Uncoupling growth and division in Chlamydomonas reinhardtii colonies: consistent cell cycle regulation under confinement.

A planar cell microcolony served as a model system to study the impact of inter-cellular crowding and cell-matrix interactions upon the cell cycle. We studied the development over several days of Chlamydomonas reinhardtii microcolonies, grown from single cells, using a bespoke experimental setup allowing timelapse fluorescence microscopy. Through precise cell segmentation and lineage tracking of a large systematic dataset, characterising individual cell growth and divisions, we uncovered how the external matrix influenced cell cycle and morphology. Experiments also revealed spatial heterogeneity amongst cells within colonies, providing insights into the effects of contact inhibition and micro-gradients of mass transfer. A radial propagation of ring-like pattern, characterised by variations in parent cell size, indicated complex spatio-temporal dynamics in the regulation of the cell cycle within the constrained environment. The mechanisms of commitment and mitotic sizing remained consistent within colonies under this mechanical confinement. These findings contribute to a broader understanding of how matrix immobilisation affects C. reinhardtii, with implications for alternative culture formats such as biofilms and hydrogel encapsulation-approaches increasingly used in biohybrid applications including biophotovoltaics and bioremediation.