Impact of microplastics on three life-history stages of stony corals

Abstract

Microplastic (MP) pollution is a growing threat to corals, particularly those are adjacent to the dense urban and industrial area. The current study aims to bridge a knowledge gap by examining life-stage-dependent responses of stony corals to microplastics, with emphasis on the early ontogenetic windows. Specifically, this work quantified the settlement, growth and photophysiological performance of Acropora tumida under controlled exposures that unravelled the roles of substrate conditioning and polymer identity. Coral planulae were reared in the laboratory and assigned to four treatments: Control, Symbiodinium (Zx; 900 000 cells L⁻¹), polystyrene (PS; 900 000 particles L⁻¹) and a Mix (900 000 cells L⁻¹ & 900 000 particles L⁻¹). Treatments were applied in parallel on biofilm-conditioned versus unconditioned ceramic tiles. In a second experiment, twelve-month-old juveniles and field-collected adult fragments were exposed for 30 days to polypropylene (PP; 10 mg L⁻¹), polyethylene terephthalate (PET; 10 mg L⁻¹) or natural sediment (10 mg L⁻¹); growth (surface area, polyp budding, buoyant weight) and photochemistry (Fo, Abs, Fv/Fm) were tracked with imaging-PAM fluorometry. Field surveys at four sites compared microplastics in Platygyra acuta skeletons with adjacent sediments in Mirs Bay. The experimental findings were stage specific. Unconditioned tiles reduced larval settlement by 74–95% although post-settlement growth increased slightly (≈ 3–8%). On conditioned tiles, the Mix treatment suppressed settlement by 38%, indicating that PS-altered symbiont cues can nullify positive biofilm signals. In juveniles, PP significantly reduced tissue growth, polyp number and photochemical efficiency, whereas PET and sediment produced negligible effects. Adults remained mostly unaffected, with similar buoyant weight and Fv/Fm values to controls. Field results showed that coral skeletons incorporated microplastic concentrations up to ten times higher than nearby sediments. The particle sizes were dominated by polyethylene fragments (30–100 µm) and reflecting stronger input from urban sources. Collectively, the findings indicated that microplastic effects depend on both coral life stage and polymer type. Comparing to Adults, juveniles are the more sensitive. These results highlight the need for reducing polymer-specific inputs and for conservation measures that prioritise the protection of early coral life stages in coastal waters with high microplastic loads.