a, The graph shows cell shape variation in terms of the degree of fore–aft asymmetry and minor/major axis ratio (see equation (1)). The parameter c denotes the degree of fore–aft asymmetry, a is the semi-major axis, b is the semi-minor axis. We highlighted the average contours (see Extended Data Table 2) for the subpopulation of downward swimmers (HA452(↓), blue), the subpopulation of upward swimmers (HA452(↑), orange) and HA3107 (green). Values of a, b and c are given in Extended Data Table 2 and Supplementary Table 2. b, Epifluorescence micrograph showing the chloroplasts. c, Three-dimensional schematic of an HA452 cell used to compute the contribution of the chloroplasts to the offset of the centre of mass relative to the contribution of the nucleus. The large sphere represents the nucleus (density ρN = 1,300 kg m−3, radius sN = 2.5 μm) and the 20 small spheres represent the chloroplasts (density ρchlo = 1,150 kg m−3, radius rchlo = 0.75 μm), which for the purpose of computing the contribution to the centre of mass were taken to be randomly distributed adjacent to the cell membrane. The contribution of the chloroplasts to the offset of the centre of mass from the centre of buoyancy was found to be <4% of the contribution of the nucleus and was thus neglected in the stability analysis.