Feeding responses of reef-building corals provide species- and concentration-dependent risk assessment of microplastic

High levels of microplastics and microrubber pollution in a remote, protected Mediterranean Cladocora caespitosa coral bed

Coral reefs are increasingly threatened by anthropogenic stressors, including plastic pollution. This study investigates the abundance and possible ecological impact of microplastics (MPs) and microrubber pollution in sediments from a Cladocora caespitosa coral bed in the north-western Mediterranean. Despite being located in a remote marine protected area with no local plastic pollution sources, our results indicate exceptionally high MP concentrations (mean: 1514 particles/kg dry weight), attributed to long-distance transport of plastics by the Northern Current. Laser Directs Infrared (LDIR) Chemical Imaging and ATR-FTIR spectroscopy were used to characterize the MPs in terms of size, shape and polymer types. Most MPs are fragments (96 %), while fibers contribute only 4 %. The most abundant polymers were polyethylene (PE, 28 %), polyethylene terephthalate (PET, 25 %), and polystyrene (PS, 19 %), with significant contributions from polyurethane (PU) and microrubber. Particle size analysis showed that 92 % of MPs were smaller than 250 μm, with a median particle size varying by polymer type. Notably, polymers with heteroatoms in their main chain, such as PET and polyurethane, exhibited significantly smaller median sizes compared to polyolefins, possibly suggesting different degradation pathways. The high MP concentrations measured in sediments within coral colonies suggests that MPs could have adverse effects on heterotrophic feeding in C. caespitosa, a critical energy source during stress events. This study underscores the urgent need for targeted research on MP effects on the resilience of C. caespitosa and for increased global and regional efforts to curb plastic pollution mitigation in order to conserve coral populations in the Mediterranean.

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.

Predicting microplastic dynamics in coral reefs: presence, distribution, and bioavailability through field data and numerical simulation analysis

Understanding distribution and bioavailability of microplastics is vital for conducting ecological risk assessments (ERA) and developing mitigation strategies in marine environments. This study couples in situ data from Lizard Island (Great Barrier Reef) and numerical modelling and simulations to determine microplastic abundances in abiotic (water and sediment) and biotic (planktivorous fish, sea squirts, sponges, corals, and sea cucumbers) compartments and predict their trajectories within this ecosystem. Results show microplastics predominantly (75%) originate from beached plastics from nearby islands and coastal areas, dispersing northward without local entrapment and settlement likely occurring on northern beaches (> 50%), including Papua New Guinea. Concentrations increased by three orders of magnitude with depth, with distinct profiles: surface waters contained more fragments and low-density polymers at concentrations of < 1 microplastics m-3, and deeper layers more fibres and high-density polymers, with concentrations peaking at the seafloor at > 100 microplastics m-3. Reflecting ecological and physiological traits of each taxon, fish exhibited microplastic contamination levels nearly twice that observed in invertebrates, and while polymers and colours had no stronger evidences on influencing bioavailability, shape and size did, with fish more susceptible to contamination by microplastic fibres and all taxa to smaller-sized microplastic particles.

Increasing microplastic concentrations have nonlinear impacts on the physiology of reef-building corals

The pollution of marine environments with plastics, particularly microplastic (MP, i.e., plastic particles <5 mm), is a major threat to marine biota, including corals. While the effects of MPs are increasingly well understood, knowledge of how different concentrations of naturally occurring MP mixtures affect reef-building corals is still limited. Therefore, we aimed to elucidate the relationship of MP concentrations and their effects on reef-building corals. For this, we exposed two reef-building coral species (Stylophora pistillata and Pocillopora verrucosa) in a 12-week experiment to MPs at a gradient of concentrations (0, 0.1, 1, 10, and 100 mg·L-1). Specifically, we examined effects on the coral host physiology (i.e., surface and volume growth, calcification, necrosis, and polyp activity), and the photosynthetic activity of the photosymbionts (i.e., effective and maximum quantum yield, maximum relative electron transport rate, minimum saturating irradiance, and light capture efficiency). To mimic natural conditions, we used a MP mixture consisting of six polymers in forms of fibers and fragments. Both coral species showed reduced growth rates, necrosis, lower polyp activity, and an upregulation of photosynthesis, which intensified with increasing MP concentrations. While the effects on the coral host mostly showed basic linear or nonlinear dose-response relationships, the effects on the photosymbionts revealed more complex nonlinear dose-response relationships, and photosynthesis was only upregulated after a species-specific threshold. We found that high and extreme pollution scenarios caused strong adverse effects on coral physiology, while current low to moderate concentrations had minor effects. Increasing concentrations had amplifying effects, likely due to the disproportionately higher frequency of entanglement, leading to more frequent direct contact and potential transfer of toxins or pathogens. These results suggest that corals can cope with current average pollution levels. However, they also highlight the need for measures to limit permanent increases of MP pollution to protect the health of coral reefs.

Bisphenol A leachate from polystyrene microplastics has species-specific impacts on scleractinian corals

Plastic waste causes pervasive environmental contamination and can result in the release of harmful chemical leachates into marine ecosystems, especially as they fragment to smaller microplastics (<5 mm). The toxicity of commonly found polystyrene (PS) microplastics and associated bisphenol A (BPA) leachate to framework-building corals Pocillopora damicornis and Dipsastraea pallida was assessed through exposure experiments. Intermittent exposure over 14-days to 1) virgin PS, 2) preformulated PS with bound BPA (BPA-PS) and 3) leached BPA-PS (L-BPA-PS; simulating early stages of weathering) showed that microplastics void of leachable BPA had minimal effect on either coral species. However, BPA leachate had negative effects on the maximal photochemical yield (Fv/Fm) and tissue composition of P. damicornis fragments (e.g., decreased chlorophyll and protein compared to controls). Conversely, BPA leachate did not compromise tissues of D. pallida fragments. These results reveal that exposure to chemicals leaching out of microplastics can drive negative effects of microplastic exposure distinct from physical mechanisms due to ingestion alone, and that effects are species specific.

Microplastic pollution in tropical coral reef ecosystems from the coastal South China Sea and their impacts on corals in situ

Coral reefs possess extremely high ecological value in tropical and subtropical waters worldwide. Microplastics as emerging and pervasive pollutants pose a great threat to the health of coral ecosystems. However, in situ studies on microplastics pollution and its impacts in coral ecosystems globally are limited. The occurrence characteristics of microplastics in the environment mediums and reef-dwelling organisms were investigated in coral reef areas from the southern Hainan Island, and the impacts of microplastics on corals in situ were evaluated in this study. Average microplastics abundance was 9.48 items L-1 in seawater, 190.00 items kg-1 in sediment, 0.36 items g-1 in coral, 1.50 items g-1 in shellfish, 0.48 items g-1 in fish gill, and 1.71 items g-1 in fish gastrointestinal tract. The prevalent microplastics in the above samples were characterized as being less than 1000 µm in size, fibrous, and transparent, with predominant polymer types as polyethylene terephthalate, polypropylene, polyethylene, and rayon. The microplastic enrichment capacity of different corals varied (Pocillopora > Acropora > Sinularia). Notably, microplastics were more abundant on the surface of corals compared to their interiors, with distinct characteristics observed, including larger-sized (>500 µm) and fiber-shaped polyethylene terephthalate microplastics on the surface and smaller-sized (20-200 µm) fragmented polyethylene microplastics within coral interiors. Furthermore, the investigation showed species-specific impacts of microplastics on corals in situ, including photosynthetic activity of photosymbionts and antioxidant and immune activities of corals. Furthermore, the ecological risks of microplastics were minor across most environmental media in the studied areas, with exceptions in the bottom seawater and surface sediment of YLW, which exhibited extreme and medium risk levels, respectively. Coral risk levels were generally medium, except for dangerous levels in DDH and high levels in LHT. The potential sources of microplastics in the marginal reefs of southern Hainan Island were primarily tourism, residential, and fishing activities.

Possible sink of missing ocean plastic: Accumulation patterns in reef-building corals in the Gulf of Thailand

Individual coral polyps contain three distinct components-the surface mucus layer, tissue, and skeleton; each component may exhibit varying extent of microplastic (MP) accumulation and serve as a short- or long-term repository for these pollutants. However, the literature on MP accumulation in wild corals, particularly with respect to the different components, is limited. In this study, we investigated the adhesion and accumulation of MPs in four coral species, including both large (Lobophyllia sp. and Platygyra sinensis) and small (Pocillopora cf. damicornis and Porites lutea) polyp corals collected from Si Chang Island in the upper Gulf of Thailand. The results revealed that MP accumulation varied significantly among the four coral species and their components. Specifically, P. cf. damicornis exhibited the highest degree of accumulation (2.28 ± 0.34 particles g-1 w.w.) [Tukey's honestly significant difference (HSD) test, p < 0.05], particularly in their skeleton (52.63 %) and with a notable presence of high-density MPs (Fisher's extract test, p < 0.05). The most common MP morphotype was fragment, accounting for 75.29 % of the total MPs found in the coral. Notably, the majority of MPs were black, white, or blue, accounting for 36.20 %, 15.52 %, and 11.49 % of the samples, respectively. The predominant size range of MP particles was 101-200 μm. Nylon, polyacetylene, and polyethylene terephthalate (PET) were the prevalent polymer types, accounting for 20.11 %, 14.37 %, and 9.77 % of the identified samples, respectively. In the large polyp corals, while MP shapes, colors, and sizes exhibited consistent patterns, remarkable differences were noted in the polymer types across the three components. The findings of this study improve the understanding of MP accumulation and its fate in coral reef ecosystems, underscoring the need for further investigation into MP-accumulation patterns in reef-building corals worldwide.

Chronic effects of exposure to polyethylene microplastics may be mitigated at the expense of growth and photosynthesis in reef-building corals

The causes of the physiological effects of microplastic pollution, potentially harming reef-building corals, are unclear. Reasons might include increased energy demands for handling particles and immune reactions. This study is among the first assessing the effects of long-term microplastic exposure on coral physiology at realistic concentrations (200 polyethylene particles L-1). The coral species Acropora muricata, Pocillopora verrucosa, Porites lutea, and Heliopora coerulea were exposed to microplastics for 11 months, and energy reserves, metabolites, growth, and photosymbiont state were analyzed. Results showed an overall low impact on coral physiology, yet species-specific effects occurred. Specifically, H. coerulea exhibited reduced growth, P. lutea and A. muricata showed changes in photosynthetic efficiency, and A. muricata variations in taurine levels. These findings suggest that corals may possess compensatory mechanisms mitigating the effects of microplastics. However, realistic microplastic concentrations only occasionally affected corals. Yet, corals exposed to increasing pollution scenarios will likely experience more negative impacts.