Spectral response of Siderastrea sp. corals under varying Iron concentrations during a Mesocosm Experiment

Coral reefs are among the most biodiverse marine ecosystems, providing essential services such as coastal protection, fisheries support, and nutrient cycling. Owing to their physiological sensitivity, reef-building corals are widely recognized as bioindicators of changes in water quality and nutrient availability. Iron is an essential micronutrient that plays a central role in coral-dinoflagellate symbiosis by regulating metabolic and photosynthetic processes. However, the combined spectral and trophic responses of corals to iron enrichment remain poorly understood, particularly in Southwestern Atlantic reef systems. This study investigated the effects of increasing dissolved iron (DFe) concentrations (0, 100, 300, and 900 μg L^-1) on the spectral and trophic responses of the coral Siderastrea sp. during a 28-day mesocosm experiment under semi-natural conditions. Physiological performance was assessed using handheld spectroradiometry targeting wavelengths associated with photosynthetic pigments, and trophic strategies were evaluated through fatty acid biomarkers, including the Photoautotrophic Trophic Marker Index (PTMI). Results revealed a dose-dependent spectral response to iron enrichment. Higher DFe concentrations (300 and 900 μg L^-1) were associated with lower reflectance values across the visible spectrum (400-700 nm), indicating increased light absorption consistent with higher pigment density. In contrast, fatty acid profiles and PTMI values remained stable across treatments, indicating no significant shift in trophic strategy over the experimental period. These findings demonstrate that iron enrichment induced measurable dose-dependent increases in photosynthetic pigmentation without trophic displacement, and highlight the potential of combining spectroradiometry and biochemical trophic markers as non-invasive tools for monitoring subtle coral physiological responses under controlled micronutrient enrichment.