Photochemical degradation of organic iron complexing ligands in seawater

Abstract

It has been shown by a number of investigators that greater than 99% of the Fe in the open ocean is complexed with organic ligands having conditional stability constants on the order of log K = 10–12. Some of these studies have shown evidence for the presence of 2 distinct ligand classes. It is hypothesized that the strong ligand class is produced by plankton while the weaker ligand is a result of decomposition of the strong ligand. Photochemical degradation in the surface ocean (upper 1–5 m) is one possible mechanism for this decomposition. Data from 2 cruises to the Gulf of Mexico demonstrate the importance of this photochemical reaction. The photodegradation of ligands appears to occur in a ligand that is less than 1 kDa in size and is ubiquitous throughout the Gulf of Mexico. This reaction may be responsible for controlling surface water Fe concentrations throughout the ocean and will help to further refine biogeochemical models of primary production.