Engineering cyanobacterial resilience for cadmium bioremediation: linking Cd speciation, cellular detoxification, and synthetic biology.

Cadmium (Cd) is a persistent, non-essential toxic metal that threatens aquatic and terrestrial ecosystems, agricultural productivity, and food safety. Cyanobacteria offer a promising platform for Cd bioremediation by combining photoautotrophic growth with extracellular metal binding, membrane transport, intracellular sequestration, thiol-mediated detoxification, and redox regulation. This review develops an integrated framework for engineering cyanobacterial resilience to Cd stress, linking Cd2+ speciation and bioavailability with cellular uptake, detoxification, and biomass-associated metal recovery. We critically evaluate the evidence for metallothioneins, metal-responsive transport systems, antioxidant responses, and omics-guided target identification, while distinguishing mechanisms directly demonstrated in cyanobacteria from strategies inferred from other organisms or metal systems. Emerging approaches, including synthetic biology, nano-bio living materials, and photobioreactor engineering, are discussed in the context of improving Cd removal without compromising photosynthetic fitness and process stability. We argue that maximizing Cd accumulation alone is an inadequate remediation objective because excessive intracellular Cd can impair photosynthesis, redox homeostasis, growth, and biomass viability. Future designs should therefore optimize Cd removal, cyanobacterial resilience, metal recovery, genetic stability, and containment as interconnected process parameters. Finally, we identify key validation requirements for translating engineered cyanobacterial systems from molecular mechanisms to scalable and environmentally responsible Cd-remediation technologies.