title: The Flux of Euglena gracilis Cells Depends on the Gradient of Light Intensity.
abstract: We have quantified the photomovement behavior of a suspension of Euglena gracilis representing a behavioral response to a light gradient. Despite recent measurements of phototaxis and photophobicity, the details of macroscopic behavior of cell photomovements under conditions of light intensity gradients, which are critical to understand recent experiments on spatially localized bioconvection patterns, have not been fully understood. In this paper, the flux of cell number density under a light intensity gradient was measured by the following two experiments. In the first experiment, a capillary containing the cell suspension was illuminated with different light intensities in two regions. In the steady state, the differences of the cell numbers in the two regions normalized by the total number were proportional to the light difference, where the light intensity difference ranged from 0.5-2.0 μmol m-2 s-1. The proportional coefficient was positive (i.e., the bright region contained many microorganisms) when the mean light intensity was weak (1.25 μmol m-2 s-1), whereas it was negative when the mean intensity was strong (13.75 μmol m-2 s-1). In the second experiment, a shallow rectangular container of the suspension was illuminated with stepwise light intensities. The cell number density distribution exhibited a single peak at the position where the light intensity was about Ic ≃ 3.8 μmol m-2 s-1. These results suggest that the suspension of E. gracilis responded to the light gradient and that the favorable light intensity was Ic.
authors.affiliation: Graduate School of Science, Hiroshima University, Higashi-Hiroshima, Hiroshima, Japan.
authors.forename: Takuma
authors.initials: T
authors.lastname: Ogawa
authors.affiliation: Graduate School of Science, Hiroshima University, Higashi-Hiroshima, Hiroshima, Japan.
authors.forename: Erika
authors.initials: E
authors.lastname: Shoji
authors.affiliation: Meiji Institute for Advanced Study of Mathematical Sciences, Meiji University, Tokyo, Japan.|Graduate School of Advanced Mathematical Sciences, Meiji University, Tokyo, Japan.
authors.forename: Nobuhiko J
authors.initials: NJ
authors.lastname: Suematsu
authors.affiliation: Graduate School of Science, Hiroshima University, Higashi-Hiroshima, Hiroshima, Japan.
authors.forename: Hiraku
authors.initials: H
authors.lastname: Nishimori
authors.affiliation: Graduate School of Science, Hiroshima University, Higashi-Hiroshima, Hiroshima, Japan.
authors.forename: Shunsuke
authors.initials: S
authors.lastname: Izumi
authors.affiliation: Graduate School of Science, Hiroshima University, Higashi-Hiroshima, Hiroshima, Japan.
authors.forename: Akinori
authors.initials: A
authors.lastname: Awazu
authors.affiliation: Graduate School of Science, Hiroshima University, Higashi-Hiroshima, Hiroshima, Japan.
authors.forename: Makoto
authors.initials: M
authors.lastname: Iima
mesh_terms: D002452:Cell Count; D005056:Euglena gracilis / Q000502:physiology*; D008027:Light; D010775:Photic Stimulation; D000071444:Phototaxis / Q000502:physiology*; D055585:Physical Phenomena