## 3 Results and Discussion
### Metallic Nanoparticle Synthesis by M. magnetotacticum Using Either Fe, Ag, Au or Zn
Thus, replacing the original nitrogen source of sodium nitrate (1.4 mM) with ammonium chloride (40 mM) did not improve nanoparticle yield. When the three original carbon sources were combined with two nitrogen sources namely ammonium sulphate (40 mM) and sodium nitrate (40 mM) resulting in MSGM 7, a yield of 18 mg/50 mL was obtained that was significantly different (\(p<0.05\)) from MSGM original indicating that there was an improvement in nanoparticle yield when compared to MSGM original. Comparison of MSGM 7 to the medium with the highest yield, namely MSGM 6, indicated that there was no significant difference (\(p>0.05\)), thus increasing the amount of nitrogen present via the use of two nitrogen sources instead of one did not give a further improvement in nanoparticle yield as the maximal nanoparticle producing capacity could have already been achieved. Upon addition of yeast extract, resulting in a medium modification referred to as MSGM 8, there was an extremely significant difference found when compared to MSGM original (\(p<0.001\)). However, when MSGM 8 was compared to MSGM 6 and MSGM 7, there was no significant difference observed (\(p>0.05\)) leading one to believe that the use of yeast extract did not make a difference in terms of nanoparticle production as there was already an abundance of nutrients present thus accounting for the maximal nanoparticle producing capacity by the bacterial cells. The addition of Isogro(tm) to the modification which had the highest yield (MSGM 6) resulted in a medium modification referred to as MSGM 9. This modification was carried out to establish whether this additive would result in a higher nanoparticle yield. The yield obtained for MSGM 9 (sodium nitrate and Isogro(tm)) was 17 mg/50 mL, which was found to not significantly differ when compared to MSGM original (\(p>0.05\)). This indicates that Isogro(tm) was in fact inhibiting nanoparticle production. The addition of Isogro(tm) may have created the optimum growth conditions and while this is good for magnetotactic bacterial growth it may have resulted in feedback inhibition of magnetosome production. As mentioned earlier, MTB use the magnetosomes for geomagnetic navigation in their aquatic habitat, i.e., magnetotaxis. It has been proposed that, in natural environments, magnetotaxis may enable the cells to locate and maintain an optimal position in the water column or in sediments, and that this aligns with their main metabolic needs [31]. The magnetic assisted taxis could help MTB in their navigation toward optimum growth conditions [7]; hence, growing MTB under optimum conditions may in fact inhibit magnetosome production. When medium modification MSGM 10 (glucose and sodium nitrate) was used, the yield obtained was 16 mg/50 mL. Although the sole carbon source and nitrogen source with the highest growth (i.e., glucose (25 mM) and sodium nitrate (40 mM)) were utilized, it was found that the use of both of these nutrient sources did not significantly improve nanoparticle yield when compared to MSGM original (\(p>0.05\)). This could be attributable to adequate nutrients being available and thus there is no need for the bacteria to produce magnetosomes in order to facilitate magnetotaxis to a different region [32]. MSGM 11 (glucose, sodium nitrate and ammonium sulphate) and MSGM 12 (glucose, sodium nitrate, ammonium sulphate and yeast extract) produced the same yield (20 mg/50 mL) and were both found to be extremely significantly different when compared to MSGM original (\(p<0.001\)). Thus, the use of glucose (25 mM) and two nitrogen sources such as sodium nitrate (40 mM) and ammonium sulphate (40 mM) had allowed \(M\). _magnetacticum_ to reach its full nanoparticle producing potential and the addition of yeast extract to MSGM 12 did not have an additional effect. Interestingly, the additional nitrogen source thus appears to play an important role in magnetosome production. These findings also indicate that a fine balance between carbon and nitrogen sources is required in order to ensure optimal magnetosome production.