Survival at a cost: Corals endure microplastic and nanoplastic pollution by sacrificing energy reserves

Plastic pollution poses an increasing threat to coral reef ecosystems, yet the physiological impacts of small-sized microplastics (MPs; ∅ 2.1 μm) and nanoplastics (NPs; ∅ 30 nm) at low mass concentrations (MPs: 5.25 × 10^-4 mg L^-1; NPs: 2.4 × 10^-2 mg L^-1) comparable to those found in situ (<0.01 mg L^-1) remain largely unknown. In this study, the effects of chronic exposure to MPs and NPs on two symbiotic scleractinian coral species, Stylophora pistillata and Turbinaria reniformis, were investigated over 5 and 10 weeks under controlled laboratory conditions. We evaluated symbiont physiology, photosynthetic performance, respiration, and energy reserve content of the holobiont. The results show that S. pistillata was highly sensitive to MPs, with progressive bleaching, reduced photosynthesis, and significant depletion of lipids, proteins, and carbohydrates. Despite transient metabolic adaptations after 5 weeks, prolonged exposure resulted in physiological decline. In contrast, T. reniformis maintained stable symbiotic parameters, but still exhibited a reduction in net photosynthesis and energy reserves, indicating sublethal physiological costs. NPs elicited milder and delayed effects in both species, with significant effects in S. pistillata occurring only after 10 weeks, possibly due to the low NP mass concentration used. Species-specific responses were likely influenced by differences in morphology, polyp size, heterotrophic capacity, and the dominant Symbiodiniaceae clade. These findings demonstrate that low mass concentrations of plastics can nonetheless disrupt coral physiology and energy balance over time. This can affect coral fitness and their resilience to additional stressors such as ocean warming.