Possible toxification mechanisms of acute and chronic pentachlorophenol to Montipora digitata: Limitation of energy supply and immunotoxicity.

Pentachlorophenol (PCP) is widely found in coastal environments and has various adverse effects, and its potential impact on coral reef ecosystems concerning. The scleractinian coral Montipora digitata was used for PCP stress experiments in this study. Phenotypes, physiological indicators, microbial diversity analysis and RNA sequencing were used to investigate the mechanisms underlying the responses of corals to acute and chronic PCP exposure. After 96 h of acute exposure, coral bleaching occurred at 1000 µg/LPCP and there was a significant decrease in Symbiodiniaceae density, Fv/Fm, and chlorophyll a content. Exposure to different concentrations of PCP significantly increased the content of malondialdehyde (MDA), leading to oxidative stress in corals. Chronic PCP exposure resulted in bleaching at 60 days, with the Fv/Fm significantly reduced to 0.461. Microbial diversity analysis revealed an increase in the abundance of potential pathogens, such as Vibrio, during acute PCP exposure and the emergence of the degrading bacterium Delftia during chronic PCP exposure. Transcriptional analysis showed that PCP exposure caused abnormal carbohydrate and amino acid metabolism in zooxanthella, which affected energy supply, induced immune responses, and disrupted symbiotic relationships. Corals respond to injury by boosting the expression of genes associated with signal transduction and immune response. Additionally, the expression of genes associated with environmental adaptation increased with chronic PCP exposure, which is consistent with the results of the microbial diversity analysis. These results indicate that PCP exposure might affect the balance of coral- zooxanthellae symbiosis in the stony coral M. digitata, impairing coral health and leading to bleaching.

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.

Contrasting effects of increasing dissolved iron on photosynthesis and O2 availability in the gastric cavity of two Mediterranean corals.

Iron (Fe) plays a fundamental role in coral symbiosis, supporting photosynthesis, respiration, and many important enzymatic reactions. However, the extent to which corals are limited by Fe and their metabolic responses to inorganic Fe enrichment remains to be understood. We used respirometry, variable chlorophyll fluorescence, and O2 microsensors to investigate the impact of increasing Fe(III) concentrations (20, 50, and 100 nM) on the photosynthetic capacity of two Mediterranean coral species, Cladocora caespitosa and Oculina patagonica. While the bioavailability of inorganic Fe can rapidly decrease, we nevertheless observed significant physiological effects at all Fe concentrations. In C. caespitosa, exposure to 50 nM Fe(III) increased rates of respiration and photosynthesis, while the relative electron transport rate (rETR(II)) decreased at higher Fe(III) exposure (100 nM). In contrast, O. patagonica reduced respiration, photosynthesis rates, and maximum PSII quantum yield (Fv/Fm) across all iron enrichments. Both corals exhibited increased hypoxia (<50 µmol O2 L-1) within their gastric cavity at night when exposed to 50 and 100 nM Fe(III), leading to increased polyp contraction time and reduced O2 exchange with the surrounding water. Our results indicate that C. caespitosa, but not O. patagonica, might be limited in Fe for achieving maximal photosynthetic efficiency. Understanding the multifaceted role of iron in corals' health and their response to environmental change is crucial for effective coral conservation.

The dispersant Corexit 9500 and (dispersed) oil are lethal to coral endosymbionts.

Endosymbionts (Symbiodiniaceae) play a vital role in the health of corals. Seawater pollution can harm these endosymbionts and dispersants used during oil spill cleanup can be extremely toxic to these organisms. Here, we examined the impact of oil and a specific dispersant, Corexit-9500, on two representative endosymbionts - Symbiodinium and Cladocopium - from the Southwestern endemic coral Mussismilia braziliensis. The survival and photosynthetic potential of the endosymbionts decreased dramatically after exposure to the dispersant and oil by ~25 % after 2 h and ~50 % after 7 days. Low concentrations of dispersant (0.005 ml/l) and dispersed oil (Polycyclic Aromatic Hydrocarbons, 1132 µg/l; Total Petroleum Hydrocarbons, 595 µg/l) proved highly toxic to both Symbiodinium and Cladocopium. These levels triggered a reduction in growth rate, cell size, and cell wall thickness. After a few hours of exposure, cellular organelles were damaged or destroyed. These acute toxic effects underline the fragile nature of coral endosymbionts.