4.1 Metabolomic response
Liquid chromatography-high-resolution mass spectrometry (LC-HRMS) allows for untargeted metabolomics, which may comprehensively examine and identify sample compounds. This study utilized an untargeted metabolomics approach to identify and quantify compounds in gill samples of S. woodiana. Several studies have shown that changing amounts of pollution at various station locations can modify the composition of metabolites in the gill tissue of mussels. Therefore, it is anticipated that there would be differences in the metabolite content between the unexposed mussels and treatment sites, including both polar and non-polar metabolite composition. The extraction in this investigation utilized 100% methanol of MS-grade quality as the solvent. Methanol is commonly recommended for metabolomics research because to its excellent efficiency in extracting polar and semi-polar molecules. In order to prevent any impact on the chromatogram for chemicals that elute early, a minimal injection volume of 3 µL was employed. To acquire clear and useful results from untargeted metabolomics, it is necessary to employ an efficient and robust statistical approach to handle the large volume of data. Chemometrics, a reliable statistical method, is widely used in metabolomics analysis, specifically for investigating data obtained from untargeted metabolomics research (Worley & Powers, 2013).
Metabolomics has been utilized as a developing tool in chemical toxicity research, particularly for evaluating the potential risks associated with low-level exposure to environmental toxins. Its high sensitivity allows it to deliver crucial information. Multiple research have been conducted on the toxicological consequences using metabolomics analysis, research on bivalves as bioindicators in active biomonitoring of stream ecosystems is still limited, and few data on bivalve metabolic mechanisms are available (Liu et al., 2022; Wei et al., 2022; Jiang et al., 2021). This study used LC-HRMS to obtain metabolite profiles of the transplanted S. woodiana gills at the unexposed mussels and exposure stations better to understand the toxic consequences of pollutans in the stream. Cappello et al. (2017) used gill tissues as selected as the target organ for metabolomic analysis on mussels since gills are the first organ to be directly exposed to its environment. This allows it to accumulate various metabolites and pollutants, making them effective indicators of environmental health. The gills are involved in critical metabolic processes, including respiration and filtration. This high metabolic activity can provide insights into the organism's overall health and metabolic state (Krishnamoorthy et al., 2019; Lobo et al., 2010).
Differences in the environmental conditions of the Winongo Stream in S1, S2, and S3 significantly impacted the metabolites and proteomes expressed by S. woodiana. As previously stated, the gill metabolite profiles exhibited site-specific and time-limited metabolic alterations, indicating the presence of toxicity in the stream (Cappello et al., 2016). Through pathway enrichment analysis of the divergent metabolites found in the mussels, exposure to pollutants at the exposure station has disrupted the metabolic regulatory networks of S. woodiana. The metabolic alterations were elucidated by the findings depicted on the heat map (Fig. 3), either directly or indirectly. There are variations in the intensity of metabolites between the unexposed mussels group and the exposure stations (S1, S2, S3), as well as changes in the duration of exposure (7 and 28 days after transplantation).
The PLS-DA score plot of the gills shows that the mussel groups from different places and periods consistently exhibited a distinct separation from the unexposed mussel group. Figure 3 displays metabolites whose degradation in red indicates an increase (near + 1.5), or blue indicates a decrease (near − 1.5) consistent with an increase or decrease in water quality; this can be viewed as a candidate biomarker. Several metabolites exhibit a similar fluctuation pattern, including 2-(diphenylmethoxy)-N-methylanamine, thymidine 5'-monophosphate, citric acid, anandamide, and ceramide (d18:1/20:0) (Fig. 3). The possible reason is that the different ability of S. woodiana groups to resist the toxicity of pollutants in the environment causes the mussel to show changes with increasing and decreasing metabolite intensity. Metabolite pathway impact (Fig. 4b) and changes in metabolite intensity can be comprehensively explained by constructing metabolic pathway modules (Fig. 7).
4.1.1 Nucleotide Metabolism
Nucleotide metabolism includes the processes involved in creating and breaking nucleotides. Nucleotides have crucial functions as precursors of DNA and RNA, as activated intermediates in various biosynthetic pathways, and as regulators of metabolism (Chandel et al., 2021; Gupta et al., 2021). This study detected many purine and pyrimidine nucleobase metabolites, including xanthine, hypoxanthine, guanine, and adenosine (Fig. 7). These metabolites exhibited a significant reduction in purine synthesis and breakdown compared to the unexposed mussels group. This implies that the amount of substances available for DNA and RNA production will be insufficient, potentially reducing the capacity of the gills to divide and repair themselves. As a result, this could impact tissue regeneration and repair (Dumas et al. 2020). Specifically, in the purine metabolic pathway, an increase in deoxyadenosine intensity was observed at stations S1 and S3, while in S2, it decreased. Deoxyadenosine is a nucleoside consisting of deoxyribose and adenine. An increase in intensity indicates that DNA metabolism or the process of deamination of adenosine into inosine is actively taking place regardless of environmental pollution conditions. This is because deoxyadenosine can occur from the salvage pathway process, where purines are recovered from nucleotide degradation products, so interference in the de novo pathway will not have a significant effect. This indicates that this metabolite is stable in the purine metabolic pathway in S. woodiana. The conditions that occurred in S2 can be explained by the research of Waller et al. (2023), who observed a decrease in deoxyadenosine in mussels Lampsilis cardium after being transplanted in the Mud River, which is a polluted area because it is located upstream of the city of Kokomo, Indiana which receives various sources of pollution. These changes may also affect other cellular operations, as nucleotides are crucial components in cellular energy production and the creation of nucleic acids.
The possible reduction of metabolites in the nucleotide metabolic pathway suggests a shift of resources from DNA/RNA production to other processes, such as protein synthesis and antioxidants (Yan et al., 2019). These processes are considered more important during times of environmental stress or adaptation. Prior research has established that adenosine is a crucial signaling molecule released under inflammatory circumstances (Fredholm, 2007). On the other hand, a decrease in pyrimidine metabolites, specifically dTMP and thymine, was observed (Fig. 7). dTMP is an important component in DNA and RNA synthesis, so a decrease in its intensity can also indicate damage to gill tissues that causes the process of DNA replication and RNA transcription to be disrupted (Chon et al., 2017). Decreased dTMP intensity can be indicated as a response of mussels to environmental stress, such as exposure to pollutants, changes in temperature, pH, or low oxygen availability (Erlania and Radiarta, 2011). These stresses can affect the ability of mussels to carry out normal metabolic processes. Several related studies have reported that the decrease in dTMP metabolite intensity results from disrupting mussel's metabolic processes induced by environmental pollutants (Walter and Herr, 2022; Krungkrai and Krungkrai, 2016). This disruption may cause S. woodiana to have limited sources of basic materials such as aspartate, glutamine, and carbon dioxide needed for dTMP synthesis through the de novo pyrimidine pathway. This makes the gill tissues more dependent on the salvage pathway that uses free bases already present in the tissue, thus not requiring basic materials such as aspartate and glutamine (Dumas et al., 2022; Hartati et al., 2020; Leija et al., 2016).
This study observed a significant decrease in thymine in all samples (Fig. 7), except in S2 and S3 on day 7 of exposure. Waller et al. (2023) and Stalin et al. (2011) found that physico-chemical environmental factors such as high temperature, unstable water pH, and low oxygen availability can affect mussel metabolism, where thymine intensity was observed. The results of this study show that S2 and S3, which have physical-environmental factors included in the heavily polluted category, show an increase in thymine metabolites. In addition, an indication of the increased intensity of these metabolites may occur due to disruption in the de novo pyrimidine pathway caused by environmental stress, especially in water quality and exposure to pollutants. A previous research report showed a similar mechanism, where the study sites categorized as heavily polluted also expressed increased thymidine and thymine metabolites indicated due to exposure to various contaminants such as pharmaceutical pollutants and heavy metals (O'Rourke et al., 2023).
4.1.2 Amino Acid Metabolism
The processes involved in the synthesis and degradation of proteins can be used to explain amino acid metabolism. An increase in protein synthesis may be indicated by an increase in amino acid proteins (Poortmants and Carpentier, 2016). Mussels rely on structural and enzymatic proteins to carry out biochemical processes and preserve the integrity of their gill tissues. The levels of some amino acid proteins, such as 5-aminolevulinic acid valine and threonine, were shown to have increased (Fig. 7). Enhancing the synthesis or utilization of amino acids related to lipid and protein pathways is greatly aided by this increase. However, this increase suggests that gills are adjusting to ensure the protein synthesis required for the repair and maintenance of cellular structure (Wang et al., 2021). In particular, the intensity of threonine and valine metabolites increased in S2 and S3 and decreased in S1, both on day 3 and 28 of exposure. Threonine synthesizes and produces proteins critical for maintaining cellular integrity and pollutant inhibitory functions (Tang et al., 2021). The study stations S2 and S3, classified as heavily and moderately polluted, can explain the increase in threonine intensity, presumably due to increased metabolic activity for detoxification and stress response. This condition was also reported by Kwon et al. (2012), where Mytilus edulis mussels transplanted in Onsan Bay, an area polluted with heavy metals, showed increased concentrations of threonine and valine, compared to mussels transplanted in the Dokdo area, which is categorized as clean or unpolluted. In addition, a research report by Ding et al. (2023), which evaluated the metabolomic response in Apostichopus japonicus, found an increase in threonine and valine concentrations in polluted environments indicated by increased protein synthesis and cellular repair processes. These mechanisms may occur in the metabolism of S. woodiana mussels to maintain homeostasis and adapt to changes in severe stress conditions. Additionally, the increased ornithine is involved in the urea cycle for ammonia detoxification to prevent the accumulation of toxic nitrogen compounds. Concurrently, there was also an increase in proline, which is important for synthesizing collagen and other structural proteins. This may suggest that the gills exhibit increased detoxification and tissue repair activity, which could indicate a disturbance caused by contaminants in the Winongo stream (Li et al., 2018; Milner, 2018).
4.1.3 Lipid Metabolism
Lipids are essential for cell signaling, energy storage, and membrane shape and function (Moreau & Bayer, 2023; Yoon et al., 2021). As inflammatory mediators, polyunsaturated fatty acid metabolism produces specialized pro-resolving lipid mediators (SPMs). According to Rasquel-Oliveira et al. (2023), SPMs are presently divided into four categories: lipoxins (LX), resolvins (Rv), maresins (MaR), and protectins (PD). Lipoxin A4 (LXB4) was present in each sample of gills (Table 1). This metabolite signals "stop" to body tissues, which serves as an immune resolver. There has been scant information about LXB4's protective mechanism in mussels' gills. However, the identification of aspirin involved in SPM synthesis, specifically the lipoxin family (LX; LXA4 and LXB4), and the discovery of leukotriene and prostaglandin metabolite groups as pro-inflammatory mediators for SPM formation suggest that SPM biosynthesis processes occur in S. woodiana gill tissues. LXB4, though, plays a significant role in the tissues of the mussels' gills. Since mussels are filter-feeding creatures that take food from the water through their gills, LXB4 might have a role in the defense mechanism of the animal by stimulating immune cells in the gills and helping to identify and remove pollution particles that enter the body through filtration. According to this study, exposure stations S2 and S3 had greater levels of LXB4 than the unexposed mussels. Station S2 showed the most vigorous intensity of LXB4 among the samples. Day 7 was the height of intensity, which S3 and S1 followed. LXB4 serves as an indicator of contamination in the water environment and indicates heightened oxidative stress in the gill tissues. Furthermore, it has been proven that LXB4 possesses antioxidant and anti-inflammatory characteristics. (Ye et al., 2019; Karra et al., 2015).
This study identified many metabolites linked to lipid metabolism (Fig. 4a), including LysoPC (17:0) and lysophosphatidylcholine (LPC). LysoPC (17:0) significantly increased in S2 and S3 from day 7 to 28, while its significantly decreased in S1 over the same period. According to previous reports, the increase in LysoPC (17:0) indicates high metabolic activity related to the mussel's effort to cope with pollutants. In contrast, the decrease in S1 may indicate an environment less supportive of metabolic activity under pollution conditions. According to Fokina et al. (2014) investigation, oil pollution and decreased salinity caused alterations in the lipid composition of mussels, resulting in elevated levels of cholesterol and triacylglycerols. Therefore, the contaminated environment in S2 and S3 is thought to cause high levels of inflammation and antioxidant metabolites.
The levels of glutathione (GSH) and carnitine significantly increased in the treatment groups, S2 and S3, which were exposed for 7 and 28 days, respectively. This signifies an augmentation in the transportation of fatty acids to mitochondria for oxidation, implying that lipids are utilized as a substantial energy supply. The antioxidative role of l-carnitine and GSH has been verified in mussel species (Danielli et al., 2017). Such antioxidative composition biologically adapts to oxidative stress, consistent with the hypothesis that oxidative stress in gill tissues increases in groups exposed to polluted streams, confirmed by the increase in DPPH and ABTS scavenging activities in the gills of S. woodiana.
4.2 Proteomic Respons
Untargeted proteomics analysis utilizes a bottom-up proteomics technique. The untargeted approach offers a thorough protein profile, facilitating the identification of a maximum number of proteins in a specific sample (Windarsih et al., 2022; Manes & Nita-Lazar, 2018). Protein digestion into peptides is optimized by performing a 21-hour digestion using proteomics-grade trypsin. Protein profiling was conducted using comprehensive or untargeted techniques to detect proteins isolated from mussel gill samples collected on the unexposed mussels and 28 (treated with S1, S2, S3). The presented figure shows a Total Ion Chromatogram (TIC) plot (Fig. 6) depicting the profile of peptides detected during the analysis, where each peak on the graph reflects a group of peptides eluted at a specific time and detected by the mass spectrometer. A comparison between the TIC profiles on unexposed mussels and day 28 for sample S1 shows a change in the distribution of peptides over time. In addition, comparing TIC profiles between different samples (S1, S2, and S3) after 28 days showed variation or consistency of proteomic responses under the same conditions. These intensity changes in TIC are related to variations in the number of peptides derived from the actin protein, the only protein detected. Although there were variations in peak intensities and areas under the TIC curves, these differences were not statistically significant among the samples tested, indicating that the proteomic response was relatively stable.
Several studies have found certain protein markers in the gills of mussels. These markers, such as Actin, Antimicrobials (AMPs), Heat Shock Proteins (HSPs), Metallothioneins (MTs), Cytochrome P450 Enzymes, Ferritin, Glutathione S-Transferase (GST), Transferrin, and Superoxide Dismutase (SOD), are used to investigate different aspects of mussel physiology and their immune responses to the environment and pathogens (Giarratano et al., 2014; Campos et al., 2013). This study revealed that the mussel protein β-actin was the sole protein detected and was present in all samples (Table 2). The β-actin protein is a multifunctional globular protein that plays crucial functions in diverse physiological activities, such as cell migration, membrane transport, and cytoskeleton construction. Research has revealed multiple proteins involved in mollusks' physiology and immune responses. One such protein is β-actin, which plays a vital role in the functioning of the immune system (Yanuhar and Khumaidi, 2017). Although specific peptides, such as β-Actin, remain stable, differences in environmental conditions between stations and sampling days may affect the expression of other proteins. This suggests that different environmental conditions may trigger changes in the proteome response, but some proteins, such as β-Actin, remain consistent. The β--actin peptides detected in S1 show a response to relatively stable environmental conditions (Fig. 6). These peptides can stay stable and consistent because there is no significant change in environmental conditions, consisting of only five peptides with 23% coverage compared to the unexposed mussels with three peptides with 17% coverage. In samples S2 and S3, β-actin peptides show the same response, each having eight peptides with 34% coverage (Table 2). The study of Bultelle et al. (2021) showed a significant increase in the abundance and coverage of β-actin protein, indicating that temperature stress increases protein expression to activate ciliary function, filtration function, and respiratory process in mussel gills. This suggests that different environmental conditions in the Winongo Stream may have an impact on actin proteins that are often found to be upregulated in stressed bivalves (David et al., 2005)
The most frequently detected β-actin protein peptides at all sites were SYELPDGQVITIGNER, TTGIVLDSGDGVTHTVPIYEGYALPHAIMR, and VAPEEHPVLLTEAPLNPK (Fig. 5). The specific peptide markers, DLYANTVLSGGSTMFPGIADR, were found in the day 28 samples S2 and S3. The peptide had a m/z value of 1093.03491 in S2 and 1093.03833 (single fill) in sample S3. It was detected during retention times of 1.44 and 4.57 minutes. Peptide sequences were found to have variations, indicating a possible link to certain post-translational modifications or protein production changes triggered by particular environmental conditions. The existence of a variety of peptides in β-actin is hypothesized that changes in environmental conditions leading to heightened levels of pollution can significantly influence protein expression in organisms. This, in turn, can facilitate the emergence of biomarkers, which serve as critical indicators of biological responses to environmental stressors. A recommended method for completing a thorough protein analysis and finding specific peptide markers in a sample is to use an untargeted proteomics approach with LC-HRMS. This approach provides a strong, dependable, and consistent means of studying the metabolic response of organisms to environmental conditions.
4.2 Free Radical Scavenging Activity
The latest investigation has focused on the antioxidant activity of several aquatic organisms (Wang et al., 2019; Miller et al., 2015). Antioxidants can protect the body of an organism against the harmful impact of free radicals and reactive oxygen species. It also prevents the development of numerous chronic illnesses and lipid oxidation. Several investigations have proven that mussels demonstrate significant antioxidant capabilities. (Mamelona et al., 2010; Esmat et al., 2013; Pachaiyappan et al., 2014). Radical scavenging activity assays such as DPPH and ABTS are widely used to evaluate the antioxidant capacity of organic extracts of aquatic organisms.
This study employed three tissues, including the gills, mantles, and digestive glands of S. woodiana, for antioxidant analysis (Tables 3 and 4). This study observed mussel gills as a tissue that absorbs particles from water; the DPPH scavenging activity decreased from day 3 to 28 in S1 but increased in S2 and S3. The DPPH scavenging activity in the mantle decreased in S1 and S3 and increased in S2 day 28. At all stations, the digestive gland tissue showed increased DPPH scavenging activity. Exposure to heavy metals Cu, Fe, Cd, Pb, and other chemical pollutants from industrial and domestic waste can cause an increase in DPPH scavenging activity (Abdel-Mohsen et al., 2024). The increase in all mussel tissues from stations S2 and S3 shows that pollutants are putting stress on the stream, which can throw off the balance of the ecosystem (Garg et al., 2015).
ABTS scavenging activity in mussels showed fluctuations in the ability of mussels to capture free radicals, which depended on the location and time of exposure. It reflected the antioxidant resistance of mussels to oxidative stress. The highest activity in the gills and mantle reflected a similar pattern to DPPH scavenging activity. Meanwhile, the digestive gland showed high activity, increasing at sites S1 and S2 and decreasing at station S3. In general, mussels filter pollutants through their gills, distributed throughout the mussel's organs, especially in the digestive glands. This process leads to the accumulation of pollutants in the digestive glands. As a result, free radical scavenging activities for both ABTS and DPPH scavenging activities were highest in the digestive glands, as observed in this study (Tables 3 and 4). This indicates that the digestive glands are the place of highest accumulation and detoxification of radicals (Faggio et al., 2018). According to other studies, the activity of DPPH free radicals leads to less harmful or harmless and can also stop radical chain reactions (Wang et al., 2019; Wang et al., 2013). The increase in DPPH and ABTS activities indicates that contaminants have accumulated in the mussel's body. In addition, the increase also shows exposure to higher concentrations of pollutants at stations 2 and 3, confirming that S2 and S3 are more polluted than S1.
This study identified ethyl eicosapentaenoic acid and 4-methoxycinnamic acid, increasing at all stations (Table 1). Ethyl eicosapentaenoic acid (EPA), an omega-3 fatty acid, is known to have antioxidant properties that can help neutralize free radicals. EPA can enhance the absorption ability of DPPH and ABTS radicals, thereby enhancing cellular protection against oxidative damage (Rajasekaran et al., 2024). 4-Methoxycinnamic acid, as a derivative of cinnamic acid, is a natural phenolic acid with multiple effects, such as neuroprotection and cancer inhibition (Wang et al., 2023). This metabolite can increase radical scavenging activity by increasing DPPH and ABTS, contributing to mussel tissues' total antioxidant capacity. This is evidenced by increased DPPH and ABTS scavenging activities in all the mussel tissues. This also proves that exposure to pollutants affects the metabolomic and proteomic responses of S. woodiana.