## 2 Materials and Methods
### Magnetic Measurements and Data Quantifying  
Magnetosomes in _Magnetospirillum_ spp. reveal large magnetic moments [5] that provide for the magnetization of the bacteria ensemble in a weak magnetic field to be close to saturation [31] and, hence, strong non-linearity of its \(M(H)\) dependence [26]. To assess their magnetic parameters, the registration of the second harmonic of magnetization \(M_{2}\) in the bacteria ensemble under the steady magnetic field \(H\) and the parallel weak _ac_ field \(h(t)=h\sin(\omega t)\) with \(h=1.1\) kA/m and \(f=\omega/2\pi=15.7\) MHz was performed. This approach provided for the elimination of the contribution from the diamagnetic environment (unlike SQUID measurements) and for a negligible contribution from possible paramagnetic centers in the bacteria (due to an almost linear character of their \(M(H)\) dependence). A homemade installation described earlier [32], adapted for studying MNPs [26; 33; 34], was used in experiments. It consisted of two _dc_-field Helmholtz coils, a radiofrequency (RF) generator with a low-frequency filter at the output, a two-mode (_f_, _2f_) resonant sensor containing the sample, and a receiver registering the second-harmonic response with a high-frequency filter at the input. High sensitivity of the setup was provided by (i) the use of the two-mode sensor, (ii) the effective record of the _2f_ signal induced by the sample with the _2f_-mode selective system, (iii) deep suppression of the _2f_ voltage from the RF generator and the \(f\) voltage at the input of the receiver by the high- and low-frequency filters, respectively, ensuring the amplitudes of the _ac_ field up to 4 kA/m, and (iv) the employment of materials and elements in the two-mode sensor which do not generate the spurious _2f_ signal. The _Q_-factor of the _2f_ mode \(Q_{2}\approx 200\) ensured the enhancement of the setup sensitivity by a factor of \(\sqrt{Q_{2}}\) compared to the nonresonance registration in the _ac_ susceptibility measurements. Both signal components, Re\(M_{2}\)(_H_,_T_) and Im\(M_{2}\)(_H_,_T_), were recorded simultaneously as functions of \(H\) at room temperature, which corresponds approximately to the bacteria habitat temperature. The sample temperature was stabilized by a flow thermostat using evaporated \(N_{2}\). The steady field \(H\) was slowly scanned in the range from -24 to 24 kA/m and backwards symmetrically relative to the point \(H=0\), with the scan frequency varied in the range of \(8\geq F_{\mathrm{sc}}\geq 0.02\) Hz, to control the field hysteresis in the signal. In accordance with the symmetry properties of \(M_{2}\), the hysteresis provides evidence for the presence of a ferromagnetic moment in the sample [35]. In the current experiments, the condition \(M_{2}\)-\(l^{2}\) was obeyed, which provided applicability of the analytical expressions for the \(M_{2}\) response obtained by the perturbation theory [35; 36] in the qualitative analysis of the experimental data. This technique was employed previously for studying biogenic MNPs in viable eukaryotic cells where a small amount of magnetosome-like structures was detected [37]. It is interesting to note that this response depended on the aggregation (suspension/peltelt) of the cells. The sensitivity of the measurements was about \(10^{-13}\)A\(\cdot\)m\({}^{2}\).