Reverse Bioconvection of Chlamydomonas in The Hyper-Density Medium

Emergent collective dynamics of bottom-heavy squirmers under gravity

We present the results of hydrodynamic simulations using the method of multi-particle collision dynamics for a system of squirmer microswimmers moving under the influence of gravity at low Reynolds numbers. In addition, the squirmers are bottom-heavy so that they experience a torque which aligns them along the vertical. The squirmers interact hydrodynamically by the flow fields of a stokeslet and rotlet, which are initiated by the acting gravitational force and torque, respectively, and by their own flow fields. By varying the ratio of swimming to bulk sedimentation velocity and the torque, we determine state diagrams for the emergent collective dynamics of neutral squirmers as well as strong pushers and pullers. For low swimming velocity and torque we observe conventional sedimentation, while the sedimentation profile becomes inverted when their values are increased. For neutral squirmers we discover convective rolls of circulating squirmers between both sedimentation states, which sit at the bottom of the system and are fed by plumes made of collectively sinking squirmers. At larger torques porous clusters occur that spawn single squirmers. The two latter states can also occur transiently starting from a uniform squirmer distribution and then disappear in the long-time limit. For strong pushers and pullers only weak plume formation is observed.

The shape effect of flagella is more important than bottom-heaviness on passive gravitactic orientation in Chlamydomonas reinhardtii

The way the unicellular, biflagellated, green alga Chlamydomonas orients upward has long been discussed in terms of both mechanics and physiology. In this study, we focus on the mechanics, i.e. the 'passive' mechanisms, of gravitaxis. To rotate the body upwards, cellular asymmetry is critical. Chlamydomonas can be depicted as a nearly spherical cell body with two anterior, symmetric flagella. The present study looks at the question of whether the existence of the flagella significantly affects torque generation in upward reorientation. The 'density asymmetry model' assumes that the cell is spherical and bottom-heavy and that the shape and weight of the flagella are negligible, while the 'shape asymmetry model' considers the shape of the flagella. Both our experimental and simulation results revealed a considerable contribution from shape asymmetry to the upward orientation of Chlamydomonas reinhardtii, which was several times larger than that of density asymmetry. From the experimental results, we also quantified the extent of bottom-heaviness, i.e. the distance between the centers of gravity and the figure when the cell body is assumed to be spherical. Our estimation was approximately 30 nm, only one-third of previous assumptions. These findings indicate the importance of the viscous drag of the flagella to the upward orientation, and thus negative gravitaxis, in Chlamydomonas.

Analysis and modeling of the inverted bioconvection in Chlamydomonas reinhardtii: emergence of plumes from the layer of accumulated cells

Bioconvection is a convective flow found in a suspension of motile cells that swim against gravity, and is a primitive form of order formation of cells, which has been studied both experimentally and theoretically. We formulate here an inverted bioconvection occurring in a suspension of phototactic cells in a high-density medium, which is illuminated from the bottom. We used a highly phototactic strain 137c of Chlamydomonas reinhardtii in the experiments. Using a custom-made lateral microscope, we observed a close view of cellular dynamics in the initiation of inverted bioconvection. In conventional bioconvection, convective flows of cells are formed spontaneously with or without formation of the surface cell layer. In inverted convection, a crowded cell layer was initially formed at the bottom, which was a prerequisite for the subsequent emergence of plumes, namely, floating populations of cells. The plume formation was a result of neither uneven initial cell density nor unequal light intensity. Based on detailed analysis of individual cells, we constructed a model of inverted bioconvection, in which each cell experiences a transition between two modes of movement: phototactically swimming cell and non-motile cell aggregate. A simulation using the CompuCell3D software reproduced basic behaviors of the plume formation. The modal transition has not been a subject of basic studies, but provides an interesting target of study of cell-to-cell interactions.

Individual Flagellar Waveform Affects Collective Behavior of Chlamydomonas reinhardtii

Bioconvection is a form of collective motion that occurs spontaneously in the suspension of swimming microorganisms. In a previous study, we quantitatively described the "pattern transition," a phase transition phenomenon that so far has exclusively been observed in bioconvection of the unicellular green alga Chlamydomonas. We suggested that the transition could be induced by changes in the balance between the gravitational and shear-induced torques, both of which act to determine the orientation of the organism in the shear flow. As both of the torques should be affected by the geometry of the Chlamydomonas cell, alteration in the flagellar waveform might change the extent of torque generation by altering overall geometry of the cell. Based on this working hypothesis, we examined bioconvection behavior of two flagellar mutants of Chlamydomonas reinhardtii, ida1 and oda2, making reference to the wild type. Flagella of ida1 beat with an abnormal waveform, while flagella of oda2 show a normal waveform but lower beat frequency. As a result, both mutants had swimming speed of less than 50% of the wild type. ida1 formed bioconvection patterns with smaller spacing than those of wild type and oda2. Two-axis view revealed the periodic movement of the settling blobs of ida1, while oda2 showed qualitatively similar behavior to that of wild type. Unexpectedly, ida1 showed stronger negative gravitaxis than did wild type, while oda2 showed relatively weak gravitaxis. These findings suggest that flagellar waveform, not swimming speed or beat frequency, strongly affect bioconvection behavior in C. reinhardtii.

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

Motile aquatic microorganisms are known to self-organize into bioconvection patterns. The swimming activity of a population of 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 bioconvection pattern formation, little quantitative investigation has been carried out. 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 three phases in the pattern formation of the bioconvection of C. reinhardtii: 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 or 2. By means of the newly developed two-axis view method, we revealed that the population of C. reinhardtii moves toward the bottom of the experimental chamber just before the pattern transition. This indicates that the pattern transition response could be caused by enhancement of the gyrotaxis of C. reinhardtii as a result of 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 C. reinhardtii is phototactically insensitive. These facts suggest that the bioconvection pattern has a potential to drastically reorganize its convection structure in response to the physiological processes under the influence of environmental cues.