 µm beads (Fig. 5 plus two more cells each in Fig. S2): strongly asymmetric flow around spontaneously immobilized cells vs symmetric flow around cells glued with Cell-Tak in Na+-supplemented solution. Simulated locomotor:oral traction ratios below 0.1 matched immobilized cells; 0.5-1 matched glued cells. · Paramecium immobilizes by inhibiting its…; Figure legends

5. SuggestedHigh-speed imaging shows many dorsal locomotor cilia stopped or passively floating even where they do not touch the surface, while mouth and oral-groove cilia keep beating at frequencies similar to glued cells; the authors report the mouth beat frequency as a new observation.

   500-700 Hz imaging of 4 spontaneously immobilized vs 5 Cell-Tak-glued cells: mouth 71±2 vs 76±8 Hz (p=0.19), oral groove 52±3 vs 46±3 Hz (p=0.11), dorsal 17±1 vs 24±2 Hz (p=0.008, one-sided Mann-Whitney). Time-averaged movies showed immobile dorsal cilia only in spontaneously immobilized cells; glued cells had none apart from a few stuck to a trichocyst. · Paramecium immobilizes by stopping locomotor…; Figure legends