Exploring Organic Metal-binding Ligands at the Lost City Hydrothermal Field

The Lost City Hydrothermal Field is located 15 km west of the Mid-Atlantic Ridge and is situated on ultramafic rocks, predominantly peridotite. When olivine, the main constituent of peridotite, interacts with seawater, it undergoes serpentinization producing hydrated minerals and alkaline fluids that form large carbonate-brucite chimneys that are low in trace metals. However, these chimneys often contain elevated organic matter, suggesting the Lost City biogeochemical system has the potential to be a significant source of organic metal-binding ligands to the deep ocean that are able to stabilize critical micronutrients. To investigate the role of organic metal-binding ligands at Lost City, we characterized a chimney sample from the base of the Poseidon complex. Using the Berger leach method in concert with inductively coupled plasma-mass spectrometry, we determined the concentrations of labile trace metals within the chimney. Both polar and nonpolar organic metal-binding compounds were explored using liquid chromatography paired with inductively coupled plasma-mass spectrometry and electrospray ionization-mass spectrometry (LC-ICP/ESI-MS). Within the chimney, iron (Fe) had the greatest labile concentration of all the metals measured, but the lowest fraction of organically-bound metal (<0.04%), and only two chromatographically resolved Fe-binding organic ligands were identified. By comparison, the nearby surrounding seawater had elevated concentrations of organic Fe-binding ligands as detected via cathodic stripping voltammetry and LC-ICP/ESI-MS. The labile cobalt and copper concentrations within the chimney were low compared to Fe and unexpectedly each associated with several distinct organic compounds. The prevalence of organic metal-binding ligands within the chimney and in the surrounding seawater suggests they may be facilitating the uptake of trace metals by microorganisms in these low metal hydrothermal vent systems. These compounds can shed light on unique microbial environments of a once-venting chimney as well as elucidate the impacts on surrounding seawater chemistry.