Molecular mechanism of dissolvable metal nanoparticles-enhanced CO₂ fixation by algae: Metal-chlorophyll synthesis

Algae-driven photosynthetic CO₂ fixation is a promising strategy to mitigate global climate changes and energy crises. Yet, the presence of metal nanoparticles (NPs), particularly dissolvable NPs, in aquatic ecosystems introduces new complexities due to their tendency to release metal ions that may perturb metabolic processes related to algal CO₂ fixation. This study selected six representative metal NPs (Fe₃O₄, ZnO, CuO, NiO, MgO, and Ag) to investigate their impacts on CO₂ fixation by algae (Chlorella vulgaris). We discovered an intriguing phenomenon that bivalent metal ions released from the metal NPs, especially from ZnO NPs, substituted Mg²⁺ within the porphyrin ring. This interaction led to 81.8% and 76.1% increases in Zinc-chlorophyll and Magnesium-chlorophyll contents within algal cells at 0.01 mM ZnO NPs, respectively. Integrating metabolomics and transcriptomics analyses revealed that ZnO NPs mainly promoted the photosynthesis-antenna protein pathway, porphyrin and chlorophyll metabolism, and carbon fixation pathway, thereby mitigating the adverse effects of Zn²⁺ substitution in light harvesting and energy transfer for CO₂ fixation. Ultimately, the genes encoding Rubisco large subunit (rbcL) responsible for CO₂ fixation were upregulated to 2.60-fold, resulting in a 76.3% increase in carbon fixation capacity. Similar upregulations of rbcL expression (1.13-fold) and carbon fixation capacity (76.1%) were observed in algal cells even at 0.001 mM ZnO NPs, accompanied by valuable lipid accumulation. This study offers novel insights into the molecular mechanism underlying NPs on CO₂ fixation by algae and potentially introduces strategies for global carbon sequestration.