Microplastics-induced energy flux in the digestive and antioxidant systems of the mussel, Mytilus galloprovincialis
Creators: name: TRESTRAIL, Charlene
Publisher: name: RMIT University
Year: 2024
Subjects: classificationCode: 410402
subject: Environmental assessment and monitoring

description: Microplastics are highly abundant in marine environments and are eaten by many marine invertebrate taxa. Ingested microplastics rarely cause death. Instead, they induce sub-lethal effects - including retarded somatic growth rates and diminished reproductive output - which reduce invertebrates' individual fitness and can affect population density. The mechanisms behind microplastics-induced sub-lethal effects are unknown; this thesis addresses this knowledge gap by identifying and testing potential mechanistic pathways.

This thesis applies the Dynamic Energy Budget model as a paradigm for interpreting sub-lethal invertebrate responses to ingested microplastics. Using the model's tenets, this thesis hypothesises that microplastics alter energy flux in the digestive and antioxidant systems, thereby diverting energy from growth and reproduction. This hypothesis was tested using the marine bivalve, Mytilus galloprovincialis, a common animal model for microplastics research. Energy flux in the digestive and antioxidant systems was assessed using system-specific enzymes and biomolecules as proxies for energetic investment.

The effects of microplastics on energy flux in the digestive system were assessed by exposing M. galloprovincialis to microplastics of different polymers (polyethylene and polystyrene), sizes (20 µm and 75 µm) and concentrations (1 x 104 and 5 x 104 microplastics L-1). Then, the activities of seven key digestive enzymes were quantified. This is one of the first attempts to understand how microplastics influence invertebrates' enzymatic digestion, and it is only the third study to investigate this phenomenon in a bivalve. Polymer type significantly affected the activities of carbohydrase enzymes: polystyrene reduced amylase and xylanase activities, and increased cellulase activity. High concentrations of microplastics increased total protease activity. The activities of laminarinase, lipases and lipolytic esterases were unaffected by any microplastics treatment. These changes to digestive enzymes can influence energy flux in the digestive system: decreased enzyme concentrations reduce mussels' ability to acquire energy from meals, whilst upregulated enzyme concentrations reflect an energetic investment that - in the absence of appropriate substrate - does not increase energy acquisition.  These microplastics-induced changes to digestive enzymes can reduce mussels' net energy acquisition from meals, potentially reducing the energy available for growth and reproduction.

The effects of microplastics on energy flux in the antioxidant system of invertebrates were systematically reviewed, and effects in M. galloprovincialis were experimentally determined by exposing mussels to low and high concentrations (1 x 103 and 1 x 106 microplastics L-1) of 75 µm polyethylene microplastics. Energy flux in the antioxidant system was assessed by measuring concentrations of antioxidant components and oxidative damage products. Both concentrations of microplastics significantly affected the activities of the antioxidant enzymes catalase and glutathione-S-transferase, and reduced the concentrations of the antioxidant biomolecule, glutathione. This suggests that both microplastics concentrations disrupted redox homeostasis in M. galloprovincialis, necessitating the depletion of antioxidant components to re-establish redox balance. The concentrations of protein carbonyls and malondialdehyde - products of protein and lipid oxidation, respectively - were unaltered by either treatment, indicating that redox homeostasis was reinstated before excessive biomolecule oxidation occurred. These microplastics-induced changes to antioxidant enzymes and biomolecules can alter energy flux in the antioxidant system because cells must return spent antioxidant components to baseline concentrations. Synthesising these components is energetically expensive and can therefore reduce the energy available for growth and reproduction.

To determine whether energy flux in the antioxidant system is influenced by microplastics' chemical leachate or physical presence, experiments were conducted with phenol-formaldehyde microplastics - a previously unstudied polymer. Phenol-formaldehyde microplastics leached phenols into the surrounding water, and standard toxicity tests demonstrated the toxicity of this leachate to aquatic invertebrates. M. galloprovincialis were exposed to microplastics' leachate, leachate-free microplastics, and leachate-laden microplastics. Energy flux in the antioxidant system was assessed by measuring antioxidant enzyme activities and malondialdehyde concentrations. Catalase activity progressively declined, and glutathione-S-transferase activity progressively increased, across treatments in the order: leachate &lt; leachate-free microplastics &lt; leachate-laden microplastics. Although these trends fell short of statistical significance, they indicate that microplastics' chemical and physical characteristics - separately and cumulatively - alter energy flux to the antioxidant system of M. galloprovincialis. Enzyme changes correlated with significant declines in malondialdehyde, suggesting that mussels spent energy to overproduce at least one, unmeasured antioxidant component, which caused lipid peroxidation to fall below the baseline levels of the control group.

Taken together, the results of this thesis present clear evidence that ingested microplastics alter energy flux in M. galloprovincialis. This work successfully used enzymes and biomolecules to indicate energy flux associated with the digestive and antioxidant systems, and it highlights the importance of identifying the characteristics of microplastics that generate biological responses. Future explorations of microplastics-induced energy flux should focus on combining biomarker measurements with mathematical application of the DEB model, so as to quantify fluxes linked with biochemical changes.