Numerous international governmental agencies that steer policy assume that polystyrene persists in the environment for millennia. Here, we show that polystyrene is completely photochemically oxidized to carbon dioxide and partially photochemically oxidized to dissolved organic carbon. Lifetimes of complete and partial photochemical oxidation are estimated to occur on centennial and decadal time scales, respectively. These lifetimes are orders of magnitude faster than biological respiration of polystyrene and thus challenge the prevailing assumption that polystyrene persists in the environment for millennia. Additives disproportionately altered the relative susceptibility to complete and partial photochemical oxidation of polystyrene and accelerated breakdown by shifting light absorbance and reactivity to longer wavelengths. Polystyrene photochemical oxidation increased approximately 25% with a 10 °C increase in temperature, indicating that temperature is unlikely to be a primary driver of photochemical oxidation rates. Collectively, sunlight exposure appears to be a governing control of the environmental persistence of polystyrene, and thus, photochemical loss terms need to be included in mass balance studies on the environmental fate of polystyrene. The experimental framework presented herein should be applied to a diverse array of polymers and formulations to establish how general these results are for other plastics in the environment.

In sunlit waters, photochemical alteration of dissolved organic carbon (DOC) impacts the microbial respiration of DOC to CO₂. This coupled photochemical and biological degradation of DOC is especially critical for carbon budgets in the Arctic, where thawing permafrost soils increase opportunities for DOC oxidation to CO₂ in surface waters, thereby reinforcing global warming. Here we show how and why sunlight exposure impacts microbial respiration of DOC draining permafrost soils. Sunlight significantly increases or decreases microbial respiration of DOC depending on whether photo-alteration produces or removes molecules that native microbial communities used prior to light exposure. Using high-resolution chemical and microbial approaches, we show that rates of DOC processing by microbes are likely governed by a combination of the abundance and lability of DOC exported from land to water and produced by photochemical processes, and the capacity and timescale that microbial communities have to adapt to metabolize photo-altered DOC.

Following the Deepwater Horizon (DWH) blowout in 2010, oil floated on the Gulf of Mexico for over 100 days. In the aftermath of the blowout, substantial accumulation of partially oxidized surface oil was reported, but the pathways that formed these oxidized residues are poorly constrained. Here we provide five quantitative lines of evidence demonstrating that oxidation by sunlight largely accounts for the partially oxidized surface oil. First, residence time on the sunlit sea surface, where photochemical reactions occur, was the strongest predictor of partial oxidation. Second, two-thirds of the partial oxidation from 2010 to 2016 occurred in less than 10 days on the sunlit sea surface, prior to coastal deposition. Third, multiple diagnostic biodegradation indices, including octadecane to phytane, suggest that partial oxidation of oil on the sunlit sea surface was largely driven by an abiotic process. Fourth, in the laboratory, the dominant photochemical oxidation pathway of DWH oil was partial oxidation to oxygenated residues rather than complete oxidation to CO₂. Fifth, estimates of partial photo-oxidation calculated with photochemical rate modeling overlap with observed oxidation. We suggest that photo-oxidation of surface oil has fundamental implications for the response approach, damage assessment, and ecosystem restoration in the aftermath of an oil spill, and that oil fate models for the DWH spill should be modified to accurately reflect the role of sunlight.

In May 2021, the M/V X-Press Pearl cargo ship caught fire 18 km off the west coast of Sri Lanka and spilled ∼1680 tons of spherical pieces of plastic or “nurdles” (∼5 mm; white in color). Nurdles are the preproduction plastic used to manufacture a wide range of end products. Exposure to combustion, heat, and chemicals led to agglomeration, fragmentation, charring, and chemical modification of the plastic, creating an unprecedented complex spill of visibly burnt plastic and unburnt nurdles. These pieces span a continuum of colors, shapes, sizes, and densities with high variability that could impact cleanup efforts, alter transport in the ocean, and potentially affect wildlife. Visibly burnt plastic was 3-fold more chemically complex than visibly unburnt nurdles. This added chemical complexity included combustion-derived polycyclic aromatic hydrocarbons. A portion of the burnt material contained petroleum-derived biomarkers, indicating that it encountered some fossil-fuel products during the spill. The findings of this research highlight the added complexity caused by the fire and subsequent burning of plastic for cleanup operations, monitoring, and damage assessment and provides recommendations to further understand and combat the impacts of this and future spills.

Sunlight exposure is a control of long-term plastic fate in the environment that converts plastic into oxygenated products spanning the polymer, dissolved, and gas phases. However, our understanding of how plastic formulation influences the amount and composition of these photoproducts remains incomplete. Here, we characterized the initial formulations and resulting dissolved photoproducts of four single-use consumer polyethylene (PE) bags from major retailers and one pure PE film. Consumer PE bags contained 15–36% inorganic additives, primarily calcium carbonate (13–34%) and titanium dioxide (TiO₂; 1–2%). Sunlight exposure consistently increased production of dissolved organic carbon (DOC) relative to leaching in the dark (3- to 80-fold). All consumer PE bags produced more DOC during sunlight exposure than the pure PE (1.2- to 2.0-fold). The DOC leached after sunlight exposure increasingly reflected the ¹³C and ¹⁴C isotopic composition of the plastic. Ultrahigh resolution Fourier transform ion cyclotron resonance mass spectrometry revealed that sunlight exposure substantially increased the number of DOC formulas detected (1.1- to 50-fold). TiO₂-containing bags photochemically degraded into the most compositionally similar DOC, with 68–94% of photoproduced formulas in common with at least one other TiO₂-containing bag. Conversely, only 28% of photoproduced formulas from the pure PE were detected in photoproduced DOC from the consumer PE. Overall, these findings suggest that plastic formulation, especially TiO₂, plays a determining role in the amount and composition of DOC generated by sunlight. Consequently, studies on pure, unweathered polymers may not accurately represent the fates and impacts of the plastics entering the ocean.