No short-term effect of sinking microplastics on heterotrophy or sediment clearing in the tropical coral Stylophora pistillata

Low incidence of microplastics in coral reefs of Kāne'ohe Bay, Hawai'i, USA

This study investigated microplastic and other micro-debris pollution in sediment, seawater, sea cucumbers, and corals from fringing and patch reefs in Kāne'ohe Bay, O'ahu, Hawai'i, USA. Microplastic pollution in Kāne'ohe Bay Bay was low compared to other tropical coral reefs. Microplastics were detected in sediments (29 %), sea cucumbers (9 %), and coral (0-2 %) samples but were not quantifiable. Seawater had quantifiable microplastic (< 0.5 mm) and macroplastic (> 0.5 mm) pollution, with mean concentrations ranging from 0.0061 to 0.081 particles m-3. Most particles detected in seawater samples were larger, floating plastic debris consisting mostly of polyethylene, polypropylene fragments, and fibers. Across the other matrices, the most detected particles were polyester, polypropylene, and cotton fibers. These results provide baseline data for this important coral reef ecosystem, and further monitoring is recommended to understand the seasonal and long-term trends in microplastic pollution and its potential future impacts.

Microplastics in coral from three Mascarene Islands, Western Indian Ocean

Little is known about microplastics (MPs) in corals from the Indian Ocean. We compared MP concentrations, morphotypes, size, colours, and polymer compositions in six coral genera from three remote Mascarene islands (Rodrigues, St. Brandon's Atoll, and Agalega) of the Republic of Mauritius, on a 1200 km transect located in the South Equatorial Current (SEC). The mean MP concentration was 0.78 n/g (53 % fibres) with no significant differences between islands. Polymers were polypropylene (78 %) and polyethylene (18 %). We conclude that the SEC's MP concentrations and compositions have homogenized over thousands of kilometres across the Indian Ocean. We discuss the lack of hazardous polyurethane MPs in coral samples given obvious sources on St Brandon. To the best of our knowledge, this study is the first to report on MPs in coral from the Western Indian Ocean and the Mascarene Islands providing a baseline for further research, monitoring, mitigation, and policy development.

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.

Responses in reef-building corals to wildfire emissions: Heterotrophic plasticity and calcification

Extreme wildfire events are on the rise globally, and although substantial wildfire emissions may find their way into the ocean, their impact on coral reefs remains uncertain. In a five-week laboratory experiment, we observed a significant reduction in photosynthesis in coral symbionts (Porites lutea) when exposed to fine particulate matter (PM2.5) from wildfires. At low PM2.5 level (2 mg L-1), the changes in δ13C and δ15N values in the host and symbiotic algae suggest reduced autotrophy and the utilization of wildfire particulates as a source of heterotrophic nutrients. This adaptive strategy, characterized by an increase in heterotrophy, sustained some aspects of coral growth (total biomass, proteins and lipids) under wildfire stress. Nevertheless, at high PM2.5 level (5 mg L-1), both autotrophy and heterotrophy significantly decreased, resulting in an imbalanced coral-algal nutritional relationship. These changes were related to light attenuation in seawater and particulate accumulation on the coral surface during PM2.5 deposition, ultimately rendering the coral growth unsustainable. Further, the calcification rates decreased by 1.5 to 1.85 times under both low and high levels of PM2.5, primarily affected by photosynthetic autotrophy rather than heterotrophy. Our study highlights a constrained heterotrophic plasticity of corals under wildfire stress. This limitation may restrict wildfire emissions as an alternative nutrient source to support coral growth and calcification, especially when oceanic food availability or autotrophy declines, as seen during bleaching induced by the warming ocean.

Physical and cellular impact of environmentally relevant microplastic exposure on thermally challenged Pocillopora damicornis (Cnidaria, Scleractinia)

Microplastic pollution is an increasing threat to coral reefs, which are already strongly challenged by climate change-related heat stress. Although it is known that scleractinian corals can ingest microplastic, little is known about their egestion and how microplastic exposure may impair corals at physiological and cellular levels. In addition, the effects of microplastic pollution at current environmental concentration have been little investigated to date, particularly in corals already impacted by heat stress. In this study, the combined effects of these environmental threats on Pocillopora damicornis were investigated from a physical and cellular perspective. Colonies were exposed to three concentrations of polyethylene microplastic beads (no microplastic beads: [No MP], 1 mg/L: [Low MP]; 10 mg/L: [High MP]), and two different temperatures (25 °C and 30 °C) for 72 h. No visual signs of stress in corals, such as abnormal mucus production and polyp extroflection, were recorded. At [Low MP], beads adhered to colonies were ingested but were also egested. Moreover, thermally stressed colonies showed a lower adhesion and higher egestion of microplastic beads. Coral bleaching was observed with an increase in temperature and microplastic bead concentration, as indicated by a general decrease in chlorophyll concentration and Symbiodiniaceae density. An increase in lipid peroxidation was measured in colonies exposed to [Low MP] and [High MP] and an up-regulation of stress response gene hsp70 was observed due to the synergistic interaction of both stressors. Overall, our findings showed that heat stress still represents the main threat to P. damicornis, while the effect of microplastics on coral health and physiology may be minor, especially at control temperature. However, microplastics could exacerbate the effect of thermal stress on cellular homeostasis, even at [Low MP]. While reducing ocean warming is critical for preserving coral reefs, effective management of emerging threats like microplastic pollution is equally essential.

The comparative effects of chronic microplastic and sediment deposition on the scleractinian coral Merulina ampliata

Despite increasing research into the effects of microplastics on corals, no study to date has compared this relatively novel pollutant with a well-established stressor such as downwelling sediments. Here, Merulina ampliata coral fragments were exposed to polyethylene terephthalate (PET) and calcium carbonate particles (200-300 μm) at two deposition levels, high (115.20 ± 5.83 mg cm-2 d-1, mean ± SE) and low (22.87 ± 1.90 mg cm-2 d-1) in specially-designed Flow-Through Resuspension (FloTR) chambers. After 28 d, there were no significant differences between fragments exposed to sediments and microplastics for coral skeletal growth, Symbiodiniaceae density, and areal or cellular chlorophyll a concentrations. There were also no significant differences between levels of treatments, or with the control fragments. More PET microplastic particles were incorporated into the coral skeletons of fragments exposed to microplastics compared to those exposed to sediment and the control fragments, but there was no difference between fragments exposed to high and low microplastic levels. Together, the results show that M. ampliata appears to be able to cope with both microplastic and sediment stress, and suggests that microplastics do not represent a more serious threat than downwelling sediments at the levels tested.

Impacts of marine debris on coral reef ecosystem: A review for conservation and ecological monitoring of the coral reef ecosystem

Coral reefs are the most spectacular underwater creation of nature. It enhances ecosystem functioning and marine biodiversity while also ensuring the livelihood of millions of coastal communities worldwide. Unfortunately, marine debris poses a serious threat to ecologically sensitive reef habitats and their associated organisms. Over the past decade, marine debris has been regarded as a major anthropogenic threat to marine ecosystems and gained scientific attention around the globe. However, the sources, types, abundance, distribution, and potential consequences of marine debris on reef ecosystems are hardly known. The goal of this review is to provide an overview of the current status of marine debris in various reef ecosystems across the world, with special emphasis on its sources, abundance, distribution, species impacted, major categories, potential impacts and management strategies. Furthermore, the adhesion mechanisms of microplastics to coral polyps, diseases caused by microplastics and are also highlighted.