Experimental exposure to microplastics does not affect the physiology of healthy or moderately bleached Anomastraea irregularis and Pocillopora verrucosa corals

Microplastics are ubiquitous in marine systems, but their effects on coral physiology are still largely unknown. This study aimed to determine if exposure to a range of sizes of microplastic (10–100 µm) over a 4-week period affected selected physiological processes of healthy and moderately bleached corals. Two separate feeding experiments in which similar sized healthy and bleached Anomastraea irregularis and Pocillopora verrucosa nubbins from the south coast of KwaZulu-Natal, South Africa (30.3167° S, 30.7333° E) were fed either 0.8 mg of zooplankton only or a mix (a combination of 0.4 mg polypropylene fragments, 0.4 mg polyester fibres, and 0.8 mg zooplankton) were conducted in closed chambers for a period of 28 nights during May and June 2018. No significant differences in the physiology (ingestion, respiration, photosynthetic and growth rates) and tissue composition (Symbiodiniaceae, chlorophyll a and lipid contents) were found between corals fed the mix and those fed zooplankton only (p > 0.05). An egestion analysis revealed that both healthy and bleached A. irregularis and P. verrucosa egested the majority of the ingested microplastics after 18 h. This study shows that corals can ingest different types of microplastics but are also able to egest them over a few hours, preventing any detrimental effects.