Interactive effects of warming and iron supplementation on O2 dynamics, trace metal content, and microbial diversity within different compartments of two Mediterranean corals

Mediterranean corals living in coastal habitats are subjected to natural fluctuations in temperature and nutrient availability. Their nutritional status is linked to the uptake of various micronutrients essential for cellular metabolism and growth, including iron (Fe), a key cofactor in many physiological processes. Although ambient Fe concentrations in seawater are typically low (< 5 nM), Mediterranean coastal waters receive substantial Fe inputs via terrestrial runoff (up to 14.5 nM). While most corals require Fe to support their metabolic functions, the assimilation rate, physiological thresholds, and spatial allocation of Fe within coral compartments, and how these factors change with elevated seawater temperature, remain poorly understood. Here, we provide the first characterization of oxygen (O ₂ ) dynamics, trace metal content, and microbial community composition in two Mediterranean corals, Cladocora caespitosa and Eunicella singularis , exposed to chronic warming (18–24°C) and Fe(III) supplementation (20 nM day ^− 1 ). We show that although these corals are not Fe-limited, increased temperature enhanced the Fe uptake in the algal symbionts of C. caespitosa . In C. caespitosa , Fe supplementation reduced the O ₂ availability within the gastrovascular cavity (GVC) and altered the composition and diversity of GVC microbial communities. In E. singularis , interactive effects of Fe and warming reduced GVC O ₂ availability within the GVC, and warming increased metal content, while the microbiome resembled the surrounding seawater. These intraspecific differences in the sensitivity of the coral holobiont to warming and Fe supplementation could have important implications for the resilience of Mediterranean corals to ongoing climate stress, underscoring the importance of considering coral compartments in ecophysiological research.