Iron organic speciation in a warmer and a more acidic ocean

Iron is an essential and toxic element for terrestrial and marine life. It is present at subnanomolar concentration in seawater, limiting the oceanic primary production and carbon export in an estimated 40 % of the ocean surface. The potentially bioavailable iron (i.e. useable for phytoplankton growth) is believed to be mostly in the dissolved fraction, which is strongly complexed (> 99 %) with organic ligands. The chemical distribution of the different iron species, or chemical speciation, is highly dependent on the pH and, to a lesser extent, temperature. The impact of the decrease in ocean pH and increase in temperature, both due to the increasing CO2 pressure in the atmosphere, will modify the iron speciation and potentially the interaction between the iron and the organic ligands, thus affecting its overall biogeochemical cycling, its residence time in the water column and ultimately its bioavailability. In view of the importance of iron on biogeochemical ecosystems, understanding how its chemical speciation is likely to change in more acidic and warmer oceans is of vital importance.
The most sensitive methods used to investigate iron speciation are based on the voltammetric adsorption on a mercury drop of iron bound to an electroactive artificial ligand. The total iron and ligands concentration and the stability constants can be retrieved but these methods often require fastidious experimental procedures. The main issue is the requirement of pH adjustment which is potentially modifying significantly the iron speciation in the samples and misleading the interpretation. In this project, we are trying to develop analytical methods flexible in pH to measure the iron speciation at current and future pH/temperature value to gain insights into the effect of ocean acidification and increased temperatures on Fe speciation.