Unravelling the mechanisms of cyanobacterial resilience in photosynthetic living materials

Background Photosynthetic cyanobacteria entrapped in artificial solid matrices, such as alginate hydrogels, can sustain active photosynthesis and bio-production of chemicals for months, yet the cellular adaptations underlying this remarkable resilience remain poorly understood. Results Here, we compared the physiological and proteomic responses of the model cyanobacterium Synechocystis sp. PCC 6803 entrapped within thin calcium-alginate hydrogel films with those of suspension-grown cells. Immobilized cells maintained relatively stable photosystem II (PSII) photochemical efficiency over three weeks despite strongly restricted biomass accumulation. By contrast, suspension cultures exhibited a progressive decline in phycobilisome connectivity and PSII photochemical yield during prolonged cultivation. To elucidate the molecular adaptations associated with immobilization, we applied a comparative label-free proteomics, which revealed extensive and time-dependent proteome remodelling following immobilization. Proteins involved in photoprotection, alternative electron sinks and respiratory terminal oxidases progressively increased in abundance, indicating an enhanced capacity for excitation-energy dissipation and redox balancing. In parallel, ribosomal proteins, chaperones and Rubisco subunits broadly decreased in abundance, whereas the stringent-response regulator SpoT increased, suggesting a regulated downshift in growth-related metabolism and reallocation of cellular resources towards maintenance. Increased abundance of inorganic carbon uptake systems, cell-surface and pilus-associated proteins and toxin-antitoxin modules further indicated acclimation to spatial confinement, diffusion limitations, and high local cell density within the hydrogel matrix. Conclusion Collectively, these findings demonstrate that entrapment in thin-layer hydrogel induces a coordinated, maintenance-oriented physiological state that preserves photosynthetic activity under growth-limited conditions, consistent with a longevity phenotype. This state shares key features with natural cyanobacterial biofilms and enables photosynthetic living materials to function as robust biocatalysts for long-term bio-production.