Waste Polystyrene Degradation in the World Oceans: Newly identied sources of Contamination

Since 1970, lumps of plastic breakage into micro/nano pieces has been clearly shown a serious and large source of ocean pollution. To clarify in detail the course of this impact, thermoplastics were decomposed at natural conditions kinetically. And eld surveys conducted on four thousand sand and water samples including these at deep-sea sites from around the world during the period, 2000 to 2015. All samples were found to contain styrene oligomers (SOs), that had been generated from drifting polystyrene (PS) degradation. Lumps of plastic not only break up into micro/nano-fragments but also degrade into their basic structure units of plastic. From 1950 to 2015, 4.2 billion metric tons (MT) of SOs were shown to be generated from drifting PS. The monomers are newly and highly signicant source of pollution of marine ecosystem directly and global warming of planet and should be given the utmost serious attention.


Introduction
The cumulative amount of all plastics produced from 1950 to 2015 has been estimated at 7.8×10 9 metric tons (MT). 1 Thermoplastics, polyethylene (PE), polypropylene (PP), polyvinyl chloride (PVC, including plasticized PVC) and polystyrene (PS) are the four major plastics produced in the greatest amounts, constituting for approximately 70% of total world production 2 . Whether accidentally or intentionally, waste PS from land sources ultimately makes its way into world oceans. PS has been shown a major constituent of drifting plastics and, due to its ability to oat, is an outstanding factor contributing to world pollution.
Carpenter and Smith 3 rst reported plastic contamination in the western Sargasso Sea and indicated 3,500 oating pieces to have a weight of 290 g km 2 . van Sebille 4 , Eriksen 5 and Cozar 6 et al. calculated that between 7.0 × 10 3 to 2.7 × 10 5 MT debris plastics to be present on ocean surfaces and to be the predominant constituent of "garbage patches" found throughout world ocean. 7 By 2050, debris plastics in oceans may possibly attain an exceeding that of all sh. 8 Drifting lump plastics in oceans have been studied in great detail with major quite emphasis on plastic breakage into progressively smaller pieces (These pieces are visual to the naked eye. Chemically, they have the structures of macromolecules). To address this most serious problem of global waste plastic pollution, it is most urgent to make quantitative determination of waste plastic presently in world oceans, as well as their amounts a oat and the extent of their degradation. This undertaking should be possible by conducting the following steps: [1]. Determination of amounts of plastic waste owing into oceans. [2]. Determination of amounts of plastic drifting on ocean surface. [3]. The quanti cation of monomers, produced by plastic degradation, which subsequently sedimentate on the ocean oor 9 . Differences in amounts of [1] within a given period [3] should serve as indication of drifting PS [2].
To clarify the amounts of degraded PS [3] in oceans, puri ed PS was decomposed within a natural temperature range kinetically and SOs were found to be the main products. 10 Unlike PS, this product has been shown to dissolve in the ocean (Supplementary Information Section I). Over 4,000 sand and water samples from coastlines in 26 different countries and isolated islands as well as open oceans, including those taken from a depth of 5,000 m, were used to determine the amounts of SOs. SOs which had been found in all samples. [11][12][13][14] Results And Discussion Worldwide SOs contamination The amount of waste PS in ow in oceans from 1950 to 2015 has been estimated as 1.5 × 10 7 MT, based on the assumption PS constitutes 7 % total global plastic production (4.8 × 10 8 MT) 2 and that 3% of this plastic ow into the world's oceans 15  and SOs (mixture of styrene monomer: SM, 2,4-diphenyl-1-butene:styrene dimer, SD and 2,4,6-triphenyl-1-hexene:styrene trimer, ST) were detected. 10 SM has been shown a breakdown product formed by cinnamon mold ora and possibly may be present in oceans as a single contaminant. 16 But SOs with constant composition ratio (puri ed PS decomposition, SM1: SD1: ST5 and eld survey in this study, SM1: SD1: ST8) is not naturally present in ocean. It is clear that SO contamination has been around for 45 years before. 10 SO contaminants have been shown present in commercial PS products such as disposable lunch boxes, food trays, and noodle containers sold around the world.
The exudation of SOs from these PS products enters rivers and oceans as contaminates. The degree of these SOs should be determined. 10,17 Using 2.5 L water and 5.0 g sand dichloromethane extracts, SOs were analyzed in detail by Gas Chromatograph equipped with Mass Spectrometry (GC/MS). [10][11][12][13][14] Mean SO value from coastal surveys worldwide were found to be 1613.4 µg kg -1 for sand samples and 2.7 µg L -1 for water samples. SO contamination of sand was 600 times higher than that of water. All sampling sites are shown in Fig.1-A and SO analytical results in Extended Data sTable 2. The composition ratio of SO was SM1: SD1: ST8 in water and sand samples, respectively. The highest regional values of SOs in sand samples were for Greece (N:37º98', 31,400 µg kg -1 , Extended Data sTable 2, site 9) and water for Washington/USA were (N:47º39', 30.4 µg L -1 , Extended Data sTable 2, site 21-2). The area with the lowest SO levels was in the Hawaii Islands/USA (N:21º18,24', 16.0 µg kg -1 in sand and 0.02 µg L -1 in water, Extended Data sTable 2, site 21-k). Comparing the SO values of the world oceans, the sand is greatest in the Indian Ocean ( Fig.1-A, site-12,13,14; 7,500 µg kg -1 ), followed by the Atlantic Ocean ( Fig.1-A, site-1,2,3,21-e,f,g,h,I and 24 east; 4,485 µg kg -1 ) and the Paci c Ocean ( Fig.1 (Fig.1-A). Beach sand is exposed directly to sunrays and so consequently surface temperatures of shallow coastal areas may reach as high as 60 °C during the day in tropical and subtropical areas, causing greater PS degradation. 17 High SO values were found along coastlines near large cities such as London, Tokyo, Los  Fig. 1-A).
Dispersion in the Northwest Paci c Ocean from Japan SO contamination along the shores of Japan and dispersion in the Northwest Paci c Ocean have been examined in great detail. The Japanese Ministry of Agriculture has divided the country into 9 areas for management. From these 9 designated areas, some 2,000 sand and water samples were analyzed during 2000 to 2015. The means SO value in sand was 591.0 µg kg -1 and amount in water 1.3 µg L -1 . Japanese SO values were almost the half in water, but the sand was 36 % of world SO contamination. The details for these determinations are presented in another paper. 14 Every year, 10,000 MT of drifting PS washed up onto the shore of Japan. 18 SO contamination in sand may possibly arise from this.
Using Japanese Government and the University of Tokyo joint operation ocean research ship Hakuho-maru along a 160° east longitude, 5,000 km north to south and 2,000 km east to west while making a HK-12-3 cruise during 2012 in the Northwest Paci c Ocean. SO contaminations were examined with 10 L were collected using a Niskin-type water sampler at xed depth from the bottom to surface. (Fig.1-A, red line, Table 1 and Extended Data sTable 3, Stn 1-18). In the open ocean, the SO composition ratio was SM1: SD3: ST26 at the surface (Extended Data sTable 3) and SM1: SD39: ST204 as the average value for data at the depths of 2,000, 3,000 and 4,000 m deep (Table 1). From these values, it is evident that ST is the major pathway for the formation from PS in the ocean. 10 At sampling station (Stn)-9 and -8 showed SO contaminations two times and ve times the values noted at the surface (Extended Data sTable 3). The size of the area examined was basically the same as that of the Northwest Paci c Garbage Patch (PGP , Fig 1-C). The surface current was 0.7 knots and the average salinity was 34.4 permil during the cruise (Extended Data sTable 3). On the other hand, the average salinity above a depth of 2,000 m is 37.0 permil and buoyancy is considered to increase (Table 1). SO was high (ca. 0.2 μg L -1 ) in the photic zone near the depth of 200m (Fig.1-B), and the means SO value being about 0.1 μg L -1 in the deeper than 2,000m where there was no horizontal movement. This deep SOs may thus possibly adhere to or be adsorbed by organic or inorganic matter. This may perhaps be the reason for the vertical SO movement.
With consideration to various marine factors along Japanese coasts, determination was made of SOs dispersion in the open ocean as shown at Fig.1-C (Supplemental Information Section III) 19 . SOs were clearly shown to disperse from the coasts of Japan into the Northwest Paci c Ocean over time, to eventually accumulate at high concentrations in the PGP, as noted also for ordinary plastic debris. SOs from the open ocean were also detected in the Northwest Paci c Ocean at about 0.2 μg L -1 at the surface. At depths of 2,000 to 5,000m the SO value was 0.1 μg L -1 at Table 1 and Fig.1-B. In all these computations, 4-6, 8,15, 21 there has always been considerable margin for error. There is really no completely relivable information on the quantities of plastics that have undergone degradation and sedimented to the ocean oor.
Based on SO-free PS decomposition, one MT of puri ed PS was found to degrade at a rate of 0.3g per year at 30 °C, at an activation energy of 42.0 kJ mol -1 . 10,17 The careful decomposition of other thermoplastics at natural temperature range kinetically are generate into monomers at low activation energy (Extended Data sTable 4): the heat resistant plastics PC and EPX 22 generate BPA, PET and PVC generate PAE and polyurethane (PU) generate diisocyanate. The toxicity of these monomers is endocrine-23 , teratogenic-24 and reproductive-23,24 affects and potentially mutagen or carcinogenic 24,25,26 is well known (Extended Data sTable 4).
Plastic/macromolecules do not metabolize though their monomers can be metabolized by living organisms. The activation energy for these thermoplastics were basically the same as for PS (Extended Data sTable 4). In actual situations, ST gradually degrades to give SM as well as SD, which degrades into SM. 10,17 We have been searching for SD and ST degrading bacteria from natural sources for over 20 years, but which still have not been isolated. Simulation results at PS degradation at 30°C indicated the total amount of degraded PS in oceans will come to 90 MT by 2015 (t = 65 year), this amount in excess of 400 MT by 2050 (t = 100 year). 10 On the other hand, the amounts of drifting PS [2] are estimated based on SOs surveys [3] and in ow [1] into oceans throughout the world. Using total ocean water volume of 1.4 × 10 21 L and SO values of 0.1 and 0.2 μg L -1 , which possibly may re ect the amounts of SO sedimentation (deep sea) and drift (surface), 4.2 billion MT of SO were shown present in ocean water columns between 1950 to 2015 (Supplemental Information Section II). This quantity of SOs was subsequently converted to the amount of PS as 2.6 × 10 6 MT (Supplemental Information Section II).
Drift plastic and generated monomer have both shown to be a signi cant source of serious threat throughout the entire global planet.
The ICPP reports that the amount of GHG increased by an average of 9% every year from 2000 to 2017 and 2017 GHG reached 600 million MT per year 27 . PE and PP have been shown seven times that of PS production 1 , suggesting that drifting PE and PP may produce seven times or much more GHG than SO 28,29 . It was suggested that generation of GHG from drift PE, PP degradation should thus be considered a serious threat throughout the entire global planet.
Drift lump PS and particles tend to sink and undergo property change over time due to weathering, embrittlement, fragmentation and corrosion 30,31 . As shown by worldwide SOs contamination (Extended Data sTable 2), there are signi cant effects on the generation of SOs by geographical factors such as latitude, weather, wave, current, temperature, sun ray, population and so on 14,15,17 . Although the amount of PS degradation at 30 °C and surveyed SO values are signi cantly different, the plastic degradation was shown to be promoted more in natural environments.

Conclusions
Thermoplastics do not remain permanently intact in the ocean with time degrade into monomers. New invisible monomer contamination is progressing in the world's oceans. Drift plastic and generated monomer have both shown to be a most serious threat not only to the marine ecosystem directly but to the global planet as a whole.

Declarations
Data and materials availability: All data is available in the main text, the extended data gures and tables, or the supplementary information. Fig. 1 -A, B, C and Table 1 Extended data gures and