Effects of exposure to mixtures of selected toxicants and microplastics in microcrustaceans
Creators: name: Dong Thi, Dung
Publisher: name: RMIT University
Year: 2024
Subjects: classificationCode: 4104
subject: Environmental management
classificationCode: 410404
subject: Environmental management

description: Contaminants, particularly microplastics (MPs) and trace metals, are increasingly found in ever-increasing amounts in aquatic environments. While the coexistence of MPs and trace metals in the aquatic environment is a growing concern, few studies investigate how binary mixtures of MPs and other contaminants, including trace metals, affect aquatic organisms. The mechanisms behind the interaction of microplastics- and trace metal-induced sublethal effects in organisms are unknown. This thesis addressed this knowledge gap by identifying and testing potential mechanistic pathways with exposure of microcrustaceans to these contaminants as single compounds and binary mixtures.
 
The effects of microplastics and Copper (Cu) were assessed by exposing two Australian native crustacean species, marine and freshwater respectively (Allorchestes compressa and Daphnia carinata) and one marine species Artemia franciscana in the laboratory to either clear sphere polystyrene (diameter 1 – 10 µm) and/or inorganic Cu. The effects of the contaminants and their mixtures on the organisms were evaluated by measuring the activities of three biomarkers (GST, MTs, and LPO), and the quantification of physical endpoints.
 
 The effect of Cu on test organisms followed a dose-dependent pattern, and the test species could successfully regulate Cu out of their body if the Cu concentrations were low. However, the crustaceans also demonstrated different ways to eliminate or deal with excessive Cu by altering biomarkers (GST, MTs, and LPO), reducing reproductive success, and increasing moulting frequency. Indeed, Cu affected all indices measured at the physiological level (biomarkers), individual level (male, female, different life stages) and population level. Specifically, when two amphipod A. compressa populations were exposed to Cu treatments of 0, 50, 100 and 250 µg/L for ten days, followed by a 10-day recovery period, the amphipod population resident in the polluted area demonstrated a higher tolerance than the population from the “cleaner” field site. Additionally, females were generally more sensitive to Cu regardless of the population.
 
Exposure to MPs did not cause lethality in the freshwater D. carinata and marine A. franciscana even at extremely high concentrations of 200 mg/L and 300 mg/L in the acute tests or a low concentration of 0.25 mg/L in the chronic tests. However, the presence of MPs in the test solutions might prolong the test organisms’ survival in the acute tests. The effect of MPs on aquatic organisms should be evaluated in long term multigenerational or sublethal tests as the concentrations used in standard ecotoxicology tests are extremely high. This study indicated that ecotoxicological studies based on acute tests might misinterpret data as the effects may occur with exposure at low concentrations for an extended period.
 
The antagonistic and synergistic interactions between MPs and Cu in binary combinations indicated possible severe effects of low MPs concentration (0.25 mg/L and 1 mg/L) and low Cu concentrations only observed in chronic tests. The antagonistic interaction between these two contaminants was indicated by an increase in the number of moults in the freshwater D. carinata. MPs and Cu could interact either directly or indirectly. In the direct interaction, MPs adsorb a certain amount of trace metals on their surface and thus act as a vector of metallic contaminants. In indirect interaction, the presence of MPs can cause a reduction in the bioavailability of trace metals as MPs to affect trace metal speciation through adsorption or by changing their physical and chemical properties. Interestingly, both antagonistic and synergistic interactions were detected in the present studies. To the best of our knowledge, this is the first study that showed both types of interactions in one study with elevated trace metal concentrations with the same concentration of MPs.
  
Results of this thesis show that the interaction of MPs and Cu can vary amongst test organisms. This work highlights the importance of identifying the importance of considering gender and historical exposure to contaminants and long-term exposure to mixtures of contaminants with different physicochemical properties like the binary mixture of MPs and Cu; in order to ensure effective ecological risk assessments.