# Microbial Biopolymers: From Production to Environmental Applications--A Review
## 2 Microbial Polymers
### Microbial Hyaluronic Acid  
A recurring mixture of D-glucuron acid and D-N-acetylglucosamine connected by \(\beta\) (1-3) and \(\beta\) (1-4) linkages, respectively, makes up hyaluronic acid, also known as hyaluron [101]. It can be isolated from animal tissues like roosters' crest, cartilage, vitreous humor, and umbilical cords through a microbial fermentation process with the help of bacterial and yeast strains such as _Streptococcus_ spp., _Mytilus galloprovincialis_, _Pichia pastoris_, and _Pseudomonas_ spp. [19; 20; 102; 103]. The D-glucuron acid and N-acetyl glucosamine moieties of hyaluronic acid are generated from glucose-6-phosphate and fructose-6-phosphate, correspondingly, by two different pathways [104]. The molecular makeup of the hyaluronicacid molecule reveals that it has several hydroxyl and carboxyl groups, which can result in a great deal of intramolecular and intermolecular hydrogen bonding in solutions. These bindings contribute to overall stiffness and exhibit gel-formation abilities [105; 106]. Nevertheless, the advancement of microbial means of extraction has been prompted due to the obstacles in managing tissues from animals, excessive expenses, and moral complications related to hyaluronic acid generated from animals. Hyaluronic acid is the least explored biopolymer for environmental applications. The main challenge involved in microbial hyaluronic acid production is its low yield because of the extremely viscous nature of the broth, which makes it challenging to regulate the mixture and rapid transfer of oxygen. There is fierce competition for the same precursor molecules that are essential for cellular growth and the generation of hyaluronic acid. In addition, the build-up of lactic acid--the main metabolic waste product in hyaluronic acid fermentation activities--has a significant negative effect on both cellular development and hyaluronic acid production. Different approaches to prevail over these challenges include the systematic screening of proficient strains, optimization of tailored culture media, and development of sophisticated cultivation procedures [107; 108; 109; 110]. [PERSON] et al. [111] recently developed a hybrid hydrogel for plant growth regulation and the adsorption of heavy metal ions. [PERSON] et al. [112] developed a composite hydrogel to remove manganese in wastewater. When it comes to fish waste (head, skin, fin) specifically, animal waste poses a serious environmental risk because almost 50% of the tissue is thrown away. It is essential to pursue initiatives related to research and development aimed at developing novel approaches that make it easier to obtain this biomaterial while also minimizing difficulties with waste management, emphasizing the significance of those initiatives. Refs. [113; 114] extracted hyaluronic acid from _Scyliorhinus ciancula_ discards with the help of _Streptococcus zoepidemics_ strains. They concluded that among all approaches, fed-batch operatory was valuable in achieving a high yield and was beneficial in reducing the overall cost of production. [PERSON] et al. [115] extracted microbial hyaluronic acid from agro-industrial by-products. They obtained a maximum yield of 3.48 g/L hyaluronic acid among three culture media. Moreover, hyaluronic acid has potential application as a nanofiber due to its water-resistant characteristics. [PERSON] et al. [116] and [PERSON] et al. [117] constructed a water-resistant nanofiber with an electrospinning and electro-blowing technique.