## 3 Results and Discussion
### Metallic Nanoparticle Synthesis by M. magnetotacticum Using Either Fe, Ag, Au or Zn  
Based on the growth curves obtained in Figures 1 and 2, a peak in _M. magnetotacticum_ growth was observed after 48 h of incubation in MSGM and MSGM medium modifications. Therefore, 48 h was selected as the point of isolation of nanoparticles from the various modifications. Dry weight yields of iron-containing nanoparticles produced by _M. magnetotacticum_ are presented in Table 1 below. \begin{table}
\begin{tabular}{l l} \hline \hline
**Medium Modifications** & **Nanoparticle Yield (mg/50 mL)** \\ \hline MSGM original & 14 \\ \hline MSGM 1 (glucose) & 14 \\ \hline MSGM 2 (sodium pyruvate) & 15 \\ \hline MSGM 3 (lactic acid) & 15 \\ \hline MSGM 4 (ammonium sulphate) & 20 \\ \hline MSGM 5 (ammonium chloride) & 15 \\ \hline MSGM 6 (sodium nitrate) & 22 \\ \hline MSGM 7 (ammonium sulphate and sodium nitrate) & 18 \\ \hline MSGM 8 (ammonium sulphate, sodium nitrate and yeast extract) & 21 \\ \hline MSGM 9 (sodium nitrate and Isogro\({}^{\text{TM}}\)) & 17 \\ \hline MSGM 10 (glucose and sodium nitrate) & 16 \\ \hline MSGM 11 (glucose, sodium nitrate and ammonium sulphate) & 20 \\ \hline MSGM 12 (glucose, sodium nitrate, ammonium sulphate and yeast extract) & 20 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Dry weight yields of iron-containing nanoparticles produced by _M. magnetotacticum_ and isolated after 48 h of incubation at 30 \({}^{\circ}\)C in different media. _M. magnetotacticum_ synthesis of Fe nanoparticles in MSGM original resulted in a yield of 14 mg/50 mL which was found to be similar (\(p>0.05\)) to yields from all three sole carbon source modifications, i.e., MSGM 1, MSGM 2 and MSGM 3 having yields of 14 mg/50 mL, 15 mg/50 mL and 15 mg/50 mL, respectively. This indicated that the original carbon sources present in MSGM (i.e., ascorbic acid, succinic acid and tartaric acid) could be replaced by any one of the three sole carbon sources without significantly impacting _M. magnetotacticum_ nanoparticle yield. However, when _M. magnetotacticum_ was grown in a modified MSGM medium with a higher concentration of sodium nitrate (40 mM) (MSGM 6), there was an extremely significant difference observed (\(p<0.001\)) with a yield of 22 mg/50 mL being obtained. This indicated that, by increasing the concentration of sodium nitrate, the nanoparticle yield improves. Thus, the role of nitrogen is very important to form proteins for the assembly of magnetosomes, and to transport and incorporate iron in MTB. These findings can be supported by [PERSON] et al. (2019) who showed that the use of sodium nitrate in particular led to a higher production of magnetosomes as observed in our study, possibly due to nitrate being a strong oxygen acceptor having redox potential [16]. In addition, a study by [PERSON] and [PERSON] (1983) found that _M. magnetotacticum_ was able to reduce nitrate mainly to N\({}_{2}\)O or N\({}_{2}\) while producing magnetosomes, indicating the existence of a functioning denitrification mechanism in MTB suggesting that it may be involved in the synthesis of magnetite and thus magnetosome production as mentioned by [PERSON] et al. (1995) and [PERSON] et al. (2003) [28; 29; 30]. Interestingly, by replacing the original 1.4 mM sodium nitrate in _M. magnetotacticum's_ growth medium (MSGM) with ammonium sulphate (40 mM) (MSGM 4), it resulted in an extremely significantly different (\(p<0.001\)) nanoparticle yield of 20 mg/50 mL. However, no significant difference was observed (\(p>0.05\)) in nanoparticle production when _M. magnetotacticum_ was grown in either MSGM 4 (40 mM ammonium sulphate) or MSGM 6 (40 mM sodium nitrate) implying that they both significantly increase nanoparticle production and thus the use of either nitrogen source is optimal for nanoparticle production. Similarly, 1.4 mM sodium nitrate was replaced with ammonium chloride (40 mM), but there was no significant difference observed between nanoparticle yield from MSGM original and nanoparticle yield from MSGM 5 (ammonium chloride).