## 1 Introduction  
The main objective of microbial biotechnology is to use microorganisms and their biological processes to obtain useful products or services [1]. Such products could be feedstocks for industry, human foods, animal feeds, biofuels, nutraceuticalticals, or bioactive materials [2; 3; 4; 5]. Also, many biotechnological services such as bioremediation of environmental wastes and hazardous or xenobiotic compounds can be performed using different kinds of microorganisms [5; 6; 7; 8; 9]. Recently, the field of nanotechnology research has become more active and attractive due to the promising industrial, agricultural, environmental, and health applications. Nanomaterials with particle sizes less than 100 nm are considered to be significant milestones of rapidly developing field of nanotechnology studies and applications [10; 11]. Nanoparticle size plays important roles in catalytic efficiency, thermal conductivity, chemical performance as well as antimicrobial activity [12]. Bio-nanotechnology is a combination of nanotechnology and biotechnology, and one of its key purposes is to develop environmentally friendly technologies for nanomaterials synthesis and applications. Bio-nanotechnology has already been applied in many environmental, industrial, and medical fields [13; 14]. Several nano-scale materials, such as iron, selenium, cadmium, platinum, titanium, zinc, copper, gold, magnesium, aminolays, and silver, have been derived [15; 16; 17; 18]. Nanoparticles can be synthesized through various approaches including physical, chemical, and biological methods. Although chemical approaches for nanoparticle production are the most used and effective methods, some chemicals are toxic or environmentally hazardous [19]. Therefore, environmentally friendly and green biosynthesis of nanoparticles using microorganisms or their extracellular metabolites as well as plant extracts has become more desirable [20; 21; 22]. Different microbial groups including bacteria, fungi, actinomycetes, and microalgae reportedly have been implemented in green nanoparticle biosynthesis [12; 17; 23; 24]. Those microorganisms can produce intracellular or extracellular (in their cultural filtrates) reducing agents that can mediate the bio-conversion of metal ions into nano-forms [25]. Microalgae cultivation can be performed for wastewater treatment, biofuel (biodiesel, bioethanol, etc.) production, animal feeds, food additives, and in pharmaceutical industries [26; 27; 28; 29]. It offers the ability to photosynthetically convert CO\({}_{2}\) into biomass via a series of interior biochemical reactions [29]. Most microalgal species can utilize different nitrogen forms such as nitrate, ammonia, and urea [28; 30]. Huge amounts of microalgae culture filtrates from large-scale production systems typically are discarded despite their many beneficial metabolites and bioactive compounds. Such bioactive compounds include enzymes and reducing agents that can be utilized to manufacture different nanometals extracellularly [31]. Extracellular substances produced by microalgae differ according to nitrogen source in the culture medium as well as by algal growth stage [32; 33]. Several microalgae such as _Scenedens_s sp., _Chlorella_ sp., and others have been reported for biosynthesis of different nanoparticles, especially silver nanoparticles (AgNPs), from appropriate metallic ion sources [19; 34; 35]. This biogenic synthesis of nanoparticles can occur either intracellularly due to interior biochemical reactions or extracellularly due to excreted polymers and enzymes [12; 34]. Silver-based materials are known to cause bacterial membrane damage induced by formed free radicals. The antibacterial activity of nanosilver has been reported to be superior due to greater microbe/nanosilver surface-area contact [12; 36]. Therefore, application of AgNPs as antimicrobial agents can be a reliable alternative solution to the problem of antibiotic-resistant pathogens that threaten humanity [35; 37].