Abstract
Approximately 4.8–12.7 million tons of plastic waste has been estimated to be discharged into marine environments annually by wind and river currents. The Ellen MacArthur Foundation warns that the total weight of plastic waste in the oceans will exceed the total weight of fish in 2050 if the environmental runoff of plastic continues at the current rate. Hence, biodegradable plastics are attracting attention as a solution to the problems caused by plastic waste. Among biodegradable plastics, polyhydroxyalkanoates (PHAs) and poly(ε-caprolactone) (PCL) are particularly noteworthy because of their excellent marine biodegradability. In this review, the biosynthesis of PHA and cutin, a natural analog of PCL, and the biodegradation of PHA and PCL in carbon cycles in marine ecosystems are discussed. PHA is biosynthesized and biodegraded by various marine microbes in a wide range of marine environments, including coastal, shallow-water, and deep-sea environments. Marine cutin is biosynthesized by marine plants or obtained from terrestrial environments, and PCL and cutin are biodegraded by cutin hydrolytic enzyme-producing microbes in broad marine environments. Thus, biological carbon cycles for PHA and PCL exist in the marine environment, which would allow materials made of PHA and PCL to be quickly mineralized in marine environments.
Highlights
Plastics are used in various aspects of our lives and are indispensable
The Ellen MacArthur Foundation warns that the total weight of plastic waste in the oceans will exceed the total weight of fish in 2050 if the environmental runoff of plastic continues at this rate [7]
Biodegradable plastics are materials degraded by the actions of microorganisms in the environment that are converted into inorganic substances
Summary
Plastics are used in various aspects of our lives and are indispensable. In 2018, the global production of plastics reached 359 million ton [1]. It has been estimated that 300 million tons of plastic waste is generated annually, of which 79% is disposed of in landfills or discharged into the environment [3, 4]. Biodegradable plastics are materials degraded by the actions of microorganisms in the environment that are converted into inorganic substances. Biodegradable plastics are hydrolyzed by enzymes produced by microorganisms [20]. Chemically synthesized biodegradable plastics do not have a dedicated degradation enzyme, unlike PHA [21]. Synthesized biodegradable plastics are recognized and hydrolyzed in the environment as substrate analogs by lipases, which use fat as a substrate, and by cutinases, which uses the polyester cutin (present in the cuticular layer of plants), as a substrate [22,23,24,25,26,27,28,29] (Table 2)
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