Incorporation of hydroxamate siderophore and associated Fe into marine particles in natural seawater

Microbially produced siderophores, some of the strongest Fe-binding agents, are commonly considered to be soluble low-molecular-weight compounds in aquatic systems. However, the presence of siderophores in marine particles/colloids or high-molecular-weight phases has been overlooked. Here, experimental evidence is provided to suggest the possibility of the incorporation of siderophore compounds into marine particles in the ocean. An incubation experiment with the ⁵⁹Fe-complexed desferrioxamine (DFO) was conducted using natural seawater to examine the partitioning of DFO, a hydroxamate siderophore (HS) compound, and its associated Fe during microbial growth. After the incubation, the incorporation of ⁵⁹Fe into suspended particles (suspended particulate matter - SPM) or aggregates (>0.45 µm) was quantified, and DFO incorporated into the particles was quantified using our modified and well-calibrated ‘Csaky’ method. Our results showed that ~55% of the ⁵⁹Fe, originally in the form of DFO-Fe, was incorporated in the SPM/aggregates, but only 0.079% (±0.004%) of the DFO was incorporated into the SPM/aggregates. This suggests that while HS facilitates iron incorporation into microbial biomass, most of the DFO was released back into the dissolved phase after they finished their Fe-transport mission. While still a minor fraction, it is still a significant fraction of the HS that could be ‘retained’ in the particulate phase via mechanisms yet to be identified. However, while siderophore compounds can indeed partition in the particulate fraction, this could have become cumulatively more important considering a more complex natural system that involves the interplay between minerals, bacteria, phytoplankton and zooplankton in the open ocean.