**2.4 Synthesis/production in algae**

The aforementioned methodology presents a viable alternative to conventional physical and chemical techniques for synthesizing nanoparticles, owing to its costeffectiveness and environmentally sustainable nature [44]. In addition, it has been observed that algae exhibit a significant ability to absorb metals. Observations have indicated that biological entities, such as marine algae, possess the ability to facilitate particular chemical reactions. The ability to perform this function is crucial for contemporary and pragmatic biosynthetic strategies [45]. According to a recent study utilizing algae extract, the alteration of hue from yellow to brown may serve as an indicator of the reduction of silver ions to silver nanoparticles. Furthermore, Rajesh Kumar et al. observed a significant deepening of the brown hue of silver nanoparticles after 32 hours of incubation. This finding suggests a positive correlation between the duration of incubation and the intensity of the observed coloration [46]. The present study reports the synthesis of silver nanoparticles via reduction of aqueous solutions of silver nitrate, utilizing powder and solvent extracts of *Padina pavonia*. Moreover, the nanoparticles obtained exhibited notable stability, rapid formation kinetics, and diminutive dimensions [47]. The production of silver nanoparticles was reported by Salari et al. via bio-reduction of silver ions, which was induced by Spirogyra variants [48].

#### **2.5 Synthesis/production in virus**

The utilization of viruses for the production of artificial nanocrystals, including but not limited to cadmium sulfide, silicon dioxide, ferrous oxide, and zinc sulfide, represents a distinctive methodology. The investigation of methods for generating semiconductor nanoparticles, including zinc sulfide and cadmium sulfide, is currently a topic of great interest in the electronics industry and the field of green

chemistry. The employment of intact viruses in the synthesis of nanomaterials has been a topic of investigation for numerous years [49]. The extrinsic capsid protein of the virus serves a beneficial function in the creation of nanoparticles through the production of a metal ion-binding interface with notable reactivity [50]. At the exterior of the tobacco-mosaic virus [TMV], the total number of capsid proteins is 2130. Peptides possess the potential to serve as interlocking components for the construction or production of three-dimensional conduits intended for diverse medical applications [51]. The reduction in size of the synthesized nanoparticles was observed upon the addition of Au and Ag salts in moderate quantities to TMV, prior to the introduction of plant extracts from *Nicotiana benthamiana* or Hordeum vulgare. The augmented quantities of free nanoparticles observed at higher concentrations of tobacco mosaic virus (TMV) in comparison to the control group indicate a relatively modest production of the nanoparticles. The utilization of TMV as a biological template for the metallization of nanowires was also observed [52]. In contrast to a scenario without the virus, the existence of a pathogen not only resulted in a decrease in the length of biosynthesized nanoparticles but also significantly augmented their synthesis.

#### **2.6 Synthesis/production in Actinomycetes**

Actinomycetes exhibit the ability to produce both intracellular and extracellular nanoparticles through their metabolic processes. The process of intracellular synthesis takes place at the mycelia surface as a result of electrostatic attraction between Ag + ions and the negatively charged carboxylate groups found in the enzyme located on the mycelia cell wall. Subsequently, the enzymes present in the cellular wall facilitate the reduction of Ag + ions, leading to the formation of silver nuclei. The aggregation of silver nuclei results in the generation of silver nanoparticles at the nanoscale level [53]. The process of synthesizing gold nanoparticles was carried out using Rhodococcus sp., which is a type of alkalotolerant Actinomycetes. The presence of nanoparticles on the Actinomycetes' cell walls was confirmed by transmission electron microscopy (TEM) images, indicating that the nanoparticles were synthesized intracellularly [54]. It has been revealed that Rhodococcus NCIM 2891 can be used for the intracellular manufacture of silver nanoparticles [55]. After 72 hours of contact with HAuCl4, the previously yellow biomass of *Streptomyces hygroscopicus* changed pink, indicating the production of extracellular gold nanoparticles [56]. The production of silver, manganese, and zinc nanoparticles by Streptomyces sp. HBUM171191 was observed upon exposure of the wet biomass to the respective metal solutions. The alteration in hue of the biomass, transitioning from a light-yellow shade to brown, dark yellow, and dark yellow, respectively, is indicative of the synthesis of silver, manganese, and zinc nanoparticles [57]. The enzymatic processes associated with the nitrogen cycle have been identified as a potential mechanism for the extracellular biosynthesis of nanoparticles. It is possible that they could assume responsibility for the enzymatic reduction of metals through electron shuttle [58]. Thea-NADH-dependent nitrate reductase is a significant contributor to the process of reducing Ag + ions to silver nuclei. Actinomycetes were responsible for mediating the extracellular creation of nanoparticles by the use of Streptomyces glaucus71MD [59]. Within 12 hours, a pure culture of Streptomyces sp. ERI-3 was able to convert a colorless solution of silver nitrate to a color that was more reddish-brown [60], indicating the creation of silver nanoparticles.
