## 4 Conclusions  
Magnetosomes produced by six MTB strains of the genus _Magnetospirillum_, BB-1, MSR-1, SO-1, AMB-1, SP-1, and LBB-42, were studied by second-harmonic nonlinear response. Each culture was measured after fixation by 5% formalin, both as-prepared and one year after preparation. A bimodal size and, correspondingly, magnetic moment distribution was found in all the species. The \"heavy\", monodisperse mode is suggested to correspond to the fixed-in-space linear chains of magnetosomes in accordance with the transmission electron microscopy data for mature cells of two strains, SO-1 and SP-1. The cell ensemble includes dividing cells at different stages of maturation, which contribute to the signals of both the \"light\" and \"heavy\" fractions. The \"light\", highly disperse, mode is implied to consist of more or less free shorter fragments. The latter are either rolled up or coupled with each other by dipolar forces to decrease the magnetostatic energy and the magnetic moment of the aggregate. From the AFM image, one can suggest degradation of a part of the cells at long-term storage of the BB-1 culture, even when fixed by formalin. The obtained mean magnetic moments, the saturation magnetizations, and the number of magnetic centers responsible for the signals noticeably differ for all the strains. The \"heavy\"-fraction parameters are only rough estimations, due to magnetic hysteresis in the \(M_{2}\) response. Electron magnetic resonance evidenced uniaxial magnetic anisotropy of the \"heavy\"-fraction magnetosome chains and its different stability in different strains.  
Slow destruction of magnetosome chains at long-term storage occurred in certain places, not randomly.  
The magnetic parameters were extracted solely from the NLR-M2 measurements, while the EMR data were analyzed phenomenologically to obtain qualitative information on the MTB strains complementing the quantitative data.  
The following supporting information can be downloaded at: [[https://www.mdpi.com/article/10.3390/app13042431/s1](https://www.mdpi.com/article/10.3390/app13042431/s1)]([https://www.mdpi.com/article/10.3390/app13042431/s1](https://www.mdpi.com/article/10.3390/app13042431/s1)). Supplementary Figure S1: The AFM image of the _M. moscoviese_ BB-1 \"old\" fixed by addition of 5% formalin to culture after a year of storage.  
Conceptualization, V.R., M.S. and K.G.; methodology, V.R. and V.D.; software, V.D. and I.L.; validation, M.S., D.G. and V.K.; formal analysis, V.D.; investigation, V.R., I.K. and Y.M.; resources, D.G., V.K., N.S. and T.Z.; data curating, V.D. and I.L.; writing--original draft preparation, V.R. and V.D.; writing--review and editing, M.S., K.G. and T.Z.; project administration, M.S. and K.G.; funding acquisition, M.S. and Y.M. All authors have read and agreed to the published version of the manuscript.  
This research was funded by the Ministry of Science and Higher Education of the Russian Federation (Agreement No. 075-15-2022-301 from 20.04.2022), by Russian Science Foundation grant No. 23-25-00273 ([[https://rscf.ru/project/23-25-00273/](https://rscf.ru/project/23-25-00273/)]([https://rscf.ru/project/23-25-00273/](https://rscf.ru/project/23-25-00273/))) magnetic measurements, studies by atomic force microscopy, and data processing were supported. The production of magnetosomes was partially supported by the Russian Federal Academic Leadership Program Priority 2030.  
Not applicable.  
The data that support the finding of this research are not publicly available, due to confidentiality constraints.  
The authors are grateful to the staff of the PNPI PIK Data Centre for its valuable technical assistance.  
The authors declare no conflict of interest.