Changes in physiological performance and protein expression in the larvae of the coral Pocillopora damicornis and their symbionts in response to elevated temperature and acidification

Transcriptomic and physiological analyses reveal the toxic effects of inorganic filters (nZnO and nTiO2) on scleractinian coral Galaxea fascicularis

The effects of sunscreen on scleractinian corals have garnered widespread attention; however, the toxic effects and mechanisms remain unclear. This study investigated the toxicological effects of two common inorganic filters used in sunscreens, nano zinc oxide and titanium dioxide (nZnO and nTiO₂), on the reef-building coral Galaxea fascicularis, focusing on the phenotypic, physiological, and transcriptomic responses. The results showed that after exposure to 0.8 mg/L of nZnO and 30 mg/L of nTiO₂ for 48 h, all coral polyps exhibited retraction. Zn and Ti ions were detected in coral tissues at concentrations of 67.18 and 24.87 μg/g, respectively, indicating the accumulation of nZnO and nTiO2 in coral tissues. The zooxanthellae density, Fv/Fm, and chlorophyll-a content decreased significantly. The activity of antioxidant enzymes showed an increasing trend. Meanwhile, glutamine synthetase and glutamate dehydrogenase activities exhibited a decreasing trend. The health status of corals was impacted as a result of nZnO and nTiO2 stress. Transcriptomic analysis showed that the toxicity mechanisms of nZnO and nTiO2 differed in corals. Following exposure to nZnO, differentially expressed genes (DEGs) in corals were mainly enriched in signaling pathways related to immune response. The genes related to innate immunity, such as MASP1, MUC5AC, TLRs, and C2, were significantly upregulated, indicating that nZnO exposure induces an innate immune response in corals. Meanwhile, following nTiO2 exposure, the upregulated DEGs were mainly enriched in signaling pathways related to transporter activity. In contrast, the downregulated DEGs were mainly enriched in energy metabolism pathways, indicating that nTiO2 disrupted the energy supply of corals, thereby leading to an increased demand for nutrient transport. This study reveals the toxic effects of nZnO and nTiO2, and their mechanisms of action on scleractinian corals, providing a reference for further assessing the toxicity of sunscreen on corals.

Symbiodiniaceae algal symbionts of Pocillopora damicornis larvae provide more carbon to their coral host under elevated levels of acidification and temperature

Climate change destabilizes the symbiosis between corals and Symbiodiniaceae. The effects of ocean acidification and warming on critical aspects of coral survical such as symbiotic interactions (i.e., carbon and nitrogen assimilation and exchange) during the planula larval stage remain understudied. By combining physiological and stable isotope techniques, here we show that photosynthesis and carbon and nitrogen assimilation (H13CO3- and 15NH4+) in Pocillopora damicornis coral larvae is enhanced under acidification (1000 µatm) and elevated temperature (32 °C). Larvae maintain high survival and settlement rates under these treatment conditions with no observed decline in symbiont densities or signs of bleaching. Acidification and elevated temperature both enhance the net and gross photosynthesis of Symbiodiniaceae. This enhances light respiration and elevates C:N ratios within the holobiont. The increased carbon availability is primarily reflected in the 13C enrichment of the host, indicating a greater contribution of the algal symbionts to the host metabolism. We propose that this enhanced mutualistic symbiotic nutrient cycling may bolster coral larvae's resistance to future ocean conditions. This research broadens our understanding of the early life stages of corals by emphasizing the significance of symbiotic interactions beyond those of adult corals.

Coral larvae increase nitrogen assimilation to stabilize algal symbiosis and combat bleaching under increased temperature

Rising sea surface temperatures are increasingly causing breakdown in the nutritional relationship between corals and algal endosymbionts (Symbiodiniaceae), threatening the basis of coral reef ecosystems and highlighting the critical role of coral reproduction in reef maintenance. The effects of thermal stress on metabolic exchange (i.e., transfer of fixed carbon photosynthates from symbiont to host) during sensitive early life stages, however, remains understudied. We exposed symbiotic Montipora capitata coral larvae in Hawai'i to high temperature (+2.5°C for 3 days), assessed rates of photosynthesis and respiration, and used stable isotope tracing (4 mM 13C sodium bicarbonate; 4.5 h) to quantify metabolite exchange. While larvae did not show any signs of bleaching and did not experience declines in survival and settlement, metabolic depression was significant under high temperature, indicated by a 19% reduction in respiration rates, but with no change in photosynthesis. Larvae exposed to high temperature showed evidence for maintained translocation of a major photosynthate, glucose, from the symbiont, but there was reduced metabolism of glucose through central carbon metabolism (i.e., glycolysis). The larval host invested in nitrogen cycling by increasing ammonium assimilation, urea metabolism, and sequestration of nitrogen into dipeptides, a mechanism that may support the maintenance of glucose translocation under thermal stress. Host nitrogen assimilation via dipeptide synthesis appears to be used for nitrogen limitation to the Symbiodiniaceae, and we hypothesize that nitrogen limitation contributes to retention of fixed carbon by favoring photosynthate translocation to the host. Collectively, our findings indicate that although these larvae are susceptible to metabolic stress under high temperature, diverting energy to nitrogen assimilation to maintain symbiont population density, photosynthesis, and carbon translocation may allow larvae to avoid bleaching and highlights potential life stage specific metabolic responses to stress.

Multiomics data integration, limitations, and prospects to reveal the metabolic activity of the coral holobiont

Since their radiation in the Middle Triassic period ∼240 million years ago, stony corals have survived past climate fluctuations and five mass extinctions. Their long-term survival underscores the inherent resilience of corals, particularly when considering the nutrient-poor marine environments in which they have thrived. However, coral bleaching has emerged as a global threat to coral survival, requiring rapid advancements in coral research to understand holobiont stress responses and allow for interventions before extensive bleaching occurs. This review encompasses the potential, as well as the limits, of multiomics data applications when applied to the coral holobiont. Synopses for how different omics tools have been applied to date and their current restrictions are discussed, in addition to ways these restrictions may be overcome, such as recruiting new technology to studies, utilizing novel bioinformatics approaches, and generally integrating omics data. Lastly, this review presents considerations for the design of holobiont multiomics studies to support lab-to-field advancements of coral stress marker monitoring systems. Although much of the bleaching mechanism has eluded investigation to date, multiomic studies have already produced key findings regarding the holobiont's stress response, and have the potential to advance the field further.

Effects of elevated temperature and copper exposure on the physiological state of the coral Galaxea fascicularis

The co-occurrence of elevated seawater temperature and local stressors (heavy metal contamination) affects the ecophysiology of phototrophic species, and represents a risk to the environmental quality of coral reefs. Therefore, we investigated the effects of both Cu alone and Cu in combination with elevated temperature (ET) on the physiology of the coral Galaxea fascicularis, and measured the parameters related to the photo-physiology and oxidative state. G.fascicularis is one of the dominant coral species in the South China Sea which exhibits strong adaptability to environmental stress. We exposed the common coral species G.fascicularis to a series of environmentally relevant concentrations of Cu at 29 °C (normal temperature, NT) and 32 °C (elevated temperature, ET) for 96 h. Single polyps were used in the experiments, which reduced individual variability when compared to the coral colonies. The results suggested that: i) Cu or ET had significant negative effects on the actual operating ability of photosystem Ⅱ (PSII), but not on the maximal chlorophyll fluorescence in darkness (Fv/Fm). ii) Symbiodiniaceae density was significantly reduced by high Cu concentrations, for Cu-NT and Cu-ET, a high concentration of Cu (40 μg/L) significantly impacted Symbiodiniaceae density, causing a 75.4% and 81.0% decrease, respectively. iii) the content of malondialdehyde (MDA) in coral tissues increased significantly under Cu-ET. iv) a certain range of copper concentration (25-30 μg/L) increased the pigment content of the Symbiodiniacea. Our results indicated that the combined stressors of Cu and ET made the coral tissue sloughed, caused the coral tissue damaged by lipid oxidation, reduced the photosynthetic capacity of the Symbiodiniacea, and led to the excretion of Symbiodiniacea.

Contamination of microplastics in tropical coral reef ecosystems of Sri Lanka

Microplastics (MPs) in different marine compartments are a global concern. This study investigated the abundance, distribution, and characteristics of microplastics from ten coral reef ecosystems in Sri Lanka, a non-quantified threat for some context. Microplastics were isolated and quantified in terms of abundance, shape, size, color, and polymer type with average abundances 546.7 ± 170.3 items kg-1, 9.8 ± 7.6 items m-3, and 46.3 ± 29.7 items kg-1 in corals, water, and sediments respectively. The most dominant microplastic type was blue, LDPE fibres. Acropora exhibited the highest amount. The significant differences in average microplastic abundances among corals suggest that they are capable of enriching microplastics depending on species-specific characteristics. Similar microplastic characteristics in corals and reef environment indicate that corals may have enriched microplastics from surface water and surface sediments. Microplastics being ubiquitous in selected reefs highlights the importance of coral reefs as a long-term microplastic sink in the ocean, contributing to the missing plastic phenomena.

Separate and combined effects of elevated pCO2 and temperature on the branching reef corals Acropora digitifera and Montipora digitata

Ocean acidification (OA) and warming (OW) are major global threats to coral reef ecosystems; however, studies on their combined effects (OA + OW) are scarce. Therefore, we evaluated the effects of OA, OW, and OA + OW in the branching reef corals Acropora digitifera and Montipora digitata, which have been found to respond differently to environmental changes. Our results indicate that OW has a greater impact on A. digitifera and M. digitata than OA and that the former species is more vulnerable to OW than the latter. OW was the main stressor for increased mortality and decreased calcification in the OA + OW group, and the effect of OA + OW was additive in both species. Our findings suggest that the relative abundance and cover of M. digitata are expected to increase whereas those of A. digitifera may decrease in the near future in Okinawa.