**3.2 Biotic degradation pathway**

Plastic degradation by microorganisms present in the marine habitat results in the biodegradation of plastic wastes. However, macroplastics (larger plastic debris) are not the ideal feedstock for biotic degrading agents owing to their size, which poses a hindrance to the degradation mechanism, either the enzymes produced by the microorganisms are not enough to degrade the macroplastics, or they are not readily bioavailable for microbial cell uptake. During the degradation process, polymeric plastics need to be first converted into monomers before they can be mineralized by the biological agents. The molecular size of plastics (i.e., polymers) is larger than the pore size of microorganism's cell membrane. Hence, they need to be depolymerized into smaller fragments before they can be absorbed and biodegraded within the microbial cells. Therefore, smaller fragments of plastic formed as a result of abiotic degradation are of the appropriate size to be further degraded by microorganisms [12]. Microorganisms predominantly present in the marine environment include bacteria, fungi, and algae.

### *3.2.1 Bacteria*

*Bacillus* species are commonly found in the marine environment, for example, *Bacillus subtilis* and *Bacillus cereus.* These bacteria were found to secrete extracellular hydrolytic enzymes such as *lipase, xylanase, keratinase, chitinase, and protease*, which lead to plastic degradation [77]. PVC, the most common plastic polymer, can be degraded by *Methanosarcina barkei*. They can adhere to the surface of the PVC surfaces and release exopolymeric substances to form a biofilm on the PVC, followed by the release of enzymes to degrade the plastic via hydrolytic cleavage of the polymeric bonds [78, 79]. Similarly, PE can be degraded by *Rhodococcus ruber*, which produces an enzyme laccase that results in PE degradation [80]. PS can be degraded by *Azotobacter spp.*, which produces hydroquinone peroxidase. PET can be degraded by *Alcanivorax*, *Hyphomonas,* and *Cycloclasticus* species, which can change the surface chemistry via hydrolysis of the ester bonds [81].
