## 3 Results and Discussion  
Carbon and nitrogen sources serve as essential nutritional requirements for bacterial cell growth. The concentration of carbon and nitrogen sources present in growth media are amongst the most important elements impacting MTB development and, as a result, magnetosome formation [14]. The present study investigated the use of different carbon and nitrogen sources in order to find suitable sole carbon and nitrogen sources that could both optimize growth and improve magnetosome production whilst being cost efficient. When analysing the effect of the carbon source modification on growth of _M. magnetotacticum_, it was found that comparatively growth was slow in the unmodified MSGM (MSGM) with no distinct peak and no distinct phases of growth (Figures 1 and 2). A graph with all growth curves on a single set of axes is available in the Supplementary Material as Figure S1. There was a gradual increase in OD as time progressed; however, the growth did not exceed an OD of 0.287 even after 336 h of incubation. Improved growth of _M. magnetotacticum_ was observed in MSGM media containing sodium pyruvate (MSGM 2) compared to MSGM original. During the exponential phase between 0 and 48 h, a maximum OD of 1.005 was observed. This finding was supported by [PERSON] et al. (2019) who used _M. gryphiswaldense_ strain MSR-1 and four different carbon sources (sodium pyruvate, sodium L-lactate, sodium succinate and sodium acetate), and  
Figure 1: Growth curves of _M. magnetotacticum_ in MSGM original and medium modifications MSGM 1, 2, 8, 10, 11 and 12 at 30 \({}^{\circ}\)C with iron as the metal source. OD readings were taken at a wavelength of 600 nm for MSGM, MSGM 1, MSGM 8 and MSGM 10–12, and at 450 nm for MSGM 2.  
reported that sodium pyruvate and sodium L-lactate significantly increased the cell growth, cellular magnetotactic response (Cmag), rate of Fe uptake and magnetosome production when compared to sodium succinate and sodium acetate as the carbon source [16]. Their results also showed that sodium pyruvate (25 mM) was much more effective at increasing magnetosome production. Results from the present study thus indicate that sodium pyruvate contributes to rapidly dividing bacterial cells; however, a rapid decline could be noted after the peak had been obtained indicating that this carbon source was depleted quite quickly. It is thus possible that limited vital nutrients could have led to death of the cells. Growth of _M. magnetotacticum_ in MSGM 2 was also greater than MSGM containing lactic acid (MSGM 3) which produced a maximum OD of 0.46 after 288 h of incubation. In addition, the results showed that the lactic acid modification (MSGM 3), compared to the original MSGM, had an r value of 0.83. The high level of correlation could be attributed to both of these media having no distinct phases of growth. The third sole carbon source used in this study was glucose as it is a well-known carbon source that is preferred for bacterial growth (e.g., _E. coli_), and is a known source of energy for all organisms as it is essential to fuel both aerobic and anaerobic cellular respiration [25,26]. Glucose is also a cheaper alternative to the three combined carbon sources used in MSGM (i.e., ascorbic acid, succinic acid and tartaric acid) and is commonly used in bacterial growth media. When glucose was utilized as the sole carbon source (MSGM 1), the highest and most sustainable growth of _M. magnetotacticum_ was observed compared to using growth media MSGM 2, MSGM 3 and even MSGM the original medium which had three carbon sources. When _M. magnetotacticum_ was grown in MSGM 1, a peak OD of 1.15 was observed after only 48 h and this peak was sustained for 336 h.  
Figure 2: Growth curves of _M. magnetotacticum_ in MSGM original and medium modifications MSGM 3–7 and MSGM 9 at 30 \({}^{\circ}\)C with iron as the metal source. OD readings were taken at a wavelength of 600 nm for MSGM, MSGM 4–7 and MSGM 9, and at 450 nm for MSGM 3. The nitrogen source modifications were thereafter performed. [PERSON] et al.