Long-time behavior of swimming Euglena gracilis in a heterogenous light environment

Abstract

The cell motion of Euglena gracilis in homogeneous and heterogeneous light environments was analyzed. Homogeneous and heterogeneous environments were prepared, with only a red color or with a red circle surrounded by brighter white regions, respectively. In a heterogeneous environment, the cells move into the red circle. Swimming orbits at 1/25 s intervals for 120 s were analyzed. The speed distribution of the 1 s-averaged cell orbits in a homogeneous environment was different from that in a heterogeneous environment, where the faster swimming fraction was enhanced. The relationship between speed and curvature radius was analyzed using a joint histogram. Histograms for short timescale motion, constructed by 1 s-averaged orbits, suggest that the cell swimming curves are not biased, while those for long timescale motion, constructed by 10 s-averaged orbits, suggest that the cell swimming curves are biased in the clockwise direction. Furthermore, the curvature radius determines the speed, which does not seem to depend on the light environment. The mean squared displacement in a heterogeneous environment is larger than that in a homogeneous environment on a 1 s timescale. These results will be the basis for constructing a model for the long-time behavior of photomovement for light differences.

Keywords: E. gracilis; curvature radius; diorama environment; heterogeneous light condition; long time scale orbit.

FIGURE 1

(A) Experimental setup. (B) Dimensions of suspension container. A silicone plate with 0.5 mm thickness is sandwiched between two rectangular glass plates. The suspension is put inside the circle (cylinder). (C) An image of E. gracilis. (D) Sub-regions for cell counting. (E) The search region of orbit analysis (a d/2×2d rectangle).

FIGURE 2

(A) Images of the cell distribution (t =30 min). The picture was divided into upper and lower parts, both of which were modified independently to visualize the cells for presentation purposes. (B) Time series of number density n _k (t) in the subregions.

FIGURE 3

(A) Cell motion for 120 s in homogeneous environment. The initial positions were set at the origin. (B) As in (A) but under heterogeneous light conditions in the laboratory frame. The boundary of the red region is indicated by a grey circle. (C) Normalized speed distribution for 1 s-averaged orbits in a homogeneous environment. (D) As in (C) but in a heterogeneous environment. The normalized distribution of the speed of the orbits inside the red region is also indicated by the solid lines.

FIGURE 4

(A) Joint distribution of speed v and curvature radius R for 1 s-average orbit in a homogeneous environment. (B) As in (A) but in a heterogeneous environment.(C) As in (A), but for 10 s-average orbit. (D) As in (C) but in a heterogeneous environment. (E) MSDs for all orbits under homogeneous and heterogeneous conditions. (F) Selected 5 s-average orbits in a heterogeneous environment, colored by local speed. The blue parts indicate speeds less than 90 μm/s and the red parts indicate speeds larger than 90 μm/s.