The Characterization of Microbially-Produced Iron-Binding Ligands Across the Mid-Cayman Rise

Iron (Fe) is an essential micronutrient for marine organisms and its availability is integral to microbial dynamics in the global ocean. Recent work has demonstrated that hydrothermal vents can be significant sources of Fe to the ocean interior, but Fe requires physicochemical stabilization for basin-scale transport. Organic ligands are ubiquitous molecules with the ability to stabilize Fe. However, the diversity, and sources of these molecules in hydrothermal vent ecosystems are poorly understood. To investigate whether strong Fe-binding organic ligands such as siderophores are present in neutrally-buoyant hydrothermal plumes, samples from seven hydrothermal sites across the Mid-Cayman Rise (MCR) were collected for ligand analysis in January and February of 2020. Ligand samples were analyzed using liquid chromatography-electrospray ionization mass spectrometry (LC-ESIMS), and siderophores were identified at each of the seven sites, with a range of confidence levels. Several different siderophores were detected, including nocardimicins and nocardichelins (Nocardia) and mycobactins (Mycobacterium) amphiphilic chelators with the potential to be anchored to the cell membrane. Pyoverdins, known to be produced by Mn(II)-oxidizing bacteria in Fe-limited conditions, were also putatively detected, suggesting possible competition for Fe by Mn-oxidizers. Very few genes for siderophore biosynthesis were detected relative to those for siderophore transport in 10 publically-available metagenomes from MCR, suggesting siderophore uptake is widespread. This study suggests siderophores may play an active role in Fe cycling at the MCR, and further work will investigate the role of siderophores and their microbial sources in additional hydrothermal plumes.