Effects of polyethylene, polylactic acid, and tire particles on the sediment microbiome and metabolome at high and low temperatures

ABSTRACT

Global warming has led to a high incidence of extreme heat events, and the frequent occurrence of extreme heat events has had extensive and far-reaching impacts on wetland ecosystems. The widespread distribution of plastics in the environment, including polyethylene (PE), polylactic acid (PLA), and tire particles (TPs), has caused various environmental problems. Here, high-throughput sequencing techniques and metabolomics were used for the first time to investigate the effects of three popular microplastic types: PE, PLA, and TP, on the sediment microbiome and the metabolome at both temperatures. The microplastics were incorporated into the sediment at a concentration of 3% by weight of the dry sediment (wt/wt), to reflect environmentally relevant conditions. Sediment enzymatic activity and physicochemical properties were co-regulated by both temperatures and microplastics producing significant differences compared to controls. PE and PLA particles inhibited bacterial diversity at low temperatures and promoted bacterial diversity at high temperatures, and TP particles promoted both at both temperatures. For bacterial richness, only PLA showed inhibition at low temperature; all other treatments showed promotion. PE, PLA, and TP microplastics changed the community structure of sediment bacteria, forming two clusters at low and high temperatures. Furthermore, PE, PLA, and TP changed the sediment metabolic profiles, producing differential metabolites such as lipids and molecules, organic heterocyclic compounds, and organic acids and their derivatives, especially TP had the most significant effect. These findings contribute to a more comprehensive understanding of the potential impact of microplastic contamination.

IMPORTANCE

In this study, we added 3% (wt/wt) microplastic particles, including polyethylene, polylactic acid, and tire particles, to natural sediments under simulated laboratory conditions. Subsequently, we simulated the sediment microbial and ecosystem responses under different temperature conditions by incubating them for 60 days at 15°C and 35°C, respectively. After synthesizing these results, our study strongly suggests that the presence of microplastics in sediment ecosystems and exposure under different temperature conditions may have profound effects on soil microbial communities, enzyme activities, and metabolite profiles. This is important for understanding the potential hazards of microplastic contamination on terrestrial ecosystems and for developing relevant environmental management strategies.

INTRODUCTION

Global warming is altering Earth’s climate, leading to more frequent extreme weather, including heat events that disrupt ecosystems (1, 2). Microorganisms, fundamental to ecosystem balance, are especially vulnerable to such changes (3). They maintain sediment health through nutrient cycling and organic matter breakdown but are affected by both temperature fluctuations and pollution.

One pervasive form of pollution is microplastics (MPs), which are now ubiquitous in terrestrial environments, including wetlands (4). Wetlands are natural filters that capture MPs, but these persistent materials, notably polyethylene (PE), polylactic acid (PLA), and tire particles (TPs), accumulate in the sediment (5), where they can linger and impact the biotic community. PE, the most common synthetic polymer found in single-use plastics, is frequently detected in aquatic sediments alongside polypropylene and polystyrene (6). Conversely, PLA, a biodegradable alternative sourced from renewable materials, is less prevalent due to its natural degradation. Notably, sediment samples from key water bodies like the Pearl River and Poyang Lake show high PE concentrations, highlighting its persistence in the environment (7, 8). Moreover, TPs, which originate from tire wear, demonstrate low degradation rates, leading to significant accumulation in various environmental settings as estimated in global sediment studies (9).

MPs provide substrates for microbes, altering the sediment microbiome (10). The changes they induce are not just biological; they also affect sediment chemistry, enzyme activity, and nutrient cycles (11, 12). Furthermore, MPs can release additives and other chemicals that compound their impact on bacterial communities and functions (13). While PE and TP are well-studied, the effects of PLA, a biodegradable alternative, are less understood. Recent findings suggest that degradable MPs like PLA might disrupt soil bacterial communities more than previously believed (14, 15).

Temperature modulates the bacterial degradation of MPs, with higher temperatures potentially accelerating MP breakdown (16). This interaction is crucial as global warming could enhance the bacterial activity affecting MPs. Conversely, extreme temperatures might hinder bacterial processes (17). Our prior research in wetland sediments revealed that bacterial diversity shifts with the seasons, linked to temperature changes (18). This demonstrates a clear gap in understanding how variable temperatures affect MPs’ impacts on the microbiome and metabolome in sediment environments.

This study aims to fill this gap by examining how MPs affect sediment ecosystems under high- and low-temperature thermal regimes. We focused on three types of MPs (PE, PLA, and TP) and assessed their effects on the sediment microbiome and metabolome at high and low temperatures. Our hypothesis is that MPs will affect the sediment microenvironment differently depending on temperature. Therefore, the objectives of this study were to investigate (i) the effects of MPs on sediment enzyme activity and chemistry at high and low temperatures, (ii) how bacterial communities and diversity respond to MPs, and (iii) the effects of MPs on metabolite species at high and low temperatures. By investigating the interactions between MPs and sediment microbiomes at high and low temperatures, this research will provide insights into the ecological implications of MPs under current and projected climate scenarios. Our results will contribute to a better understanding of the environmental behavior of MPs and inform mitigation strategies for their impact on sediment ecosystems.

MATERIALS AND METHODS

MP and sediment collection

PE and PLA MPs were purchased from Dongguan Jiecheng Plastic Chemical Co., Ltd., and the TP was purchased from Dujiangyan Huayi Rubber Co. Scanning electron microscopy (SEM) and Fourier transform infrared reflectance were used to characterize the morphology and surface functional groups of MPs before and after the incubation experiments, and the results were shown in Fig. 1.

Fig 1.

(a) SEM before and after 2 months of incubation at 15°C and 35°C. (b–d) FTIR spectra of PE, PLA, and TP MP surfaces before and after 2 months of incubation.

Sediment samples were collected from a wetland in Pingdingshan, Henan Province, China (113°12′ E and 33°46′ N). Sediment samples were collected from 0 to 20 centimeters at approximately 0.5 meters below the surface. A sum of 20 kg of sediment was collected, the sediment was thoroughly mixed and homogenized, and the fragments were removed by forceps. Then, they were naturally air-dried through a 2-mm sieve and stored at 5°C before the experiment.

Experimental design

Beaker experiments were conducted in a climatic incubator for a 60-day exposure period. Approximately 300 g of sediment was placed in a 500-mL beaker, then 9 g (3% wt/wt) of MPs (PE, PLA, and TP) was added, and control check (CK) was also set for control. To maintain a uniform distribution of MPs in the sediments, the MPs and sediments were initially mixed completely in a beaker, and then deionized water was added slowly along the beaker wall. Deionized water was added to the beaker and checked daily to maintain the water level at 1 cm above the sediment, and water additions were reduced before the end of the incubation to facilitate sampling. Incubation conditions were kept dark and incubated at low (15°C) and high (35°C) temperatures, respectively. The experimental MP content (3% wt/wt) was referenced from a previous study in Kamilo Beach (3.3% wt/wt) (19). The size (less than 1 mm) and concentration (3% wt/wt) of the MPs used in this research lie within the common range used in laboratories to assess their ecological effects. The increase in concentration to 3% wt/wt is intended to provide a clearer approach to assessing ecosystems and to help characterize possible effects and mechanisms. At the end of the incubation, sediment cores were randomly collected and homogenized at three locations within the microcosm for metabolite detection and bacterial community analysis, respectively. The remaining sediment samples were used for the isolation of MPs and the determination of environmental factors. After 60 days of incubation, the MPs were extracted by density separation method using 1.37 g/mL of ZnCl solution as flotation solution, which was referred to the previous study (20).

Sediment enzymatic activity and chemical properties

After incubation, the sediments were naturally air-dried and ground and then passed through a 2-mm sieve. Sediment chemistry properties including pH, total organic carbon (TOC), nitrate nitrogen (NO₃-N), nitrite nitrogen (NO₂-N), total phosphorus (total P), total Kjeldahl nitrogen (TKN), and ammonia nitrogen (NH₄-N) were determined by reference to previous methods (21). In addition, sediment enzyme activities were determined, including catalase, urease, dehydrogenase (DHA), and neutral phosphatase (NP). Methods for determining sediment chemistry and enzyme activity were shown in Text S1.

DNA extraction, high-throughput sequencing, and sequencing data analysis

Sediment DNA was extracted using the E.Z.N.A MagBind Soil DNA Kit (Omega, M5635-02, USA) following the manufacturer’s instructions. The region of bacterial diversity identified was the V3-V4 region of the 16S rRNA gene (primers: 338F 5′-ACTCCTACGGGAGGCAGCAG-3′; 806R 5′-GGACTACHVGGGTWTCTAAT-3′) (18). After PCR amplification and purification, sequencing was performed in Illumina MiSeq platform (22). High-throughput sequencing was conducted by Sangon Biotech (Shanghai) Co., Ltd. The sequences were clustered into different operational taxonomic units (OTUs) based on the identity between sequences with an identity cutoff of 97%. A detailed description of DNA extraction, high-throughput sequencing, and sequencing data analysis is shown in Text S2. The raw reads were deposited into the NCBI Sequence Read Archive database with the accession numbers: PRJNA941436.

Sediment metabolite analysis

Briefly, an extract solution (methanol: water = 3:1 (vol/vol) with isotopically labeled internal standard mixture) was first added to the sediment sample, and then the supernatant was taken for analysis using a Vanquish ultra-performance liquid chromatograph (UPLC) with a Waters ACQUITY UPLC HSS T3 (2.1 × 100 mm, 1.8 µm) liquid chromatographic column, which was used for the chromatographic separation of the target compounds. Data acquisition was performed using an Orbitrap Exploris 120 mass spectrometer, and then the raw data were annotated in a database (23). The results were analyzed using the statistical method of orthogonal partial least squares-discriminant analysis (OPLS-DA) (24). Differential metabolites (DEMs) were required to satisfy both the variable importance in the projection (VIP) of the first principal component of the OPLS-DA model >1 and the Student’s t-test (P value) <0.05 (13). The difference comparison strategies were PE15 vs CK15, PLA15 vs CK15, TP15 vs CK15, PE35 vs CK35, PLA35 vs CK35, and TP35 vs CK35. In addition, commercial databases including KEGG (http://www.genome.jp/kegg/) and MetaboAnalyst (http://www.metaboanalyst.ca/) were used for pathway enrichment analysis. Detailed steps for metabolite extraction and data analysis are shown in Text S3.

Data analysis

Environmental properties and enzymatic activities of sediments in each treatment group were analyzed using SPSS 25.0 software, and these analyses were performed using one-way ANOVA and post hoc Duncan tests. Pearson’s correlation analysis was performed by the SPSS version 25.0. Principal component analysis (PCA) was used to analyze the differences in sediment metabolism between each treatment.

RESULTS

MP characterization

As shown in Fig. 1, the surface morphology and functional groups of MPs (including PE, PLA, and TP) changed after 2 months of incubation. The surface of the incubated MPs was aged with more cracks and roughness. In addition, the results of FTIR spectroscopy showed the changes in the characteristic peaks of MPs due to aging. For example, the higher transmittance exhibited in PE MPs (2,910, 2,850, 1,470, and 720 cm⁻¹) can be explained by the breakage of the C-C double bond, leading to an increase in methyl groups. The FTIR spectrum at the end of the incubation showed ester bond stretching vibrational peaks (1,730 cm⁻¹) and hydroxyl group characteristic peaks (3,400 cm⁻¹) due to PLA degradation. The enhancement of the characteristic TP MP crystallization peaks (1,420 and 2,800 cm⁻¹) also showed changes in the structure of the surface crystalline polymers.

Effect of MPs and temperature on enzyme activity and sediment properties

Catalase activity was significantly higher (P < 0.05) at 35°C than at 15°C. PE showed inhibition of catalase, and TP showed promotion compared to control at 15°C and 35°C, especially the difference was (P < 0.05) significant at 15°C (Fig. 2a). The addition of TP inhibited the urease activity, particularly (P < 0.05) significantly at 15°C, PE was significantly increased (P < 0.05) at 35°C compared to the CK group, and nothing else was significant (Fig. 2b). Compared with the control, PE significantly increased (P < 0.05) urease activity at 35°C, whereas the other treatments were not significant. DHA activity was higher at 15°C than at 35°C, and different types of MPs did not stimulate DHA significantly (Fig. 2c). The three MP additions did not significantly affect NP activity at 15°C, whereas all significantly increased (P < 0.05) NP activity at 35°C (Fig. 2d). The results showed that catalase, urease, DHA, and NP were co-regulated by high and low temperatures and MPs and exhibited significant differences.

Fig 2.

Effect of three MPs on sediment enzyme activity (a–d) and properties (e–j). The analyses were applied by one-way ANOVA with post hoc Duncan’s test. Different letters represent significant differences between treatments.

Compared to the control, MPs increased the pH of the sediments at 15°C and 35°C, especially PLA and TP MPs significantly increased (P < 0.05) the pH (Fig. 2e). The addition of the three types of MPs significantly increased (P < 0.05) the TOC of the sediments (Fig. 2f). NO₃-N was significantly higher (P < 0.05) at 15°C than at 35°C. In addition, NO₂-N in the sediments was all below the detection limit (0.15 mg/kg) and therefore not as shown in Fig. 2. The addition of MPs decreased total P and NH₄-N concentrations and increased TKN concentrations in the sediments compared to the control.

Bacterial diversity and community structure

As shown in Fig. 3a, 1,728 (CK15), 1,687 (PE15), 1,594 (PLA15), 2,236 (TP15), 2,505 (CK35), 2,785 (PE35), 2,993 (PLA35), and 3,303 (TP35) OTUs were detected in the sediments. The results showed that temperatures significantly affected sediment bacterial α- and β-diversity. Principal coordinate analysis (PCoA) showed that high and low temperatures formed distinct clusters (Fig. 3b), whereas three MP additions did not (Fig. 3c). Similar results were shown for α-diversity (Fig. 3d), with no significant differences between the three MP treatment groups under the same temperature conditions, while the differences between the high- and low-temperature treatment groups were significantly different. The three MP additions did not significantly alter the Shannon index at 15°C, whereas PLA and TP MPs significantly increased (P < 0.05) the Shannon index at 35°C. TP MP addition significantly increased (P < 0.05) the chao1 index at both 15°C and 35°C, whereas PE and PLA MP addition had no significant effect compared to the control. Except for TP MPs significantly reduced (P < 0.05) Simpson index at 35°C, there was no significant effect on Simpson index under other MP treatments.

Fig 3.

Effect of MP addition on sediment bacterial community structure. (a) Effect of three MP additions on the number of sediment OTUs. (b and c) PCoA showing the differences in the composition of bacterial communities in sediments after three MP additions. (d) Effect of MP addition on the α-diversity index of bacterial communities. (e) and (f) Relative abundance of bacterial communities in the sediment at the phylum and genus level.

Proteobacteria (37%–47%), Bacteroidetes (27%–44%), Firmicutes (2%–8%), Acidobacteria (2%–8%), and Chloroflexi (0.4%–7%) were the five dominant phyla in all treatments (Fig. 3e). MPs and temperature affected sediment bacterial ecosystems, leading to changes in the relative abundance of bacterial communities. The relative abundance of Bacteroidetes was significantly higher (P < 0.05) in CK15 than in CK35, indicating that Bacteroidetes was susceptible to temperature, whereas there was no significant difference in the relative abundance of Proteobacteria, Firmicutes, Acidobacteria, and Chloroflexi between CK15 and CK35 (Table S1). Chitinophaga, Ralstonia, Phenylobacterium, and Herbaspirillum were the main dominant genera. Compared to CK35, TP35 significantly decreased (P < 0.05) the relative abundance of Bacteroidetes and significantly increased (P < 0.05) the relative abundance of Chloroflexi (Table S2). Furthermore, TP15 also significantly increased (P < 0.05) the relative abundance of Firmicutes compared to CK15. Significant differences in the relative abundance of Chitinophaga and Gp6 with temperature were observed in CK15 and CK35, but not in the other dominant genera. Chitinophaga, Ralstonia, Phenylobacterium, Herbaspirillum, and Mucilaginibacter all had significantly lower (P < 0.05) relative abundance in TP35 than in CK35, whereas Geobacter, Flavisolibacter, and Gp6 were significantly higher. Moreover, the relative abundance of Gp7 in TP15 was significantly lower (P < 0.05) than in CK15.

The treatment groups were compared to obtain biomarkers (Fig. S1). Compared to 35°C, few species differed at 15°C. At 35°C, Desulfocapsa, Amycolatopsis, and Syntrophomonas were biomarkers for TP35, Gp7 for PLA35, and Tissierella for PE35, and Phenylobacterium, Mucilaginibacter, and Phyllobacterium are biomarkers for CK35.

Effect of MP addition on the metabolic profile of sediments

A total of 388 metabolites were identified by liquid chromatography-mass spectrometry. Quality control was effectively based on PCA modeling analysis (Fig. 4a and b). PCA analysis showed that PE and PLA overlapped with CK, whereas the TP group was significantly separated from the other treatment groups (Fig. 4a). The metabolites did not form significant clusters about temperature, under incubation at both temperatures (Fig. 4B). The OPLS-DA results showed significant differences between the PLA and TP MP treatment groups and the control, whereas the PE treatment group was not significant, indicating that the three types of MP exposures affected the metabolites differently (Fig. 4c through h). In addition, Volcano plots showed 13 (10 upregulated and three downregulated), 22 (21 upregulated and one downregulated), 125 (116 upregulated and nine downregulated), 12 (10 upregulated and two downregulated), 17 (16 upregulated and one downregulated), and 145 (141 upregulated and four downregulated) DEMs (Fig. 4i through n). DEMs of the sediments consisted mainly of lipids and lipid-like molecules, organoheterocyclic compounds, and organic acids and their derivatives, and in particular, unique metabolites such as organic oxygen compounds, alkaloids and derivatives, and hydrocarbons were also produced in the TP-added treatment groups (Fig. S2).

Fig 4.

Effect of MP addition on sediment metabolites. Note: DEM selection thresholds: VIP >1 and P < 0.05. (i–n) Volcano plot each point represents a peak, and scatter size represents the VIP value of OPLS-DA model; the larger the scatter, the larger the VIP value. Significantly upregulated metabolites are shown in red, significantly downregulated metabolites are shown in blue, and non-significantly different metabolites are shown in gray.

Effect of MP addition on bacterial function

The prediction results of the 16S-based KEGG and FAPROTAX functions were shown in Fig. S5 and S6. The results showed in Table S3 KEGG function prediction indicated that these metabolic pathways were mainly involved in amino acid metabolism, carbohydrate metabolism, membrane transport, and replication and repair. However, no significant differences were observed among the treatment groups. The results are shown in Table S4, and FAPROTAX predictions showed significant differences in biogeochemical elemental cycling processes such as ureolysis, nitrogen fixation, chemoheterotrophy, respiration of sulfur compounds, sulfate respiration, and fermentation among the treatments. No significant differences were observed between CK15 and CK35 in functional genes, suggesting that the effect of temperature on functional genes was not significant. There was also no significant difference between PE and PLA MPs at 15°C and 35°C compared to the control. Compared to the control, at 15°C, TP MPs significantly increased (P < 0.05) chemoheterotrophy, respiration of sulfur compounds, sulfate respiration, and dark oxidation of sulfur compounds, whereas at 35°C, respiration of sulfur compounds, iron respiration, and aromatic compound degradation significantly increased.

Correlation analysis of environmental factors and sediment bacterial community structure

The relationship between sediment properties, enzyme activities, and bacterial community structure was shown in Fig. 5. The redundancy analysis (RDA) model showed that the total contribution of the first two axes at 15°C and 35°C was 95.74% and 92.85%, respectively. No physicochemical parameters had a significant effect at 15°C, whereas pH and NH₄-N had a significant effect on bacterial communities at 35°C. Pearson correlations between sediment environmental properties and dominant microorganisms are shown in Fig. S3 and S4; Chitinophaga, Mucilaginibacter, Gp7, Geobacter, and Gp6 were closely correlated (P < 0.05) with mineral-nutrient metabolism and enzyme activities at 15°C, whereas at 35°C, Chitinophaga, Ralstonia, Phenylobacterium, Herbaspirillum, Mucilaginibacter, and Bacillus were mainly involved in mineral nutrient metabolism. In addition, Mantel’s test showed that total P and urease of sediment significantly (P < 0.05) affected dominant phylum, bacterial diversity, and KEGG function at 15°C, whereas pH and NH₄-N significantly (P < 0.05) affected dominant phylum and bacterial diversity at 35°C. The results indicated that the three types of MPs and temperature regulated bacterial community structure by affecting physicochemical properties and enzyme activities.

Fig 5.

(a and b) RDA analysis revealed the effect of sediment properties and enzyme activities on the bacterial community at 15°C and 35°C, respectively. (c and d) Mantel test revealed the correlation of sediment properties and enzyme activity with the dominant phylum (top 10), α-diversity, and KEGG function at 15°C and 35°C, respectively.

DISCUSSION

Enzymatic activities in sediment are vital for ecosystem functionality and health (25). Catalase, urease, DHA, and NP play significant roles in organic matter decomposition, nutrient cycling, energy flow, and environmental pollution mitigation (26). Our study reveals that catalase activity is temperature-sensitive, exhibiting significant (P < 0.05) variation with changes in ambient temperature. Meanwhile, TP MPs significantly inhibited (P < 0.05) urease activity across temperatures, possibly due to the release of toxic substances like benzothiazoles and heavy metals (27). Conversely, PE and PLA MPs influenced urease activity differently at the two temperatures, suggesting that they interact distinctly with microbial metabolic processes under thermal stress. Regarding DHA activity, our findings align with previous research showing MP-induced inhibition, particularly at high PE concentrations, hinting at a possible antimicrobial effect of these particles (28). This study extends such observations by identifying that the impact on DHA activity varies not only with MP type but also with temperature. In this investigation, the two temperature conditions yielded different responses in soil enzymatic activities following MP addition. In a previous study, PE MP incorporation significantly increased soil acid phosphatase activity (29). At a moderate 15°C, NP activity remained unaffected by MPs. However, at a warmer 35°C, NP activity significantly increased with MP addition (P < 0.05). This is likely due to heightened temperatures fostering a more diverse and active bacterial community, which, in turn, augments NP synthesis and secretion. Additionally, the presence of more organic matter at the higher temperature acts as an energy and carbon reserve, further bolstering bacterial vitality and sustaining NP activity levels.

Additionally, MP interaction with sediment pH appears multifaceted (30). While some studies have noted an increase in pH with high PLA concentrations and a decrease with PE addition (31), our findings show a rise in sediment pH across all three MP types studied. This rise can be attributed to biological reactions such as microbial metabolism, leading to the production of alkaline substances (32). MPs also significantly increased TOC at both temperatures (P < 0.05), suggesting that MP degradation and subsequent release of organic carbon may enhance bacterial activity and, thus, sediment TOC content (33). The impact of MPs and temperature on nitrogen cycling is similarly complex (34). Ammonia nitrogen’s conversion to nitrate nitrogen, via nitrification, is more active at lower temperatures (35), while the release of Kjeldahl nitrogen, associated with organic matter decomposition, is facilitated at higher temperatures.

MP particles offer surfaces for microorganism colonization, potentially altering sediment bacterial community structure and diversity. The physicochemical properties of MPs, like surface charge and hydrophobicity, are crucial in determining microbial adhesion and biofilm formation. While some studies have noted a reduction in bacterial diversity and abundance with the introduction of PE and polyvinyl chloride (36), our study finds that PLA, in contrast to PE, can foster greater bacterial diversity under certain conditions (e.g., 35°C) (37). Moreover, the study showed that degradation products of different MPs (PE, PLA, and TP) had a significant effect on soil bacterial communities, with temperature being a critical factor. PE and PLA reduce bacterial diversity at lower temperatures, potentially due to harmful chemical release and acidic byproducts, respectively (38). However, at higher temperatures, diversity recovers, suggesting that warmer conditions may bolster bacterial resilience and degradation capabilities. TP uniquely boosts both diversity and richness across temperatures, possibly due to a selection for microbes that can tolerate or metabolize its complex additives (27). This indicates that MP impacts on soil bacteria are complex and contingent on both the composition of MPs and environmental conditions.

The temperature dependence of bacterial responses to MPs is a recurring theme, influencing bacterial competitiveness and adaptability. At higher temperatures, some bacteria exhibit enhanced growth and metabolic activity, possibly facilitating more effective degradation of MPs, which can promote diversity and abundance within bacterial communities. TP MPs, however, have been observed to increase bacterial diversity at both temperature extremes, suggesting that TP acts as an additional carbon source that promotes the development and massive growth of biofilms composed of fast-growing bacteria (39). We also explored the interaction between MPs and bacterial populations over time. Long-term exposure to MPs results in distinct shifts in bacterial community structure (40), with the persistence of MPs leading to significant changes. Interestingly, some microorganisms, like those belonging to Bacilli and Proteobacteria, have shown potential in degrading MPs and other pollutants, underscoring their ecological significance (41).

Finally, sediment metabolites serve as indicators of bacterial responses to external pollutants, reflecting bacterial activity, ecosystem health, and contaminant biodegradation processes. Past research identified the carbohydrate and amino acid content in inter-root soils as pivotal for the prevalence of certain bacterial species like Bacteriophage, Rhizobium, and Sphingomonas (13). Specifically, PE has been shown to significantly impact the soil metabolite composition by altering levels of organic compounds containing oxygen, nitrogen, lipids, and derivatives of organic acids (41). While temperature was not a major factor influencing metabolites in the current investigation, distinct MPs yielded unique metabolomic profiles, particularly TP MPs which interfered most significantly with sediment metabolites. The incorporation of PE, PLA, and TP MPs was observed to substantially modify sediment metabolite profiles. These changes included alterations in the presence of lipids and lipid-like molecules, organoheterocyclic compounds, organic acids and derivatives, phenylpropanoids, and polyketides. Moreover, these MPs disrupted specific metabolic pathways, such as those involved in amino acid and carbohydrate metabolism. These alterations lend credence to the hypothesis that certain microorganisms can leverage the organic carbon from MPs for their growth and metabolic functions (42).

In summary, MPs significantly affect sediment enzyme activities and chemical properties, with these effects modulated by environmental temperature. MPs can serve as a substrate for microorganism attachment, altering community distribution and diversity. The temperature-dependent nature of these responses highlights the complexity of interactions between MPs, bacterial communities, and ecosystem processes. Understanding these dynamics is crucial for assessing the ecological implications of MP contamination and developing strategies to mitigate their impact on sedimentary environments.

Conclusions

This study revealed the response of sediment enzymatic activities, physicochemical properties, bacterial community composition, and metabolic profiles to three types of MPs at high and low temperatures. Sediment enzymatic activity and physicochemical parameters were influenced by a combination of temperature and MP type. The three MPs showed changes in surface morphology and functional groups after 60 days of incubation. PE and PLA inhibited bacterial diversity at low temperatures, whereas they promoted bacterial diversity at high temperatures, and TP showed promotion at both temperatures. Temperature and MP concentration were important factors influencing the structure of bacterial communities, forming two clusters at high and low temperatures. In addition, metabolomic results showed that PE, PLA, and TP particles significantly altered the metabolite profiles of the sediments, including lipid and lipid-like molecules, organic heterocyclic compounds, and organic acids and their derivatives, and TP MP additions formed unique metabolite clusters.

DATA AVAILABILITY

Data will be made available on request.

SUPPLEMENTAL MATERIAL

The following material is available online at https://doi.org/10.1128/aem.02016-23.

Supplementary Texts S1 to S3, Tables S1 to S4, Fig. S1 to S6. aem.02016-23-s0001.docx.

Supplementary material.

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