Development and multi-site validation of a rapid biodegradation test method for seafloor conditions using extracted seawater with microbes from sediments

Within the framework of a circular economy of polymers, recycling is a major strategy, but for applications prone to environmental release, replacing them with marine biodegradable plastics is highly desirable. Since plastics sink to the seafloor due to biofouling, evaluating their biodegradability in seafloor environments is crucial. Standardized methods such as ISO 19679 and ISO 18830, which are designed to assess biodegradability under simulated seafloor conditions, use seawater and sediment as inocula, but often require long testing periods and may show variability among tests. In this study, we developed a novel biodegradation testing method using "extracted seawater", in which microorganisms are ultrasonically extracted from sediment into seawater and supplemented with inorganic nutrients to enhance microbial activity. This approach enables rapid evaluation of biodegradability with good consistency under conditions relevant to the seafloor. Biodegradation tests conducted at 14 different locations in Japan, France, and Thailand showed that cellulose achieved 60 % biodegradability (t₆₀) in less than 50 days at all locations. For polycaprolactone (PCL), t₆₀ was also within 50 days at most locations, with only one location requiring 80 days. Furthermore, for poly(butylene succinate-co-adipate) (PBSA), a polymer known for its relatively slow degradation under marine conditions, t₆₀ was consistently reached within 50 days at three different locations. These findings indicate that the proposed method enables practical and time-efficient evaluation of biodegradability, performing consistently with inocula from diverse origins.