A systematic review of microplastics in coral reef ecosystems: Abundance, distribution, toxicity, and future research directions

Coral reefs, among the most biodiverse ecosystems providing critical ecological services, are increasingly exposed to microplastics (MPs), an emerging contaminant of concern found throughout freshwater and marine environments. Despite the growing number of studies, the lack of harmonized methodologies and reporting units for MP abundance in coral ecosystems hinder meaningful comparison across studies. This inconsistency poses challenges for ecotoxicology, where coral toxicity studies rely on accurate and comparable environmental data. Here, we adopted a systematic quantitative literature review approach to analyze 125 publications including 69 studies on the abundance of MPs in coral reefs environment and 56 studies on the toxicity of MPs on corals. Various MP forms, polymer types, colors and sizes have been reported. Reported MP concentrations in coral reef environments varied among sampled locations, spanning over six orders of magnitude in water (0.046 to 389,280 MPs/m³), up to five orders of magnitude in sediment (3.35 to 431.67 MPs/kg), and over two orders in coral tissue (0.03 to 14.18 MPs/g). To date, knowledge of the impact of environmentally relevant MPs on coral health remains limited. However, studies suggest these effects can be both species-specific and depend on polymer type and MP shape. Reported impacts include reduced growth and calcification, altered photosynthetic efficiency, increased mucus production, and in some cases coral bleaching and tissue necrosis. Furthermore, this review highlights the lack of appropriate quality assurance and quality control (QA/QC) protocols and discusses recommendations for future studies to enhance the data quality and relevance.