Microplastics (nanoplastics) pollution has been a major ecological issue threatening global aquatic ecosystems. However, knowledge of the adverse effects of nanoplastics and the effects on freshwater ecosystems is still limited. To understand the impacts of nanoplastics on freshwater ecosystems, it is essential to reveal the physiological changes caused by nanoplastics in freshwater organisms, especially at their early life-history stages. In the present study, the larval channel catfish Ietalurus punetaus were exposed to gradient concentrations (0, 5, 10, 25 and 50 mg/L) of 75-nm polystyrene nanoplastics (PS-NPs) for 24 h or 48 h, and changes in contents of energy metabolites, metabolic enzyme activities and transcriptome were assessed. The results showed glucose and triglyceride contents increased after 24 h of exposure to 10 or 25 mg/L of PS-NPs but decreased with increased concentrations or prolonged exposure duration. Activities of most metabolic enzymes analyzed decreased in the larvae after 48 h of exposure, especially in 25 or 50 mg/L of PS-NPs. These suggested that PS-NPs caused huge energy consumption and disturbed the energy metabolism in larval fish. Transcriptomic analysis showed that 48 h of exposure to 50 mg/L PS-NPs affected the expression of genes involved in protein digestion and induced response of proteasomes or heat shock proteins in the larval I. punetaus. The genes involved in peroxisome proliferator-activated receptors (PPAR) pathway and biosynthesis of amino acids were activated after the exposure. PS-NPs also depressed the expression of the genes involved in gonad development or muscle contraction in the larval I. punetaus. Overall, acute exposure to 75-nm PS-NPs disrupted the energy metabolism by consuming  the energy reserves, and affected a series of molecular pathways which may further affect the development and survival of fish. This study provided the information about adverse effects of nanoplastics on the fish larvae and revealed the molecular pathways for the potential adverse outcomes.

Thermal plasticity on different timescales, including acclimation/acclimatization and heat hardening response – a rapid adjustment for thermal tolerance after a nonlethal thermal stress, can interact on organisms to improve the resilience to thermal stress. However, little is known about the physiological mechanisms mediating this interaction. To investigate underpinnings of heat hardening responses after acclimatization in warm season, we measured thermal tolerance plasticity, compared transcriptomic and metabolomic changes after heat hardening at 33 or 37oC followed by recovery of 3 h or 24 h in an intertidal bivalve Sinonovacula constricta. The clams showed explicit heat hardening responses after acclimatization in warm season. The higher inducing temperature (37oC) caused a less effective heat hardening effect than the inducing temperature that was closer to seasonal maximum temperature (33oC). Metabolomic analysis highlighted the elevated contents of membrane glyceropholipids in all heat hardened clams, which may help to maintain structure and function of membrane. Heat shock proteins (HSPs) tended to be up-regulated after heat hardening at 37oC but not at 33oC, indicating that there was no complete dependency of heat hardening effects on up-regulated HSPs. Enhanced energy metabolism and decreased energy reserves were observed after heat hardening at 37oC, suggesting more energy costs during exposure to higher inducing temperature which may restrict heat hardening effects. These results highlighted the mediating role of membrane lipid metabolism, heat shock responses and energy costs in the interaction of heat hardening response and seasonal acclimatization, and benefit the mechanistic understanding of evolutionary change and thermal plasticity during global climate change.

Summary 1. Animal survival and species distribution in the face of global warming and increasing occurrences of heatwave largely depend on how heat tolerance shifts with plastic responses at different spatiotemporal scales, including long-term acclimation/acclimatization and short-term heat hardening. However, knowledge about the interaction of these plastic responses is still unclear. 2. To understand how plastic responses at different timescales work together to adjust heat tolerance of organisms, we examined the effect of heat hardening on the upper thermal limits of an intertidal mudflat bivalve, the razor clam Sinonovacula constricta, for different seasons by using heart rate as a proxy. 3. We observed a stronger heat hardening response of S. constricta in warm seasons, implying that heat hardening worked synchronously with seasonal acclimatization to increase resistance of the clams to high temperatures in warm seasons. In warm seasons, heat hardening increased heat tolerance by 2-4°C and showed a 24-h temporal dependence, suggesting an adaptation to the diel fluctuation of thermal regimes in summer. 4. Furthermore, thermal stress resembling seasonal maximum environmental temperature induced stronger heat hardening effects, indicating that heat hardening is an essential plastic response to extreme hot weather, complementing seasonal acclimatization. 5. Our results suggest that high temperature risk can be alleviated jointly by seasonal acclimatization and heat hardening, and emphasize the importance of considering physiological plasticity on both long-term and short-term temporal scales in evaluating and forecasting vulnerability of organisms to climate change.