Colony complexity affects microplastic loads in Pocillopora corals

Microplastic (MP) pollution poses a significant threat to marine ecosystems. Coral reefs, often located near land-based sources of these pollutants, act as potential sinks due to their complex three-dimensional structures. While the interactions between reef-building corals and MPs have been increasingly investigated, the role of coral structural complexity in MP accumulation remains poorly understood. This study investigated the influence of coral structural complexity on MP trapping efficiency under natural conditions, specifically aiming to: I) quantify and characterize MPs trapped by Pocillopora corals, II) compare MP distribution across coral compartments (surface, tissue, and skeleton), and III) assess the relationship between seven metrics of coral complexity (i.e., S/V ratio, fractal dimension, compactness, convexity, sphericity, packing, and rugosity) and MP loads. Six Pocillopora sp. colonies, comprising 36 fragments, were sampled from a reef in Kāne'ohe Bay, Hawai'i. MPs were extracted from the coral surface, tissue, and skeleton for quantification and characterization using microscopy and FTIR spectroscopy. Coral complexity was assessed using photogrammetry and 3D scanning. MPs were found at an average of 0.029 ± 0.079 particles per g coral, mostly at the coral surface (61 %). Compact, thick-branched coral morphologies showed higher MP accumulation, likely due to increased formation of stagnant water regions and reduced turbulence. Our results demonstrate that coral complexity plays a significant role in MP deposition under natural conditions, with potential implications for coral health and the transfer of MPs to other reef sinks. This highlights the importance of considering coral morphological complexity when evaluating the risk of MP pollution.