Slow modulation of the contraction patterns in Physarum polycephalum
The slime mould Physarum polycephalum has emerged as a model for self-organisation and coordination of contractile activity at large spatial scales. This self-organisation largely results from cytoplasmic flows generated by propagating contractile waves of the actomyosin cortex. In addition to these relatively fast travelling waves, complex slow modulations of the contractile activity have been observed on timescales much longer than the primary oscillation period; these slow dynamics are however scarcely characterised. Here we characterise these slow modulations by confining organisms inside annular geometries. We quantify contractile activity simultaneously across the entire organism on long time scales, exhibiting correlations between contractile wave direction, amplitude modulation, and the moving mean vein diameter. We observe travelling and alternating wave patterns: travelling wave periods scale clearly with system size, while alternating wave periods remain broadly distributed and probe larger values as the system size increases. Strikingly, the measured periods align with integer multiples of an intrinsic modulation time scale obtained independently from statistical analysis. These observations support the hypothesis that transport of a slowly advected chemical agent, which locally modifies membrane/cortex mechanical properties, underpins the observed slow modulation dynamics, accounting for the coordination across the organism on long time scales.