## 3 Results and Discussion
### NLR-M2 Results  
To determine the magnetic parameters of magnetosomes inside the bacteria (without their destruction) in freshly grown cultures without fixation with formalin and in the medium with the addition of formalin for fixation, 1-mL cuvettes were used, filled, and sealed in the nitrogen atmosphere. The experiments were carried out at room temperature. To characterize the magnetosome magnetic state (single-domain/multi-domain), the dependences \(\mathrm{Re}M_{2}(H)\) and \(\mathrm{Im}M_{2}(H)\) were registered at three steady field scanning frequencies \(F_{sc}\) = 8, 0.25, and 0.02 Hz. The increase in the scanning period provides more time for the relaxation of magnetization. The dependence of the magnetic hysteresis on the scan frequency would indicate its dynamic character, suggesting the stable single-domain state of magnetosomes and the presence of magnetic anisotropy [31; 34; 36; 41]. The absence of such dependence, in contrast, would indicate the formation of multidomain aggregates due to the adhesion of the magnetosomes after the destruction of their lipid shells. The magnetic relaxation in this case is determined by the motion of the domain walls [31; 41] rather than by the rotation of the nanoparticle magnetic moment. The hysteresis in this case is associated with the pinning of the magnetic moment on structural inhomogeneities and does not depend on the time allotted for relaxation (the scanning period of the field \(H\)). The phase components \(\mathrm{Re}M_{2}(H)\) and \(\mathrm{Im}M_{2}(H)\) recorded at \(F_{\mathrm{sc}}\) = 8 and 0.25 Hz, as well as \(\mathrm{Re}M_{2}(H)\) recorded at \(F_{\mathrm{sc}}\) = 0.02 Hz (the latter together with their best fit) for some strains (MSR-1, AMB-1, LBB-42, and SP-1), both for the \"old\" and \"new\" bacterial samples, are shown in Figures 3-6. As was found in the NLR-M2 measurements, the addition of 5% formalin to the as-grown cultures in the medium does not lead to a detectable change in the \(M_{2}\) response in five days, except for the MSR-1 strain (not shown). This implies only a weak influence of such a concentration of formalin on the internal organization of magnetosomes in the cells for most of the strains, at least during this time interval. As for the MSR-1 strain, in five days after the formalin administration, the amplitude of the \(M_{2}\) response grew approximately two-fold in both phase components, accompanied by a considerable increase in the field hysteresis, indicating some unexpected tendency for this strain. From Figure 3, this tendency persists for a longer storage period. However, it is much less pronounced with a lower scan frequency, as seen from the ratio of the signals in panels (c) and (f) compared to that in the panels (a) and (d). Figure 2: TEM images of \(M\). _cucaseum_ SO-1 (**a**) and \(M\). _marisnigri_ SP-1 (**b**) with a chain of magnetosomes. Figure 4: Dependences on static field H of phase components of M2 response, ReM2(H) (open blue and solid red circles) and ImM2(H) (open green and solid black stars), for LBB-42 strain recorded at frequencies of H-scan Fsc= 8 and 0.25 Hz are presented in panels (**a,b,d,e**). Solid symbols present the curves recorded at direct H-scan and open symbols are used for curves registered at the reverse sweep. For clarity, this is additionally indicated by arrows having the same color as the color of the corresponding symbols. Panels (**c,f**) display ReM2(H) dependences registered at Fsc= 0.02 Hz and their best fits (black lines). Red and green lines correspond to two components of entire signal (see text). The upper panels (**a–c**) reflect the magnetic state of “fresh” (”new”) bacteria and lower panels (**d–f**) characterize the state of bacteria after one year of storage (“old”). The hatched peaks in (**f**) present M2 response simulated for the NPs 40 nm size.