Fig. 1: Sample deployment at the deep-sea floor.

a Map of in situ biodegradation test sites. The map was made with GeoMapApp (www.geomapapp.org) / CC BY / CC BY (Ryan et al., 2009)³⁸. See Table 1 for abbreviations regarding ___location. b BMS and BHT are close to heavily populated cities and shipping routes to/from Tokyo Bay with a high accumulation of observed plastic debris. BHT and BMJ are sites containing unique chemosynthetic communities near cold seepage and hydrothermal vents, respectively. AKR is south of the Kuroshio Extension recirculation gyre (KERG), where a high accumulation of plastic debris has been reported. AMN is another abyssal environment, but distant from populated areas. PJM is a coastal environment (Port of JAMSTEC Yokosuka Headquarters), and serves as a reference site that is close to large cities and rivers. The sample sets were deployed on the deep-sea floor by the human-occupied submersible Shinkai 6500, except for AMN, where the deployment was performed by a lander system, and the sample sets were placed 2 m above the sea floor. c Injection-molded sample specimen used in this study and sample set in a custom-made sample holder (closed with 1 mm gap polyethylene (PE) mesh). d Melt-pressed sample specimen used in this study and the sample set with its sample sealed with 1 mm gap PE mesh. e The test samples deployed at BHT. f The test samples at BHT 4 months after deployment. g Recovery of sediments under the samples.