Evaluation of locally isolated aquatic bacteria as nanofactories for green synthesis of iron oxide and silver nanoparticles

The Malaysian tropical aquatic environment embodies a vast ecological system of immense biodiversity. Its largely untapped microbial population represent a rich bioresource that could be exploited for commercial applications replacing unsustainable conventional processes that require high investment and release toxic residues. Iron oxide nanoparticles (IONP) and silver nanoparticles (AgNP) have demonstrated tremendous potential in a variety of applications. Unfortunately, high investment costs, low yield and failure for effective scale-up using current protocols calls for alternative eco-friendly, cost-effective solutions. Green synthesis mechanisms using microorganisms as nanofactories offer a facile, sustainable and environment-friendly approach. Thus, the objectives underscored in this thesis was to isolate Malaysian aquatic bacteria with the ability for extracellular, aerobic generation of IONP and AgNP, to optimise synthesis parameters including temperature, precursor and biomass concentration using Response Surface Methodology (RSM) for maximum NP yield and potential throughput scale up and to explore the application potential of biosynthesized AgNP for antimicrobial activity and IONP as a nanoadsorbent for industrial dyes. As a result of this study, 24 prokaryote strains were isolated from the 15 tropical shallow riverbed sediment samples while 34 prokaryote strains were isolated from among 15 mangrove marine sediment sample isolates which demonstrated extracellular generation of IONP and AgNP respectively. Molecular characterization using 16S rRNA sequencing showed phylogenetic similarities of MS2 and MA6 to Spectromyces sp. and Bacillus sp respectively. HrTEM images of biosynthesized IONP (size range: 12.5–18.21 nm) and AgNP (size range: 9.81 – 23.81 nm) depicted monodispersed, spherical nanostructures with clear crystalline lattice fringes. EDX peaks of IONP showed Fe to O ratios of 3 :4. Statistical optimization of temperature, biomass concentration and precursor concentration showed highest IONP yield using 55.58% MS2 culture supernatant (CS) with 2.46 mM FeCl3.6H2O at 55.75 ᵒC while highest AgNP yield was predicted by incubating 80% MA6 CS with 2.0 mM AgNO3 at 60 ᵒC. MS2 generated IONP demonstrated nanoadsorbent potential with 62.87%, 57.4% and 12.25% removal of 10 mg/L Crystal Violet (CV), Methylene Blue (MB) and Methyl Orange (MO) from aqueous solutions respectively. MA6 generated AgNPs demonstrated antimicrobial activity for Eshcericha coli (inhibiton zone: 1.2 ± 0.05 cm) and Salmonella typhi (inhibition zone: 1.23 ± 0.11 cm). It is therefore concluded that MS2 and MA6 are valuable microbial nanofactories which could be developed as a sustainable alternative to existing IONP and AgNP synthesis methodology.

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