Drastic reorganization of bioconvection pattern of Chlamydomonas: Quantitative analysis of the pattern transition response

Abstract

Summary Motile aquatic microorganisms are known to self-organize into bioconvection. The swimming activity of the population of the microorganisms leads to the emergence of macroscopic patterns of density under the influence of gravity. Although long-term development of the bioconvection pattern is important in order to elucidate the possible integration of physiological functions of individuals through the bioconvection pattern formation, little quantitative investigation has been done. In the present paper, we present the first quantitative description of long-term behavior of bioconvection of Chlamydomonas reinhardtii, particularly focusing on the "pattern transition response." The pattern transition response is a sudden breakdown of the steady bioconvection pattern followed by re-formation of the pattern with a decreased wavelength. We found the three phases in the pattern formation of the bioconvection of Chlamydomonas: the Onset, Steady State 1 before the transition and Steady State 2 after the transition. In Onset, the wavelength of the bioconvection pattern increases with increasing depth, but not in Steady States 1 and 2. By means of the newly developed two-axis view method, we revealed that the population of Chlamydomonas moves toward the bottom of the experimental chamber just before the pattern transition. This fact indicates the pattern transition response could be caused by enhancing the gyrotaxis of Chlamydomonas due to the changes in the balance between the gravitactic and gyrotactic torques. We also found that the bioconvection pattern changes in response to the intensity of red-light illumination, to which Chlamydomonas is phototactically insensitive. These facts suggest the bioconvection pattern has a potential to drastically reorganize its convection structure in response to the physiological processes under the influence of the environmental cues.