**7. Strategies to choose to face the obstacle related to a circular economy and industrial ecosystem**

Keen interest in cyanobacteria is because of the production of different metabolites which works with more than one type of compound as a salable product this type of application use is called a "cradle to cradle" system (turning waste into a new product) that is bioplastic [107]. Another instance of turning waste into a new product is using microalgae, reusing the effluents from the refining of olive oil in the cultivation of microalgae for biodiesel and biopolymers [29].

Another beneficial environmental effect that makes the adoption of a circular bioeconomy more real is the uptake of ambient carbon dioxide for conversion into biotechnological products. Using by-products and leftovers from microbiological production, it is possible to integrate the creation of bioplastic with the manufacture of other desirable goods to reduce the cost of microbial PHB. An effective alternative is the cyanobacterial genus *Nannochloropsis sp.,* which produces eicosapentaenoic acid, and the cyanobacterial genus *Spirulina platensis*, which produces linoleic acid. This species is important for its expressive biomass output, which has a high protein content and can be used to make animal feed or nutraceuticals.

The construction of a biorefinery, merging *Synechocystis salina's* PHB synthesis with commercially valuable pigments, notably the commonly abundant phycocyanin and chlorophyll, and carotenoids, showed encouraging results. Since the quality of the resulting polymer is directly influenced by purification, which includes the removal of pigments that can be employed in manufacturing chains of higher value, the extraction of pigments without their degradation is not only feasible but also necessary. In addition to pigments, *S. salina* biomass contains carbohydrates, lipids, and proteins that can be used as animal feed as long as the necessary nutritional standards and laws regarding the presence of contaminants like heavy metals or mycotoxins are observed. In this case, cyanotoxins are given priority over cyanobacteria that do not produce toxins [108].

Cyanobacterial dietary supplements are also advantageous for animal health, with *Spirulina sp.* biomass enhancing hens' humoral and immunological responses. For cyanobacteria and microalgae in general, the dual benefit of production connected with bioremediation has already been discussed, with a focus on the creation of biodiesel. The same idea can be used to explain how naturally transformable organisms like cyanobacteria can produce biopolymers, opening new opportunities for genetic engineering.

### **8. Conclusion**

PHA has turned out to be a substitute for conventional plastics. Cyanobacteria is becoming the alternative source of PHA production. The major cause of the production of PHA using microalgae is to reduce the cost. Now, cyanobacteria aids in the production of PHA as it collects a huge amount of PHS through photosynthesis which ultimately requires less nutritional content for growth. Cyanobacteria have a very low
