Results of the present study showed a decrease in reactive oxygen species (ROS) content in the brain and gills at treatment group F after the 14 d exposure to MPs. This may imply that once aquatic organisms are stressed by exposure to MPs, they activate their antioxidant defense mechanisms, allowing them to cope with the oxidative stress caused by MPs. Also, this may be due to the fact that aquatic organism at group F did not suffer oxidative damage by MPs exposure. The exposure of chlorella sp. to MPs did not induce any significant increase in the ROS generation when compared with the ROS generated by the control, which was in agreement with the present findings (Thiagarajan et al, 2019). MPs are able to generate ROS in Chlorella sp. by limiting light and nutrient transfer, according to (Prata et. al., 2018). During the exposure periods, ROS in the brain and gills at group C showed an initial increase followed by a decrease. This could be due to the effective catalyzing of the antioxidant enzyme responsible for catalyzing ROS to a lesser toxic substance H2O2 and then into H2O and O2. Furthermore, after 14 d, ROS in the gills and brain showed a continuous increase at exposure group D compared to the control. This may suggest that oxidative damage may have occurred due to oxidative stress, caused by the inability of ROS catalyzing enzyme to balance the production and elimination of ROS. The brain recorded the highest ROS production at treatment group D than in the gills, liver and intestine suggesting that MPs at concentration 750 µm after the 14 d exposure period was able to cause oxidative stress more in the brain than the other tissues. Throughout the exposure period, the production of ROS in the liver across all the concentrations increased as the days increased implying that the liver could have suffered oxidative stress. Apart from the control, ROS in the intestine saw an initial increase and later a reduction in all the exposure groups except group F after the entire exposure period, which may indicate that there was enough antioxidant enzyme activity to manage over production of ROS in the intestine. At group F in the intestine, an increase in ROS content was observed on 10 d and 14 d. There may be a possibility that in the intestine. As the days increased, chlorella adsorbed more MPs thereby activating the over production of ROS when being ingested by the fish. This finding is in conformity to the research conducted by (Zhang et al., 2017), stating that all forms of functionalized MPs physically adsorb on the surface of Chlorella sp. resulting in stress.
SOD convert superoxide anions (O2−) into H2O and O2 thereby protecting organisms from over-production of ROS induced by xenobiotics. In this study, significant reductions of SOD activity in the gills were recorded at exposure group B/F as the days increased. Based on this result, we can infer that MPs could have suppressed the catalytic capacity of SOD in both the algae and the fish. There may also be the possibility that, ROS production in the gills at group B/F were not enough to trigger higher SOD activities. In comparison to the controls, elevated SOD activity in the brain and gills of GIFT were observed at exposure group D, suggesting a sensitive enzyme response to MPs. Increased SOD activity may also imply the excess generation of ROS by MPs exposure as well as the stimulation of the antioxidant systems against oxidative stress. Notably in this study, SOD activities in the intestines increased at group F. Thus, it is likely that the rise in ROS related enzyme (SOD) was a compensatory mechanism to decrease the ROS level in the target cells. Remarkably, in this study, SOD at exposure group C in the intestine and group F in the liver showed a temporal variability of an upsurge followed by a decline. This may be considered an indication of an elevated antioxidant status, which functioned to maintain partial oxygen reduction. SOD activities were reduced once ROS surpassed the antioxidant defense system’s self-clearing threshold, as the impact of antioxidant activation diminished with prolong exposures. MPs-induced disturbances in antioxidant enzymes have been revealed in a number of aquatic species, including mussels (Mytilus spp.) exposed to MPs at a concentration of 32 mg L− 1 (Paul-Pont et al., 2016; Detree and Gallardo-Escarate, 2018), zebrafish (D. rerio) subjected to MPs of 5 mm at concentrations 20, 200, and 2000 ug L− 1 (Lu et al., 2016) and monogonont rotifer (B. koreanus) introduced to MPs (0.05, 0.5 and 6 mm) at a concentration of 10 mg L− 1 (Jeong et al., 2016). Currently, however, there are no or limited studies reporting on the effects 0f MPs on antioxidant related enzymes in GIFT which necessitates further investigations.
Inflammation is characteristic of the innate immune response. IL-1ß is considered a pro-inflammatory cytokine, which plays roles as a vital facilitator of the inflammatory responses and exert anti- inflammatory activities (Zheng et al., 2017, 2019a). In the present study, the activity of IL-1ß in the brain at exposure concentrations 75 nm, 750 µm and in the gills at exposure concentration 750 µm were up-regulated after the 14 d exposure period, which suggested a possible increase in inflammation associated with tumor invasiveness in the brain and gills. Group C in the brain exhibited an initial rise in IL-1ß levels followed by a decrease as the exposure time increased. This finding is consistent with the ‘hormesis effect’, which states that a short duration of exposure to environmental contaminants can promote a stimulatory response in organisms, while a longer exposure time elicits an inhibitory or toxic effect. This suggest that MPs at short time of exposure may cause fish to try adjusting to the associated adverse situation with an immune response, although the continuous presence of MPs may impede the hypothetic recovery of homeostasis, especially during a long exposure time. During the 14 d exposure time, IL-1ß activity was up regulated in the liver in response to MPs across all the exposure groups suggesting that long-term exposure to MPs in the liver may result in continuous inflammatory response by GIFT. Additionally, the relative expression of IL-1ß in the brain was significantly the highest at treatment group D at the end of the 14 d exposure time. This increase may possibly that MPs at group D could result in the overexpression of IL-1ß in GIFT brain, which may be a sign of tumor progression in the brain. MPs are known to prompt immune responses similar to parasites, such as immune cell recruitment, alterations in immune gene regulation and stress and immune cell activation.
TNF-α is a pro-inflammatory cytokine, which initiates inflammatory processes and intensifies additional inflammatory routes by stimulating other inflammatory molecules. The outcome of this study showed an up- regulation in TNFα level in the brain at concentrations of group B/D/E and in the gills at 750 µm. This data may postulate an increase in the risk of inflammatory conditions posed by the presence of MPs. Exposure at 75 nm after the 14 d period led to an intriguing result in the gills with an initial increase in TNF-α level followed by a decrease then an increase. TNF-α expression rises right after inflammation, causing an inflammatory response and then falls. A later elevation in TNF-α expression may be indicative of the onset of infection caused by MPs. Furthermore, the results of this study revealed an up- regulation of TNF-α in the liver at exposure groups B/C/D, and in the intestine at 75 nm after the 14 d period. This study postulated that, the persistent increase in the expression of TNF-α after MPs exposure might possibly be due to an increased inflammation in these tissues, which consequently may result to cell damage. TNF-α in the intestine at group D showed an initial increase then later a decrease suggesting that probably enough TNF-α was released to remove MPs hence, infection was reduced. Interestingly, there were differences in the responses to the same concentration of MPs among the different tissues. The highest TNF-α level was recorded in the brain at exposure concentration of 750 µm indicating that MPs could possibly increase inflammation more in the brain compared to the other tissues. Xenobiotics including MPs trigger immune responses in invertebrates, according to previous researches (Liu et al., 2019a). MPs at 5 µm and 70 nm when exposed to zebrafish had toxic effects and caused inflammation (Lu et al., 2016; Hamed et al., 2020). The fact that MPs exposure could interfere with GIFT’s immune response may have consequences for their ability to respond to parasites, as immune response resources may be depleted leading to increased susceptibility. More investigations should be done on the effects of MPs on immune related genes in GIFT to bridge the already existing gap.