## 3 Results and Discussion
### Metallic Nanoparticle Synthesis by M. magnetotacticum Using Either Fe, Ag, Au or Zn
Overall, it could be noted that the highest nanoparticle yield was obtained for MSSM 6 (sodium nitrate) having a yield of 22 mg/50 mL; however, when compared to MSGM 4, 8, 11 and 12 with yields of 20 mg/50 mL, 22 mg/50 mL, 21 mg/50 mL, 20 mg/50 mL and 20 mg/50 mL, respectively, it could be noted that there was no statistically significant difference (\(p>0.05\)) observed indicating that any of these modifications could be used for optimal nanoparticle production having yields that are significantly different (\(p<0.01\)) when compared to MSGM original. In addition, it could be seen that increasing the source of nitrogen beyond a certain point does not increase nanoparticle yield unless there is a component added to the medium that contributes to adequate nutrient supply to the bacteria which could result in inhibition of magnetosome formation since the inherent use of magnetosomes, as described by [PERSON] et al. (2020), is to facilitate transport of the bacterial cells from an area of low nutrient availability to one that has a better amount of nutrients [32]. Since MSGM 6 (40 mM sodium nitrate) had quantitatively shown the highest dry weight of iron-containing nanoparticles, this MSGM modification was then chosen for further analysis. Nanoparticle isolation at all three phases of bacterial growth was carried out using MSGM 6, to ascertain whether the exponential phase (48 h) did indeed result in the highest yield of nanoparticles. According to Table 2 below, the lag phase (24 h) had the lowest yield (16 mg/50 mL) suggesting that the bacterial cells were getting accustomed to their environment in preparation for cell division (i.e., exponential phase) [33]. The exponential phase (48 h) had a yield of 22 mg/50 mL which was the highest yield obtained when compared to the other phases of growth. This can be attributed to the bacteria being the most metabolically active at this phase of growth. The last phase (stationary/death phase) resulted in a yield of 18 mg/50 mL which was indicative of the bacterial cells reaching a plateau in growth. It could then be confirmed that the exponential phase did result in the highest yield (\(p<0.001\)) and was therefore the most effective for isolation of nanoparticles. Furthermore, a study by [PERSON] et al. (2017) found that when bacteria start synthesizing magnetosomes at the exponential phase of growth they are magnetically closer to stoichiometric magnetite rather than when they are synthesized at the stationary phase [34]. This further suggests that the synthesis of magnetosomes after 48 h of incubation is more beneficial when compared to the other phases of growth which could also enhance the properties of the nanoparticles produced for potential downstream applicability. According to a study conducted by [PERSON] and [PERSON] (2003), a maximum yield of 0.0063 mg/mL was achieved for _M. magnetotacticum_ using a medium that varied the iron, nitrogen and carbon source concentrations, containing 27 mM lactate, 4 mM sodium nitrate and 100 mM ferric citrate [6]. It could thus be noted that the medium modification with the highest yield of 0.44 mg/mL obtained in our study (MSGM 6) showed a significant increase in nanoparticle yield as opposed to the study conducted by [PERSON] and [PERSON] (2003) [6]. The difference in yield could be attributed to the different iron and carbon sources as well as the lower concentration of sodium nitrate [6]. Another study conducted by [PERSON] et al. (2008) made use of 60 mM ferric citrate, 11.25 mM ammonium chloride and 15 mM of sodium lactate as the iron, nitrogen and carbon sources, respectively [35]. A maximum of 0.0417 mg/mL of iron-containing magnetosomes was achieved after 60 h of incubation using _M. gyrophiswaldense_. When comparing our media having the highest yield to their yield, it could be noted that our medium (MSGM 6) had an approximate 11-fold increase in yield which was significantly higher. In addition, another study by [PERSON] et al.