## 3 Results and Discussion
(2019) investigated three nitrogen sources which included sodium nitrate, ammonium chloride and ammonium sulphate. It was found that sodium nitrate increased the cellular magnetotactic response (Cmag), rate of Fe uptake and magnetosome production significantly when compared to the other two nitrogen sources. The concentration used for sodium nitrate in the optimized culture medium which was found to increase cell growth and magnetosome production was 40 mM [16; 19; 27]. In addition, nitrates such as sodium nitrate are a commonly used nitrogen source for _Magnetospirillum_ bacteria [3]. The nitrogen source present in MSGM original, 1.4 mM sodium nitrate, was modified and increased to 40 mM and the type of nitrogen source was also investigated in this study, by using ammonium sulphate (40 mM) and ammonium chloride (40 mM). The modifications using ammonium sulphate (40 mM) (MSGM 4) and ammonium chloride (40 mM) (MSGM 5) showed a very similar growth trend to that of the MSGM original (containing 1.4 mM sodium nitrate), with r values of 0.91 and 0.97 (Table S4 in Supplementary Material), respectively, indicating a very high positive correlation to MSGM original. This indicated that there were no distinct phases of growth when either ammonium sulphate or ammonium chloride were used, and thus these two nitrogen sources even at a high concentration of 40 mM did not improve bacterial growth. However, when the concentration of sodium nitrate was increased from 1.4 mM (MSGM original) to 40 mM (MSGM 6) there was a significant increase in bacterial growth, with the highest OD being 0.590 after 48 h of incubation. An r value of 0.15 was obtained when compared to MSGM original, which indicates a negligible correlation to that of MSGM original possibly due to the distinct exponential phase of growth between 0 and 48 h of incubation. The present study thus supports the findings of [PERSON] et al. (2019) [16]. However, according to a study by [PERSON] et al. (2010), ammonium chloride was identified as a better nitrogen source for growth and the discrepancy in findings may be attributed to the nature of the experiment [27]. The experiment by [PERSON] et al. (2010) took place on a larger scale with the concentration of nitrogen being auto-controlled at a constant level via the use of pH-stat feeding thus enabling ease of manipulation and elimination of the possibility of nutrient exhaustion during the culture process which would otherwise occur in a laboratory-based experiment such as in the present study [27]. The use of two nitrogen sources namely ammonium sulphate (40 mM) and sodium nitrate (40 mM) used in combination (MSGM 7) was also investigated. However, _M. magnetotacticum_ still showed better growth when sodium nitrate (40 mM) was the sole nitrogen source (MSGM 6) as per Figure 2. The peak OD of _M. magnetotacticum_ grown in MSGM 7 was below that of the MSGM modification which used sodium nitrate (40 mM) as the sole nitrogen source (MSGM 6). Due to an increase in nitrogen availability in MSGM 7, it could be argued that more magnetosomes were produced and as a result nutrient depletion occurred faster, resulting in slower growth of _M. magnetotacticum_ in MSGM 7 than in MSGM 6 media. However, slower growth due to the potential toxic effect of the magnetosomes themselves and/or their excretion can be ruled out as growth curves were also compared to nanoparticle yield as shown in Section 3.2 below; thus, nutrient depletion is the most probable factor. Yeast extract was then used as an additional nutrient source (medium modification MSGM 8) and notably the second highest growth was obtained (OD of 1.27), establishing that use of an additive benefits _M. magnetotacticum_ growth. Since the additive yeast extract improved growth, we set out to establish whether another additive Isogro(tm) could also perform similarly.