Detection of hydroxamate siderophores in coastal and Sub-Antarctic waters off the South Eastern Coast of New Zealand

In this study, we report siderophore-type compounds found in coastal and Sub-Antarctic waters within a 60 km transect off the south east coast of Otago Peninsula in New Zealand. The presence of siderophore activity was detected using chrome azurol S assay (CAS) from organic compounds extracted from ~ 200 to 480 L seawater by pre-concentration on XAD-16 resin. Csaky and Rioux assays indicated the presence of more than one functional type of siderophore or siderophore-like compound. Strong Fe-binding ligand (L1) concentrations, the sum of ligands (ΣL), and their stability constants were measured from ambient seawater samples using competing ligand equilibration-cathodic stripping voltammetry (CLE-CSV). In the study area L1 ranged from 0.23 to 0.69 nM (logK′FeL1,Fe3+ = 22.0–22.8), whereas ΣL values ranged from 0.53 to 1.23 nM (logK′FeΣL,Fe3+ = 21.4–22.2). Dissolved Fe (DFe) concentrations were measured by CSV and ranged from 0.22 to 0.45 nM. This is the first application of a new liquid chromatography–tandem mass spectrometry method using the natural iron-isotope pattern to characterize siderophore-type compounds isolated from natural seawater. Altogether six siderophore-type compounds with hydroxamate functionality were detected in one coastal, as well as Sub-Antartic surface and subsurface samples. This new method and the derived results provide a foundation for future investigations of sources and structures of strong ligands in the Southern Ocean and elsewhere.

Research highlights
► We found six siderophore-type compounds in coastal and Sub-Antarctic waters.
► We used HPLC–MS/MS, assays and electrochemical techniques for characterization.
► All compounds show hydroxamate functionality.
► Some compounds have characteristics of cyclic ferrioxamines.
► Siderophores found in iron-limited and deplete waters.