Effects of selenium on the model cnidarian Exaiptasia diaphana and its symbiotic algae

Characterising toxicity thresholds for coral reefs is essential for understanding and safeguarding these ecosystems. Coral reefs are highly sensitive to environmental changes, including pollutants and increased trace element concentrations. Corals and other cnidarians form symbiotic associations with photosynthetic algae (Symbiodiniaceae) allowing for diverse nutrient acquisition methods and effective nutrient transformation and recycling between the host animal and their Symbiodiniaceae. Selenium (Se), an essential element, supports crucial physiological functions in marine taxa but it can become toxic at elevated concentrations. Currently, Se exposure thresholds for cnidarians and Symbiodiniaceae remain unknown. To assess the impact of high inorganic Se concentrations on cnidarians and Symbiodiniaceae, we conducted toxicity tests using the model sea anemone, Exaiptasia diaphana, exposing individuals to Se-enriched seawater using Na₂SeO₃ (76 - 1100 µg Se/L) for 96 h. Mortality occurred in the highest concentration of Se (1100 µg/L) for all replicates, but 100 % survival was recorded in all lower concentrations, including 570 µg/L. This latter concentration exceeded environmentally relevant levels, negating the need to acquire more refined mortality data. In addition, decreases in oral disk and reduced tentacle length at higher Se exposures indicated potential sublethal effects and physiological stress where E. diaphana exposed to concentrations ranging from 245- 570 µg/L decreasing in size by ∼15-20 %. These findings contribute to our understanding of cnidarian physiology and stress responses, highlighting the importance of trace elements in coral reef environments. This knowledge is crucial for developing effective management strategies to protect and preserve vital ecosystems in the face of environmental challenges.