Photochemical Formation of Hydroxyl Radicals in Tissue Extracts of the Coral Galaxea fascicularis

Temperature-dependent responses of the hard corals Acropora sp. and Pocillopora verrucosa to molecular hydrogen

Coral reefs are increasingly threatened by mass bleaching events due to global ocean warming. Novel management strategies are urgently needed to support coral survival until global efforts can mitigate ocean warming. Given the strong antioxidant, anti-inflammatory and anti-apoptotic properties of molecular hydrogen, our study explores its potential to alleviate the negative effects of heat stress on corals. We investigated the ecophysiological responses of two common hard corals (Acropora sp. and Pocillopora verrucosa) from the Central Red Sea under ambient (26 °C) and elevated seawater temperatures (32 °C), with and without hydrogen addition ( ~ 150 µ M H2) over 48 h. Our results showed that at 32 °C without hydrogen addition, P. verrucosa exhibited high temperature tolerance, whereas Acropora sp. showed significant reductions in photosynthetic efficiency and maximum electron transport rate compared to the ambient condition (26 °C). The addition of hydrogen at 32 °C increased the maximum electron transport rate of Acropora sp. by 28%, maintaining it at levels compared to those at 26 °C. In contrast, the addition of hydrogen at 26 °C caused a significant decrease in the photophysiology of both Acropora sp. and P. verrucosa. This suggests that the short-term response of the coral holobiont to molecular hydrogen is temperature-dependent, potentially benefiting the coral holobiont under heat stress, while impairing the photophysiology under ambient temperatures. Our findings therefore provide the foundation for future long-term studies uncovering the mechanisms behind molecular hydrogen, potentially informing the development of new management strategies to enhance coral resilience to ocean warming.

Photophysiological and Oxidative Responses of the Symbiotic Estuarine Anemone Anthopleura hermaphroditica to the Impact of UV Radiation and Salinity: Field and Laboratory Approaches

The estuarine anemone Anthopleura hermaphroditica and its symbiont Philozoon anthopleurum are continuously exposed to intense fluctuations in solar radiation and salinity owing to tidal changes. The aim of this study was to evaluate the effects of the tidal cycle, solar radiation, and salinity fluctuations on the photosynthetic and cellular responses (lipid peroxidation, total phenolic compounds, and antioxidant activity) of the symbiont complex over a 24 h period in the Quempillén River Estuary. Additionally, laboratory experiments were conducted to determine the specific photobiological responses to photosynthetically active radiation (PAR), ultraviolet radiation (UVR), and salinity. Our field results showed that the photosynthetic parameters of the symbiont complex decreased with increasing ambient radiation; however, no relationship was observed with changes in salinity. Increased peroxidative damage, total phenolic compound levels, and antioxidant activity were mainly related to increased UVR and, to a lesser extent, PAR. During the dark period, only PAR-exposed organisms returned to the basal levels of photosynthesis and cell damage. Laboratory exposure confirmed the deleterious effects of UVR on the photosynthetic response. The present study suggests that the ability of A. hermaphroditica to acclimate to natural radiation stress is mediated by the concerted action of various physiological mechanisms that occur at different times of the day, under varying levels of environmental stress.

Responses of the sea anemone, Exaiptasia pallida, to ocean acidification conditions and zinc or nickel exposure

Ocean acidification, caused by increasing atmospheric carbon dioxide (CO₂), is a growing concern in marine environments. Land-based sources of pollution, such as metals, have also been a noted problem; however, little research has addressed the combined exposure of both pollutants to coral reef organisms. In this study we examined tissue metal accumulation and physiological effects (activity of anti-oxidant enzymes, catalase and glutathione reductase) in the sea anemone, Exaiptasia pallida after exposure to increased CO₂, as well as zinc (Zn) or nickel (Ni). After exposure to four concentrations (nominal values=control, 10, 50, 100μg/L) of Zn or Ni over 7days, both metals accumulated in the tissues of E. pallida in a concentration-dependent manner. Anemones exposed to elevated CO₂ (1000ppm) accumulated significant tissue burdens of Zn or Ni faster (by 48h) than those exposed to the same metal concentrations at ambient CO₂. No differences were observed in catalase activity due to Zn exposure; however, 50μg/L Ni caused a significant increase in catalase activity at ambient CO₂. No significant effect on catalase activity from CO₂ exposure alone was observed. Glutathione reductase activity was affected by increased Zn or Ni exposure and those effects were influenced by increased CO₂. Results of this study provide insight into the toxic mechanisms and environmental implications of CO₂ and Zn or Ni exposure to the cnidarian E. pallida.

The influence of salinity and copper exposure on copper accumulation and physiological impairment in the sea anemone, Exaiptasia pallida

Copper is a common pollutant in many aquatic environments, particularly those surrounding densely populated areas with substantial anthropogenic inputs. These same areas may also experience changes in salinity due to freshwater discharge and tidal influence. Biota that inhabit near-shore coastal environments may be susceptible to both stressors. Although copper is a noted concern in marine environments, effects of copper and varying salinity on symbiotic cnidarians are only scarcely studied. The sea anemone, Exaiptasia pallida, was used to investigate effects of copper on physiological impairment (i.e. activities of anti-oxidant enzymes) at two different salinities (20 and 25ppt). E. pallida was exposed to a control and three elevated copper concentrations for up to 21d, and copper accumulation and activity of the enzymes: catalase, glutathione reductase, glutathione peroxidase, and carbonic anhydrase were measured in the anemones. Photosynthetic parameters in E. pallida's symbiotic dinoflagellate algae were also quantified. Over the course of the exposure, E. pallida accumulated copper in a concentration-dependent manner. Higher tissue copper concentrations were observed in anemones exposed to the lower salinity water (20ppt), and physiological impairment was observed as a consequence of both increased copper exposure and decreased salinity; however, changes in salinity caused a greater response than copper exposure, at the levels tested. In general, antioxidant enzyme activity increased as a consequence of decreased salinity and/or increased copper exposure. These results clearly demonstrated the influence of two local stressors, at environmentally realistic concentrations, on a sensitive cnidarian, and highlight the importance of characterizing combined exposure scenarios.

Metal accumulation and sublethal effects in the sea anemone, Aiptasia pallida, after waterborne exposure to metal mixtures

The marine environment is subjected to contamination by a complex mixture of metals from various anthropogenic sources. Measuring the biological responses of organisms to a complex mixture of metals allows for examination of metal-specific responses in an environmentally realistic exposure scenario. To address this issue, the sea anemone, Aiptasia pallida was exposed to a control and a metal mixture (copper, zinc, nickel, and cadmium) at three exposure levels (10, 50, and 100 μg/L) for 7 days. Anemones were then transferred to metal-free seawater for an additional 7 days after the metal exposure to assess metal depuration and recovery. Metal accumulation, activity of the enzymes catalase, glutathione reductase, and carbonic anhydrase, as well as, cell density of the symbiotic zooxanthellae were measured over 14 days. Metal accumulation in A. pallida occurred in a concentration dependent manner over the 7-day exposure period. Altered enzyme activity and tentacle retraction of the host, as well as decreased zooxanthellae cell density were observed responses over the 7 days, after exposure to a metal concentration as low as 10 μg/L. Metal depuration and physiological recovery were dependent on both the metal and the exposure concentration. Understanding how A. pallida and their symbionts are affected by metal exposures in the laboratory may allow better understanding about the responses of symbiotic cnidarians in metal polluted aquatic environments.