Deissena polymorpha, (Pallas, 1771) (Pallas, 1771)

Results

Morphometric data of the mussels used in the study were as follows: weight = 1.092 ± 0.27 g, length = 20.276 ± 2.09 mm, width = 10.13 ± 0.94 mm and height = 9.741 ± 1.07 mm).

Changes in SOD activity were determined in D. polymorpha exposed to different Cd concentrations ( Fig. 1 View Fig ). Statistically significant differences were detected in SOD activity at the 24th and 96th hours compared to the control (p <0.05). Due to the increase in concentration, the maximum decrease was found to be at 96 hours and 20 µg/L Cd concentration. Concerning the CAT activity ( Fig. 2 View Fig ), there was no statistically significant difference of the activities between the control at the 24th and 96th hours experiments (p>0.05) as well as between the increasing concentrations. Changes in GPx activity ( Fig. 3 View Fig ) showed a statistically significant increase (p<0.05) after 24 h. However, there was no statistically significant change (p>0.05) in the increasing concentration groups compared to the control during the 96-hour exposure period. Changes in TBARS levels as a marker for lipid peroxidation (LPO) were determined in D. polymorpha exposed to different Cd concentrations ( Fig. 4 View Fig ). It was determined that TBARS levels increased statistically significantly at both the 24th and 96th hours compared to the control (p<0.05). It increased due to the increase in concentration. The highest increase was detected at 40 µg/L, i.e. in the most concentrated group. Changes in GSH levels in D. polymorpha exposed to different Cd concentrations ( Fig. 5 View Fig ) demonstrated that GSH levels increased statistically significantly at the 24th and 96th hours compared to the control (p<0.05). The highest increase was detected at 40 µg/L Cd (the most concentrated group).

Discussion

Since mussels are filter-feeders, they can accumulate high levels of xenobiotic substances in their tissues. For this reason, they are a group of organisms used as biomarkers in monitoring studies in the field of ecotoxicology. These effects can be observed in the form of physiological, behavioural, cellular, biochemical and molecular changes ( Faggio et al. 2018). Aquatic toxicity studies in this context are conducted to ascertain the concentration at which a contaminant causes harm to aquatic organisms ( Karataş 2005).

Pollution in aquatic environments can lead to harmful effects, such as lipid peroxidation by increasing the generation of reactive oxygen species (ROS) due to an imbalance between ROS concentration and the antioxidant defence system ( Regoli et al. 2004). The activity of important antioxidant enzymes and the levels of non-enzymatic antioxidants are affected by several individual pollutants that are known to raise levels of ROS ( Valko et al. 2006, Ryter et al. 2007, Serdar et al. 2021). Oxidative stress is defined as the imbalance in the antioxidant defence mechanism of the aerobic organism due to excessive ROS production caused by xenobiotics. Antioxidant and biotransformation enzymes, including SOD, CAT, GPx and Glutathione Stransferase (GST), protect organisms against ROS. Various environmental pollutants can exert toxicity through the induction of oxidative stress.

Lipid peroxidation is a reliable measure of oxidative harm to cellular components and is the initial stage in the degradation of cellular membranes. It is triggered by pesticides, metals and other substances ( Gamble et al. 1995, Regoli et al. 1998). Duman & Kar (2015) reported that the accumulation of Cd in organisms led to an increase in TBARS content, which was dependent on the concentration and period of exposure. Serdar et al. (2019) reported that there was an increase in TBARS levels in the model organism Gammarus pulex exposed to Cd with increasing concentration and exposure time. Chandran et al. (2005) examined the methods for altering the TBARS levels of Cd and Zn in Achatina fulica ; according to their data, the levels of TBARS rose as the concentration of Cd increased.

Batmaz (2019) investigated the biochemical effect of the zinc pyrithione (ZnPT) on the model organism Dreissena polymorpha . He reported that TBARS levels as a marker for lipid peroxidation in the gills and digestive gland were higher in the experimental group than in the control group. It was concluded that ZnPT taken from the water environment caused lipid peroxidation in gill cells. Since the digestive gland is responsible for the detoxification of xenobiotic substances taken into the body, they concluded that lipid peroxidation occurs in the digestive gland cells when the cells are exposed to this substance.

In the present study, we found that the TBARS level in Cd concentration data in D. polymorpha increased depending on time and concentration. Similar studies have shown that metals can overproduce ROS through depletion of excess sulfhydryl groups, LPO and DNA damage ( Stohs & Bagchi 1995). However, antioxidant enzymes (SOD and GPx) can reduce ROS and, therefore, ultimately reduce oxidative damage, which is consistent with this study. SOD and GPx activities in organisms are significantly stimulated by pollutant exposure ( Wang & Wang 2010). In this study, Cd exposure may force D. polymorpha to suffer from oxidative stress through ROS excess and the organism may initiate antioxidant systems to counteract this stress in turn. This suggests that lipid peroxidation damage occurs in membrane lipids due to the oxidative stress.

Batmaz (2019) determined that the glutathione (GSH) levels in the digestive gland and gill tissues of the freshwater mussel Unio mancus eucirrus , which was exposed to sublethal zinc pyrithione for 96 hours, increased significantly (p<0.05) compared to the control. He stated that the increase in reduced glutathione levels, which is the main antioxidant and cell defence mechanism, showed the negative effect of ZnPT on cellular antioxidant defence mechanisms. During metal exposure, GSH is in an inhibi-

tory state. Moreover, its antioxidant and detoxifier response is faster than enzymes such as SOD, GPx and GST ( Wang & Wang 2010). GSH can undertake the initial defence against metal attack by direct complexation with metal or by the participation of GPx or GST in the detoxification process ( Sies 1999). Similarly, some authors attribute the toxic effects of Ni to a decrease in cellular GSH and a concomitant increase in GSSG, altering the redox state of cells. Thus, GSH/GSSG may be a suitable biomarker for oxidative stress or injury in biological organisms ( Hwang et al. 1992). Moreover, the strong positive correlation of GSH level with the GSH/ GSSG ratio suggests that the remarkable depletion of GSH may reflect the oxidative state caused by pollutant attack associated with interfering with the cellular redox state ( Wang & Wang 2010).

SOD is a group of metalloenzymes that play a key role in protecting aerobic organisms from the harmful effects of superoxide radicals. SOD catalyses the conversion of superoxide radicals into hydrogen peroxide (H 2 O 2) and oxygen (O 2), which is essential for the antioxidant system. This process is crucial for maintaining cellular health and preventing oxidative damage ( Kappus 1985, Kohen & Nyska 2002). The study by Cheung et al. (2001) confirmed that the presence of xenobiotics leads to an increase in antioxidant activity. Chandran et al. (2005) examined the enzyme activities of Achatina fulica by exposing it to Cd and Zn; they found that there were decreases in the SOD activities of organisms exposed to Cd compared to the control. Duman & Kar (2015) reported that there were significant changes in SOD activity in the Gammarus pulex exposed to Cd. Pandey et al. (2008) reported that SOD activity decreased in Channa punctata exposed to multiple trace metals applied in increasing concentration groups and compared to the control group. Many studies reported decreases in SOD activity in aquatic organisms exposed to various pollutants ( Tutuş 2016, Tunca 2017, Tatar et al. 2018, Serdar et al. 2018). In this study, it was observed that SOD activity decreased in D. polymorpha after exposure to Cd and this depended on the concentration increase and the exposure time.

The CAT enzyme is ubiquitous in aerobic organisms. It facilitates the breakdown of hydrogen peroxide, leading to the production of water and oxygen ( Chelikani et al. 2004). The upregulation of these antioxidant enzymes is essential for reducing cellular damage ( Rajeshkumar et al. 2013). Conversely, the activity of CAT may either rise or fall in polluted surroundings, depending on the specific material present ( Sobjak et al. 2017). Prior research has indicated that reactive oxygen species (ROS) can impede catalase (CAT) activity (Kono & Fridovich 1982, Escobar et al. 1996, Duman & Kar 2015). The investigation revealed that the activity of CAT rose as the concentration increased. However, it was determined that this rise was not statistically significant (p>0.05). The antioxidative stress activity can differ based on factors such as gender, physiological stage and species ( Felten et al. 2008, Zhang et al. 2011). Nevertheless, it was discovered that the concentration and duration of Cd exposure also modify the activity of antioxidative stress ( Duman & Kar 2015). Additionally, it was shown that brief exposure to organic chemical contaminants results in the activation of antioxidant enzymes in aquatic creatures. Nevertheless, the activity of CAT was adversely impacted by compounds that stimulate redox cycling ( Pandey et al. 2008, Rajeshkumar et al. 2013). Serdar et al. (2019) reported that statistically significant increases were detected in CAT activity in the model organism Gammarus pulex exposed to Cd compared to the control with increasing concentration and exposure time. The study found that the activity of the CAT enzyme was hindered in organisms that were subjected to stress from exposure to Cd. The literature on the activity of this enzyme reveals that potential antioxidant variations can be explained by factors such as species and environments ( Glusczak et al. 2007).

GPx is a constituent of an intricate system that defends against harmful oxidizing agents. Its reaction is probably linked to the reactions of other enzymes and chemicals that scavenge for these harmful agents. However, its activation may serve as a sign of protection against oxidative stress ( Tsangaris et al. 2007). The decrease in GPx activity could indicate the ineffectiveness of the antioxidant system when exposed to pollution ( Ballesteros et al. 2009) or could be attributed to the direct impact of superoxide radicals or pollutants on the production of the enzyme ( Bainy et al. 1993). The study found that the level of Cd exposure in D. polymorpha rose as the exposure period and concentration of Cd increased compared to the control group. The observed variations in GPx activity in this study align with the findings by Kutlu & Susuz (2004). Serdar et al. (2019) reported that there was a decrease of GPx activity in Gammarus pulex exposed to Cd (compared to the control with increasing concentration and exposure time). Zhang et al. (2011) reported that CAT activity increased with Cd exposure and this increase suppressed the increase in GPx. Wang & Wang (2010) determined the response of GPx activity in the copepod Tigriopus japonicus exposed to Ni concentrations; they reported that a significant induction effect occurred with increasing Ni concentration exposure (p<0.05). In this study, statistically significant differences were found in the changes in GPx activity of Cd on D. polymorpha compared to the control. The rise in GPx activity can be attributed to the alteration in CAT activity. In this respect, the study is similar to the other mentioned studies.

Organisms show behavioural and physiological responses to pollution in the ecosystem. The situation is more critical for the aquatic environment and aquatic organisms, which are the final stop of all ecosystem pollution. All kinds of physicochemical changes in water affect the vital activities of aquatic organisms such as reproduction, nutrition, shelter and migration. Researching, determining, and eliminating the effects of polluting factors on aquatic organisms is important for a clean environment and the well-being of aquatic creatures.

In aquatic ecosystem, bivalve molluscs (mussels), which are sediment-dependent sessile organisms, are the most affected animals by the pollution because they are filter-feeders. Metal pollution in water is caused by agricultural and industrial wastes and leaks from old mines. Rainwater also causes metals to leach from the surrounding soil. Metals with the most common pollution effects in studies on aquatic organisms physiology are Cu, Zn, Sn, Cd, Hh, Cr, Pb, Ni, As and Al. While the order of heavy metals in terms of toxic effects in salmon is Hg ≥ Cd> Cu, the order in terms of accumulation in the body is Hg >> Pb> Cr and Cd ( Atamanalp & Yanık 2003).

According to the present findings, Cd affects the oxidative status of D. polymorpha . It was concluded that SOD, CAT and GPx were useful markers in investigating the toxic effects of Cd on the water filterfeeding test organism D. polymorpha . The results obtained showed that the response of the test organism to the toxic substance varies with the concentration of the toxic substance and the duration of application.