Oysters under anthropogenic pressure: A cellular perspective on the interactive effects of microplastic pollution and climate change

Anthropogenic driven global change is affecting all areas of the world, including the marine realm, and several factors affect the organisms at a given time. Among these, habitat temperature is one of the key driving factors for marine ectotherms. In addition, environmental pollution caused by microplastics (MP; particles < 5mm), which are particularly bioavailable to the smallest filter feeders, has become a global threat. Despite its undeniable ecological relevance, however, most previous studies have neglected potential synergistic effects of MP exposure and climate warming. The present study therefore aimed at enhancing the understanding of the interactive effects of combined exposure to environmentally relevant MP concentrations and climate warming on the cellular stress response of two ecologically and economically important key species from the German North Sea coastal ecosystem: The Pacific oyster (Crassostrea gigas) and the European flat oyster (Ostrea edulis). In two laboratory-based exposure scenarios, this study particularly addressed whether (i) MP exposure induces cellular stress in intertidal C. gigas, (ii) MP exposure enhances the vulnerability of C. gigas to atmospheric heating during low tide, and (iii) combined MP exposure and projected warming by + 3 °C induce interactive effects on cellular stress response of subtidal O. edulis. To address the thesis objectives, polystyrene MP microspheres (a mix of 4, 7.5, and 10 µm in size) at environmentally relevant concentrations (0.025 µg L-1 and 25 µg L-1) served as model MP. In a first experiment, intertidal C. gigas were exposed to MP for 16 days under a simulated semidiurnal tidal cycle to investigate possible dose- and time-dependent MP effects at ambient temperature (16 °C). On day 16, oysters were exposed to a gradual atmospheric heating during the last low tide simulation (16–26 °C; 3 °C h-1), with returning seawater (16 °C), in order to analyse possible MP-induced susceptibility to atmospheric heatwaves. In a second full-factorial experimental approach, subtidal O. edulis were exposed to MP for 28 days, either at ambient 20 °C or at + 3 °C elevated temperature (i.e., 23 °C) to investigate possible interactions between chronic MP exposure and future warming. (i) In-depth analysis revealed dose-dependent MP-induced oxidative stress in the gills of C. gigas, which was evident both at the metabolite level and at the enzymatic level. The antioxidant capacities though appeared to counteract the oxidative stress sufficiently, as no significant oxidative damage to lipids and DNA was observed. (ii) The simulated heating independent of MP induced analogous alterations in C. gigas gill metabolites to those observed following exposure to 25 µg MP L-1 at ambient temperature. Increased intra-specific variability in biomarker responses of MP-exposed oysters after the heating further suggests that heating-induced oxidative stress in the gills of some individuals may be exacerbated by chronic MP exposure. (iii) In contrast to absent effects of both MP exposure and warming when considered individually, the combination of both factors resulted in increased oxidative stress in gills of O. edulis over time. The upregulated antioxidant capacity appeared to be insufficient to fully mitigate the stress, as evidenced by macromolecular oxidative damage to lipids. The present results demonstrate an increased vulnerability of O. edulis to climate warming in response to chronic MP exposure, which should be considered in site selection for successful oyster restoration efforts. In conclusion, in terms of progressive anthropogenic global change, the present study is among the first to provide valuable insights into the interactive potential of combined MP and habitat-specific warming in inducing cytotoxic effects in gills of intertidal C. gigas and subtidal O. edulis.