Controlled Low‐Oxygen Supply Enables Magnetosome Size Tuning by Uncoupling Magnetite Nucleation and Crystal Growth in <scp> <i>Magnetospirillum gryphiswaldense</i> </scp>

) nanocrystals, known as magnetosomes. Owing to their uniform size, purity and superior magnetic properties, magnetosomes represent highly attractive nanomaterials for biotechnological and biomedical applications. However, their bioproduction is limited by demanding cultivation requirements, largely because magnetite biomineralization is highly sensitive to environmental parameters, particularly oxygen. While elevated oxygen concentrations are known to inhibit magnetosome formation, quantitative analyses under defined low-oxygen conditions are scarce. Here, we cultivated MSR-1 in bioreactors under precisely controlled dissolved oxygen (DO) levels and quantified growth behaviour, substrate uptake and magnetosome characteristics. Cells harvested during late exponential growth revealed that magnetite crystal numbers per cell were similar across a wide DO range (0%-5%), whereas crystal sizes decreased with increasing oxygen levels. The data further indicate that oxygen inhibits biomineralization primarily through direct oxidative interference rather than indirect metabolic effects. These findings provide a mechanistic basis for optimizing oxygen control strategies in MTB cultivation and demonstrate that fine-tuning DO levels enables targeted modulation of magnetosome size and properties. This advances both the bioprocess development of high-yield magnetosome production and the application of tailored magnetic nanoparticles in biotechnology and medicine.