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.

Methods
This study aimed to explore the effects of Fe and Mn supplementation on the physiological parameters of two coral species, Turbinaria irregularis and Montipora mollis, under both normal and heat stress treatments. The experimental design, which was based on previous research (Biscéré et al., 2018), incorporated two temperature levels (25°C ± 1°C and 30°C ± 1°C) and two trace metals (Fe2+ and Mn2+) at three concentrations each (50, 100, and 250 nM), as outlined by Reich et al. (2020) and Rodriguez and Ho (2018). A control not exposed to metals was also included. Each treatment was replicated, and the experiment ran for a duration of 14 days. On-site measurements of temperature, salinity, dissolved oxygen (DO), and pH were conducted daily using a handheld multiparameter water quality meter (ProDSS multiparameter digital water quality meter, YSI; Xylem Inc., USA). Digital photographs of coral branches were taken at the experiment’s outset and conclusion for later analysis. Following the experiment, each set of coral fragments was individually enveloped in aluminum foil and placed in a freezer set to -20°C for subsequent analysis of physiological indices. Tissue was rinsed using high-pressure water, and the resultant homogenate was measured to 40 mL for subsequent physiological index analyses. Symbiotic algae density and chlorophyll a (Chl a) content were further normalized based on the sample surface area, and these parameters were compared and analyzed using standardized values. The analyzed methods for each variable can be found from the paper. Statistical analysis in this study was performed using SPSS 14.0 software (https://spss.software.informer.com/14.0/), with graphing software by Grapher (v.16.0.314, Golden Software, USA). A two-way ANOVA was used to compare the various measurement parameters between different temperatures and trace metal treatments to determine whether there were any differences. Tukey’s test was employed for post hoc analysis. If the basic assumptions were not met, the Kruskal–Wallis and Mann–Whitney U tests were used instead. The Pearson and Spearman correlation tests were used to determine the strength of correlations. A p-value of less than 0.05 (p < 0.05) was considered to indicate a significant difference. Data are presented as the mean (± SE).