  Photosynthetic algae play a significant role in oceanic carbon capture. Their
performance, however, is constantly challenged by fluctuations in environmental
light conditions. Here, we show that the non-motile single-celled marine
dinoflagellate Pyrocystis lunula can internally contract its chloroplast
network in response to light. By exposing the cell to various physiological
light conditions and applying temporal illumination sequences, we find that
network morphodynamics follows simple rules, as established in a mathematical
model. Our analysis of the chloroplast structure reveals that its unusual
reticulated morphology constitutes properties similar to auxetic metamaterials,
facilitating drastic deformations for light-avoidance, while confined by the
cell wall. Our study shows how the topologically complex network of
chloroplasts is crucial in supporting the dinoflagellate's adaptation to
varying light conditions, thereby facilitating essential life-sustaining
processes.
