Dynamical States of Self-Organised Waves in a Giant Single-Celled Organism Feeding on Light

Living Systems often seem to follow an intrinsic predictive model of the world — a defining trait of Anticipatory Systems. Here we study Caulerpa, a marine green alga, which appears to predict the day/night light cycle. Our experimental results indicate that self-organized waves propagating throughout the organism are coupled to an effective self-sustained oscillator, and respond to time-dependent illumination.

Caulerpa consists of differentiated organs resembling leaves, stems and roots. While an individual can exceed a meter in size, it is a single multinucleated giant cell. Active transport has been hypothesized to play a key role in organismal development; it has been an open question whether the rhythmic transport is of autonomous circadian nature. Using Raspberry-Pi cameras, we track over weeks the morphogenesis of tens of samples concurrently, while tracing at resolution of tens of seconds the variation of the green coverage. The latter reveals waves propagating over centimetres within few hours, and is attributed to chloroplast redistribution at whole-organism scale. By coarse-graining in space we achieve a reduced description to a dynamic macroscopic observable.

Our observations of algal segments regenerating under 12-hours-light / 12-hours-dark cycles show that the initiation of the waves precedes the external light change. The temporal-spectrum of the green pulses contains a circadian period; the latter persists over days even under constant illumination. Furthermore, we identify distinct dynamical states subject to the applied illumination protocol, as well as an effect on morphogenesis.

Time-keeping and synchronization are recurring themes in the research of Living Systems as contributors to Homeostasis — biological self-stabilization far-from-thermal-equilibrium, as well as to Anticipatory Behaviour. Studying this seemingly primitive light-fed Homeostatic System offers an opportunity to explore relations among biological oscillators, metabolism and morphogenesis.