Cold seep areas function as notable MP sinks
Fig. 1b shows that the mass abundance of MPs has drastically increased from the early 1900s, which is the era of initial plastic production, to the present. Both the onset of industrial plastic production in the 1930s32 and the massive production occurring in the 1950s5 exhibited significant growth, indicating that the historical trajectories of MPs deposited in the Haima cold seep reflect the plastic production history. The MP burial rate in nonseepage areas exponentially increased from the 1930s to the 2020s, which agrees with the exponential growth in the global plastic production rate (Fig. 1c-1d). The exponential growth tendency of the burial rate at the middle cold seep development stages was much weaker (Fig. 1e-1f), and the burial rate at the early stage decreased under fluctuations, suggesting that the MP accumulation rate in the sediment decreased with methane seepage. This occurred because the upwelling methane fluid flow could carry sediment particles into the overlying hydrosphere33, and MPs may be utilized by active microorganism communities in active cold seep systems. The current burial rate (2001-2020) in the Haima cold seep (average: 248.73± 213.30 mg plastics m−2 year−1) is several times higher than that in coastal areas (64±4 mg plastics m−2 year−1), suggesting that deep-sea sediment functions as a much stronger MP sink. The total abundance of MPs is tens of thousands of items per kilogram of dry sediment, which is much higher than that in other marine environments1,2. This significant difference occurs because deep-water zones provide a very high capacity to accommodate long-term MP transportation, and the fine-grained clay sediments in the Haima cold seep areas contribute to MP accumulation34.
Principal coordinate analysis (PCoA)35 revealed that both the number and mass abundance exhibited a significant difference between the seepage and nonseepage areas. The PERMANOVA F value for the number abundance (2.343) was higher than that for the mass abundance (1.805) of MPs, and the corresponding P value was much more significant (Fig. 2a-2b). The number abundance of MPs in the nonseepage areas was higher than that in the seepage areas (Fig. 2c), and this result verified the previous conclusion that the accumulation ability of MPs in the methane seepage areas was lower. The number abundance of MPs at the M&S-S stages was the highest among all the seepage locations because strong methane bubble flows were beneficial to particle accumulation near the seepage areas. The mass abundance varied among the different locations with a similar trend to that of the number abundance, except that the Early-S stage exhibited the highest abundance among the seepage areas originating from large-scale MPs with high densities of polycarbonate, polyvinyl chloride, and fluororubber settling on the surface (Fig. 3a).
A comprehensive risk assessment was conducted to gain insight into the environmental risks of MPs. The pollution load index (PLI) at all the diving locations was relatively low, while the polymer risk assessment index (PHI) and Nemerow pollution index of the number abundance (NPI-N) in this work were significantly high in all the areas and higher than those in other environments36. This occurred because seventeen categories of MPs were detected at all five diving locations. The PLI and potential ecological risk index (PRI) were the lowest at the M&W-S stage because of long-term and weak methane seepage with fewer disturbances prior to MP degradation. The NPI-M value in the deeper layers decreased, suggesting that long-term transport and transformation of MPs in the deeper sediments could reduce the potential environmental and ecological risks.
MP sources have evolved over the last several decades
To trace the evolution of MP sources, we analyzed the changes in MP categories between stratigraphic decades. RN/M, the ratio of the number abundance (right side of Fig. 3a) to the mass abundance (left side of Fig. 3a), can be employed as an effective indicator to explain the fragmentation level of MPs. PC was primarily discovered in the upper 4 cm, and PC demonstrated obvious fragmentation with increasing RN/M value. Figure 3b also verifies that PC has only accumulated in recent years. PA and silicon resin, which are widely used in engineering plastics and coatings in ships and marine engineering, significantly increased with the depth. These two MPs also exhibited a fragmented trend at sediment depths below 10 cm. RN/M of most MPs was higher in the methane seepage areas than that in the nonseepage areas, indicating that the nonseepage areas provided a higher capacity for the sequestration of large-scale MPs. In general, the MP categories primarily evolved as light industrial products from the top surface to heavy industrial products in the inner bottom. In the top 2 cm (approximately 2016-2021), the increase in the number abundance of MPs was closely correlated with the extensive use of virus transmission prevention products, such as disposable protective clothing, safety gloves, and disposable masks, since the COVID-19 pandemic (Fig. 3c). In the deeper layers, the MP abundance was closely correlated with the number of fishing activities expressed as marine catches (Fig. 3d) and the number of fishing boats (Fig. 3e). Specifically, anthropologic activities and offshore operations were the primary sources of MPs deposited in cold seepage environments.
MP fragmentation and degradation are influenced by methane seepage
Figure 4a shows that the size distribution of MPs in the methane seepage and nonseepage areas exhibited high β diversity based on the nonmetric multidimensional scaling (NMDS) method, as the stress value was lower than 0.1. The PERMANOVA test method indicated that methane seepage induced a significant difference in the size distribution of MPs (P = 0.022). Fig. 4b shows that the diversity of the size distribution of MPs increased due to the presence of small- and medium-scale MPs (<500 μm) in the cold seep sediments, and these small-scale MPs were prone to settlement in the nonseepage areas. The changes in the ratio of the particle abundance of small-scale MPs to that of other morphologies indicated that small-scale particles were the major MP type in cold seepage areas, and this ratio increased in the methane seepage areas (Fig. 4c). In addition, the projected areas of MPs in the methane seepage areas were smaller, verifying that the fragmentation degree responded more sensitively to methane seepage (Fig. 4d). The higher value at the early-S stage was attributable to newly sequestered PVC MPs at the surface. The abundance of colored MPs synchronously varied with that of colorless MPs along the sediment depth (Fig. 4e) with a strong correlation (Fig. 4f). Both the colored and colorless MP abundance levels peaked at depths of approximately 10 and 25 cm, which agreed with the abundance peak of MP-depredating microorganism Pseudomonadales37 (Fig. S6). This microorganism level exhibited a positive correlation with colored MPs in the methane seepage areas (Fig. 4f), suggesting that MPs were more likely to be decomposed with methane seepage. The abundance of fine-scale MPs (<50 μm) relative to large-scale colored MPs in this zone correspondingly increased, providing further evidence that colored MPs were degraded and fragmented.
MP abundance is controlled by geochemical and microorganism characteristics
The MP abundance was controlled by environmental factors, as it was demonstrated that the depth and concentration of total organic carbon (TOC), dissolved inorganic carbon (DIC), iron (Fe) ions, manganese (Mn) ions, and phosphate in the sediment pore water were significantly correlated with the MP abundance based on Mantel’s test method (Fig. 5a). Environmental factors were more closely correlated with the mass abundance than with the number abundance because these parameters could influence the MP degradation ability. The scanning electron microscopy-energy dispersive spectroscopy (SEM-EDS) test method indicated that Fe and Mn ions, which are electron acceptors of the anaerobic oxidation of methane (AOM) reaction, had accumulated on the surface of MPs38 (Fig. 5b), providing indirect links between MPs and methane transformation. Network analysis revealed the relationship between the MP abundance, geochemical factors, and microorganism populations. There occurred a significant correlation between the number abundance of polypropylene (PP), polyamide (PA), polysulfone (PSF), and PC and the population abundance of Pseudomonadales, Betaproteobacteriales, Actinomarinales, and certain uncultured bacteria. In addition, these microorganism populations were primarily correlated with the sediment depth and dissolved oxygen (DO), TOC, total carbon (TC), silicate, and phosphate pore water concentrations (Fig. 5c). This result suggested that these microorganisms could provide the potential to degrade the associated MPs. In addition, Methylomirabilales indirectly affected MP degradation because they are closely linked with environmental factors influencing potential MP-degrading microorganisms. In regard to the mass abundance, more types of polymers were affected by more microbial species, as the abundance of PC, PA, polyethylene terephthalate (PET), PSF, PP, and PSF were significantly correlated with the microorganism abundance and concentration of environmental indicators (Fig. 5d).
To further elucidate the biodegradation mechanism of MPs in the Haima cold seep environment, metabolic pathways of methane oxidation and MP degradation were proposed. As shown in Fig. 6a, in the AOM process, methane was stepwise transformed into methanol, formaldehyde, and formic acid and finally into serine and acetyl coenzyme A in the tricarboxylic acid cycle. Metallic minerals, sulfate-reducing bacteria (SBR), and Acidobacteria were active in the AOM process, in which cytochrome C facilitated the production of more Fe and Mn ions and bicarbonate radicals. These substrates could enter the 3-hydroxypropionic acid cycle, in which more glyoxylic acid, glutamic acid, and other amino acids were formed to promote the synthesis of extracellular PET-degrading enzymes39. According to the module of ethylene terephthalate degradation, PET could enter the benzoate degradation pathway through the intermediate degradation products of methyl 3,4-dihydroxybenzoate and 4-carboxyl-2-hydroxyl muconic acid40, and this process could be promoted by extracellular degrading enzymes. Considering the role of oxygen radicals and monooxygenase released by microflora in the sediments, oligomeric polyethylene (PE) could be degraded into a dimer and monomer41.
To uncover the relationships between microorganisms and MP degradation, microorganism populations and associated functional genes were analyzed in depth. In the strong methane seepage area, the microorganism populations significantly differed among the four vertical uniformly spaced zones, with a linear discrimination analysis (LDA) value >4 (Fig. S6). As shown in Fig. 6b, in the upper 8 cm, groups of Syntrophobacterales, Anaerolineales, and Micrococcuss species were dominant, and Anaerolineales could degrade PET. At a depth from 8-16 cm, the dominant population order was JS1 > Acidobacteria > Alphaproteobacteria, in which JS1 could degrade alkanes and plastics42. At a depth from 16-24 cm, the dominant populations included Pseudomonadales, Betaproteobacteriales, and Coriobacteriia. Plastic-degrading Pseudomonadales were the most dominant species. The bottom layer was primarily colonized by Lactobacillales, which exhibited the potential to biodegrade plastics43,44, and Hydra vulgaris, which produced a cellulolytic enzyme45. In summary, microorganisms in the strong methane seepage area could degrade plastics. We further compared the functional gene distribution among the different sampling locations (Fig. 6c). Functional genes that could degrade the considered MP monomers were discovered at all five locations, such as the PA4-degrading gene (puuE|gabT|gabD), PE-metabolizing gene (|paaK|ACSL, fadD|abmG|), and PET-degrading gene (ligB|ligA|ligC|ligI|todF|ligK, galC|ligJ|). Other indirect genes promoting MP metabolism, as shown in Fig. 6a, consisted of methane metabolism, metal electron transport, and carbon fixation genes, and they were also widely discovered. Figure 6c shows that the gene abundance for PA degradation in the methane seepage zones was higher than that in the nonseepage zones. The early seepage area contained the most PE-degrading genes. From the perspective of a metal-driven AOM process, the methane seepage area contained abundant genes for metal electron transport and carbon fixation, which provided the potential to produce more indispensable serine, cysteine, and ornithine to promote the synthesis of extracellular MP-degrading enzymes.