The impact of variable Fe concentrations on Fe-binding ligands, dissolved organics and microbial communities in hydrothermal plumes – an experimental study

Iron (Fe) plays an important role in aquatic environments as an essential, often biolimiting micronutrient but at very high concentrations can potentially be toxic. Consequently, microbes have evolved capabilities to influence Fe bioavailability through production of organic molecules, so called ligands, which can enhance iron bioavailability or be used for detoxification mechanisms. Hydrothermal vents represent a major source of Fe to the oceans and host specialized microbes that are likely capable of influencing Fe speciation through ligands. Through abiotic decomposition of marine dissolved organic matter (DOM) or abiotic synthesis, hydrothermal systems might themselves constitute an additional source of Fe-binding ligands. Iron complexation in these systems is likely crucial in mediating Fe distribution to the water column but the interdependencies are still not well understood. Here we present first insights from experiments that incubated hydrothermal plume microbes in an artificial seawater dilution over a range of different Fe concentrations. The results show how variable Fe levels in conjunction with dissolved organics control Fe-binding ligand systematics and ultimately how this relates to the structure of the microbial community. At lower Fe concentrations the final community structure is more diverse with certain Epsilonproteobacteria as the most dominant group. Overall, ligand concentrations remain relatively low but the diversity of documented Fe-binding DOM formulas is high. In contrast, high Fe incubations are dominated by a group of Gammaproteobacteria and show high ligand concentrations but a very limited diversity of Fe formulas. These findings are further discussed in context of DOM characteristics and ligand stability constants.