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, including substantial iron (Fe) inputs via terrestrial runoff (up to 14.5 nM). While Fe is essential for coral and symbiont metabolism, the assimilation rate, physiological thresholds, and spatial allocation of Fe within coral compartments, and its interactive effects with warming, remain poorly understood. Here, we provide the first characterization of oxygen (O2) 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 O2 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 O2 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.