Organic additive release from plastic to seawater is lower under deep-sea conditions

Global plastic production is immense and a significant portion enters the ocean annually. This plastic undergoes photo- and biodegradation, creating micro- and nanoplastic particles. Plastic degradation releases dissolved organic carbon (DOC), including additives like phthalates (PAEs), organophosphate esters (OPEs), and bisphenols (BPs). These additives are endocrine disruptors with potential toxic effects on marine organisms and humans. Most studies focus on surface waters, yet a large proportion of plastic sinks to deeper ocean environments. This research addresses the knowledge gap regarding additive release under deep-sea conditions, comparing leaching rates in surface and deep seawater, considering the influence of hydrostatic pressure and prokaryotes.

Previous research has shown that plastic additives, such as PAEs, OPEs, and BPs, are released into the marine environment. These additives have been detected in coastal and open ocean environments and are known endocrine disruptors with potential health impacts. The release of water-extractable additives is influenced by polymer-water partitioning. Marine prokaryotes play a role in additive release. Modeling suggests that only a small percentage of plastic waste floats on the surface, indicating that much of it sinks to deeper depths, accumulating on the seafloor. However, information on additive release under deep-sea conditions is limited. Studies have highlighted the differences in water chemistry, pressure, and prokaryotic content between surface and deep-sea environments.

Polyethylene (PE) and polyvinylchloride (PVC) pellets were exposed to surface and deep seawater under various conditions: atmospheric pressure (0.1 MPa) and high pressure (10 MPa) simulating 1000 m depth; biotic (with natural prokaryotes) and abiotic (sterilized with HgCl2) conditions. Incubations lasted 30 days at 13°C (Mediterranean deep seawater temperature). Twenty-five organic additives (including PAEs, OPEs, and BPs) and seven PAE monoester metabolites were monitored. Prokaryotic abundance was measured by flow cytometry, and DOC, dissolved organic nitrogen (DON), and dissolved organic phosphorus (DOP) were analyzed. First-order kinetic modeling was used to analyze additive release kinetics, with confidence intervals used for statistical comparisons. Dimethyl phthalate (DMP) and tris(2-ethylhexyl) phosphate (TEHP) served as model compounds due to their contrasting physico-chemical properties.

The study found that five of the 25 monitored additives leached from PE, while DiNP and BPs were detected in PVC leachates. Most additives were released within the first week. Cumulative additive release from PE remained below ppm levels regardless of biotic conditions and pressure. DiNP release from PVC was significantly higher than from PE. High hydrostatic pressure significantly inhibited the leaching of heavier, more hydrophobic additives like TEHP and DiNP under abiotic conditions. However, the presence of prokaryotes, both surface and deep-sea assemblages, significantly increased additive release regardless of pressure. While OPEs have short half-lives in phosphorus-limited seawater, the study used non-phosphorus-limited seawater. Biodegradation is estimated to account for a maximum of 50% of the apparent additive release, excluding OPEs. Two PAE metabolites (MMP and MEHP) were found at higher concentrations in biotic leachates, suggesting bacterial degradation of DiNP and DEHP. The presence of PVC reduced prokaryotic abundance by a factor of 4 compared to controls, possibly due to toxic effects of leached compounds. DiNP accounted for 18% of the measured DOC release under surface and biotic conditions. The release of additives from PE and PVC in deep seawater was significantly lower than in surface seawater.

The results demonstrate that high hydrostatic pressure reduces the leaching of hydrophobic additives from plastics. This is likely due to an increase in Kpw (polymer-water partitioning) or a decrease in the water accessible polymer surface layer thickness under pressure. The presence of prokaryotes significantly promotes additive release, possibly through polymer oxidation and increased surface area. The study confirms that additive release and biodegradation are more efficient in surface waters than in deep-sea environments, even considering the activity of deep-sea prokaryotes. The lower release in deep-sea waters despite microbial presence suggests the dominant role of hydrostatic pressure on limiting additive release from the plastic matrix. The findings highlight the long-term release of additives from plastics and the complex interaction between physical and biological processes.

This study provides empirical evidence that organic additive release from plastics is more efficient in surface waters than in the deep ocean. Hydrostatic pressure inhibits leaching of hydrophobic compounds, while prokaryotes promote release. Despite lower release rates in the deep sea, the accumulation of plastic in this compartment could lead to long-term, significant exposure for deep-sea organisms. Future research should focus on the effects of UV radiation on deep-sea plastic degradation, longer-term studies, and the detailed role of specific microbial communities.

The study used only two types of plastic (PE and PVC) and a limited number of target additives. While this provides useful information, it may not represent the full range of plastic types and additives in the marine environment. The duration of the experiment (30 days) might not fully capture the long-term dynamics of additive release. The experiment was conducted in the dark, excluding the impact of UV radiation which is a major driver of plastic degradation in surface waters.