Widespread siderophore production among Symbiodiniaceae-associated bacteria.

Nutritional exchanges fuel the evolutionary and ecological dominance of multi-partner symbioses among reef-building corals and microbial associates in oligotrophic tropical marine ecosystems. Mutualistic relationships with endosymbiotic dinoflagellates (Family Symbiodiniaceae) are central to coral holobiont metabolism, yet their metabolic contributions are sensitive to nutrient availability. Symbiodiniaceae may compensate for oligotrophic environments via metabolic exchanges with prokaryotic partners. Bacterial production of ligands with affinities for otherwise insoluble elements promotes uptake and exchanges. Bacterial secretion of small molecules with high ferric (Fe3+) iron affinities, herein referred to as siderophore production, presents one example of microbial metabolic cooperation. We isolated 78 pure bacterial culture lines from 14 Symbiodiniaceae cultures to screen for siderophore production using a Chrome Azurol S (CAS) overlay assay. Colorimetric changes observed on CAS overlays indicated ubiquitous siderophore production across 22 bacterial genera. Many of the isolated bacterial cultures corresponded to known 'core' Symbiodiniaceae microbiome. These results suggest an avenue of bacterial metabolism may facilitate biotic iron exchange among coral holobiont partners. Future characterization of the identity siderophores secreted will inform predictions on their impacts on iron exchange within the coral holobiont. Ultimately, a greater ability to acquire iron via siderophore production may improve the coral holobiont's tolerance to environmental stressors.