Plastic pollution is an emerging stressor that increases pressure on ecosystems such as coral reefs that are already challenged by climate change. However, the effects of plastic pollution in combination with global warming are largely unknown. Thus, the goal of this study was to determine the cumulative effects of microplastic pollution with that of global warming on reef-building coral species and to compare the severity of both stressors. For this, we conducted a series of three controlled laboratory experiments and exposed a broad range of coral species (Acropora muricata, Montipora digitata, Porites lutea, Pocillopora verrucosa, and Stylophora pistillata) to microplastic particles in a range of concentrations (2.5–2500 particles L⁻¹) and mixtures (from different industrial sectors) at ambient temperatures and in combination with heat stress. We show that microplastic can occasionally have both aggravating or mitigating effects on the corals’ thermal tolerance. In comparison to heat stress, however, microplastic constitutes a minor stressor. While heat stress led to decreased photosynthetic efficiency of algal symbionts, and increased bleaching, tissue necrosis, and mortality, treatment with microplastic particles had only minor effects on the physiology and health of the tested coral species at ambient temperatures. These findings underline that while efforts to reduce plastic pollution should continue, they should not replace more urgent efforts to halt global warming, which are immediately needed to preserve remaining coral reef ecosystems.

Microplastic ingestion has been widely documented in marine zooplankton, but the retention time of microplastics in their digestive gut are still poorly studied, especially among species from different climatic zones and marine habitats. This study evaluated the ingestion and gut retention time of four sizes of fluorescent microplastic beads (1.3, 7.3, 10.6, and 19.0 μm) in stage II naupliar larvae of nine barnacle species from different habitats (epibiotic on turtles, mangroves, coral reefs, and rocky shores) and climatic zones (subtropical/tropical and temperate). Microbeads were not lethal to all species (climatic zones/habitats) tested from the four sizes of non-fluorescent virgin microbeads (1.7, 6.8, 10.4 and 19.0 μm, each at concentrations 1, 10, 100, and 1000 beads mL⁻¹). Gut retention time of microplastic beads in barnacle naupliar larvae significantly increased with decreasing size. Microbeads resided in digestive tracts generally 3–4 times longer in rocky shore and coral reef barnacles than in muddy shore and epibiotic ones. However, species from different climatic zone did not differ in retention time. Our results suggested nauplius larvae from rocky shore and coral reef barnacles appear to be more susceptible to the impacts of longer retained microplastics (e.g., toxic chemicals present on the surface).

Global warming and local disturbances such as pollution cause several impacts on coral reefs. Among them is the breakdown of the symbiosis between host corals and photosynthetic symbionts, which is often a consequence of oxidative stress. Therefore, we investigated if the combined effects of thermal stress and copper (Cu) exposure change the trophic behavior and oxidative status of the reef-building coral Mussismilia harttii. Coral fragments were exposed in a mesocosm system to three temperatures (25.0, 26.6 and 27.3 °C) and three Cu concentrations (2.9, 5.4 and 8.6 μg L⁻¹). Samples were collected after 4 and 12 days of exposure. We then (i) performed fatty acid analysis by gas chromatography-mass spectrometry to quantify changes in stearidonic acid and docosapentaenoic acid (autotrophy markers) and cis-gondoic acid (heterotrophy marker), and (ii) assessed the oxidative status of both host and symbiont through analyses of lipid peroxidation (LPO) and total antioxidant capacity (TAC). Our findings show that trophic behavior was predominantly autotrophic and remained unchanged under individual and combined stressors for both 4- and 12-day experiments; for the latter, however, there was an increase in the heterotrophy marker. Results also show that 4 days was not enough to trigger changes in LPO or TAC for both coral and symbiont. However, the 12-day experiment showed a reduction in symbiont LPO associated with thermal stress alone, and the combination of stressors increased their TAC. For the coral, the isolated effects of increase in Cu and temperature led to an increase in LPO. The effects of combined stressors on trophic behavior and oxidative status were not much different than those from the isolated effects of each stressor. These findings highlight that host and symbionts respond differently to stress and are relevant as they show the physiological response of individual holobiont compartments to both global and local stressors.

There is evidence that microplastic (MP) pollution can negatively influence coral health; however, mechanisms are unknown and most studies have used MP exposure concentrations that are considerably higher than current environmental conditions. Furthermore, whether MP exposure influences coral susceptibility to other stressors such as ocean warming is unknown. Our objective was to determine the physiology response of corals exposed to MP concentrations that have been observed in-situ at ambient and elevated temperature that replicates ocean warming. Here, two sets of short-term experiments were conducted at ambient and elevated temperature, exposing the corals Acroporasp. and Seriatopora hystrix to microspheres and microfibres. Throughout the experiments, gross photosynthesis and net respiration was quantified using a 4-chamber coral respirometer, and photosynthetic yields of photosystem II were measured using Pulse-Amplitude Modulated (PAM) fluorometry. Results indicate the effect of MP exposure is dependent on MP type, coral species, and temperature. MP fibres (but not spheres) reduced photosynthetic capability of Acropora sp., with a 41% decrease in photochemical efficiency at ambient temperature over 12 days. No additional stress response was observed at elevated temperature; photosynthetic performance significantly increased in Seriatopora hystrix exposed to MP spheres. These findings show that a disruption to coral photosynthetic ability can occur at MP concentrations that have been observed in the marine environment and that MP pollution impact on corals remains an important aspect for further research.

Scleractinian coral are experiencing global and regional stressors. Microplastics (<5 mm) are an additional stressor that may cause adverse effects on coral. Experiments were conducted to investigate ingestion size limits and retention times of microspheres in a two-day exposure as well as observing growth responses in a 12-week exposure in two Atlantic species, Pseudodiploria clivosa and Acropora cervicornis. In the two-day exposure, P. clivosa ingested a higher number of microspheres ranging in size from 425 μm–2.8 mm than A. cervicornis. Both species egested the majority of microspheres within 48 h of ingestion. In the long-term exposure, calcification and tissue surface area were negatively affected in the treatment group of both species. Exposure also negatively affected buoyant weight in A. cervicornis but not in P. clivosa. The results indicate that microplastics can affect growth responses, yet additional research is warranted to investigate potential synergistic impacts of microplastics and other stressors.

Swift degradation of the coral reef ecosystems urges the need to identify the reef decline drivers. Due to their widespread use, bioaccumulative and toxic characteristics, chlorinated organic compounds, such as chlorinated paraffins (CPs), are regarded as specific pollutants of concern. Yet little is known about the occurrence of CPs in the coral reef ecosystems. This study focuses on the short-chain chlorinated paraffins (SCCPs) and medium-chain chlorinated paraffins (MCCPs). Their distribution and congener pattern were investigated in the water-SPM-sediment system and in the corals of the Larak coral reef for the first time. Chlorinated paraffins were detected in all the coral species. Their total loadings ranged from 42.1 to 178 ng g⁻¹ dw in coral tissue, from 6.0 to 144 ng g⁻¹dw in the skeleton, and from 55.0 to 240 ng g⁻¹dw in zooxanthellae. Soft corals were found to accumulate more CPs than Scleractinian corals. Zooxanthellae and mucus accumulated more CPs than tissue and skeleton. In most cases, congener group patterns were dominated by C₁₃ (for SCCPs) and C₁₇ (MCCPs) groups, respectively. The congener patterns of CPs altered to some extent between mucus and the remaining coral compartments. High loadings of CPs were detected in the skeleton of the bleached corals. Moreover, a significant negative correlation between the levels of CPs and the symbiodinium density was observed.

Coral reefs are keystone coastal ecosystems that can be exposed to petroleum hydrocarbons from multiple sources, and when selecting spill response methods to limit environmental damages, corals represent one of the highest valued resources for protection. Because previous research to characterize the sensitivity of coral species to petroleum hydrocarbon exposures is limited, a continuous-flow passive dosing system and toxicity testing protocol was designed to evaluate the acute effects of two representative petroleum compounds, toluene and phenanthrene, on five coral species: Acropora cervicornis, Porites astreoides, Siderastera siderea, Stephanocoenia intersepta, and Solenastrea bournoni. Using analytically confirmed exposures, sublethal and lethal endpoints were calculated for each species, and used as model inputs to determine critical target lipid body burdens (CTLBBs) for characterizing species sensitivity. Further, quantification of the time-dependent toxicity of single hydrocarbon exposures is described to provide model inputs for improved simulation of spill impacts to corals in coastal tropical environments.

Characterizing benthic habitat sensitivities of rapidly-developing countries is of paramount importance. Recent efforts defining the benthic habitat of Qatar's coastal zone with a high-resolution, ground-truthed benthic habitat map has provided a framework on which to develop a habitat sensitivity map. Here we present a sensitivity analysis catered towards identifying habitats with varying sensitivities to natural and anthropogenic stressors on a quantitative scale from 1 to 5, low to high. We have identified that the majority of the coastal area is low risk habitat (i.e. sandflats). However, there are six areas which are assigned as high risk by virtue of their rich occurrences of seagrass meadows, mangrove forests, and coral reefs - [1]. The reefs offshore of Al Zubarah on the west coast of the peninsula; [2] The Al Ruwais fringing reef on the northern tip of the peninsula; [3] The east-coast bays of Al Khor and Al Dhakira; [4] The shore-attached reef complex off Mesaieed (east coast); [5] The Jazirat al Bushayriyah offshore high; and, [6] The entire shelf of Halul Island. The sensitivity analysis presented here builds on a recently-developed benthic habitat map of Qatar's coastal zone and covers a total are of 4500km². Results gleaned from this analysis can be used to support marine spatial planning objectives and ecosystem-based management decision making.

The tropical waters of the Northern Arabian Gulf have a long history of maritime resource richness. High levels of biodiversity result from the complex matrix of coastal habitats, coral reefs and sea grass beds that characterise the region. Insight into the ongoing health of such habitats and the broader Kuwait maritime environment can be gauged by the status of indicator species found within these habitats. Here we review information on the occurrence, distribution and threats to key marine habitats and associated indicator species to provide an updated assessment of the state of the Kuwait's marine biodiversity. Critical evaluation of historic data highlights knowledge gaps needed inform the focus of future monitoring and conservation efforts. This assessment is designed to evaluate performance against environmental policy commitments, while providing a solid foundation for the design of comprehensive marine ecosystem management strategies.

Seafloor macrolitter is ubiquitous in world's oceans; still, huge knowledge gaps exist on its interactions with benthic biota. We report here the colonization of plastic substrates by the Mediterranean red coral Corallium rubrum (L. 1758), occurring both in controlled conditions and in the wild at ca. 85 m depth in the Western Mediterranean Sea. Juveniles settled on seafloor macro-litter, with either arborescent or encrusting morphology, ranging from 0.6 to 3.5 mm in basal diameter and 0.2–7.1 years of age, also including a fraction (20%) of potentially sexually mature individuals. In controlled conditions, larvae settled and survived on plastic substrates for >60 days. Our insights show that marine plastic debris can provide favourable substrate for C. rubrum settlement either in controlled conditions or in the wild, suggesting their possible use in restoration activities. However, we pinpoint here that this potential benefit could result in adverse effects on population dynamics.