Physiological responses of scleractinian coral to trace metal enrichment and thermal stress

Coral bleaching events are increasingly frequent due to global climate change and marine pollution. Trace metals, such as manganese (Mn) and iron (Fe), though toxic at high concentrations, are vital for coral physiology, supporting photosynthesis and antioxidation. This study investigates how thermal stress and trace metal exposure interact to influence the physiology of the scleractinian corals Turbinaria irregularis and Montipora mollis. Corals were exposed to Mn and Fe at varying concentrations under control (25 °C) and elevated (30 °C) temperatures. Mn enhanced photosynthetic efficiency, an increase of 1.7°% in M. mollis at 250 nM and 1.4°% in T. irregularis at 30 °C (p < 0.05). Fe improved photosynthesis by 1.8°% in M. mollis at 50 nM and growth rates by 2.1°% in T. irregularis at 25 °C (p < 0.05). Both metals mitigated bleaching, as seen in reduced relative gray intensity and increased symbiotic algal density, particularly at moderate concentrations. However, elevated temperatures suppressed growth and photosynthetic efficiency, with decreases up to 1.6°% in M. mollis (p < 0.01). These results highlight the pivotal role of trace metals in coral health and stress resilience, while emphasizing the importance of species-specific differences in trace metal uptake, thermal tolerance, and physiological responses. Further studies are necessary to elucidate the mechanisms and long-term impacts of these interactions in the face of ongoing climate change.