title: Analytical challenges, development and application of CLE-ACSV for the determination of the organic speciation of iron in marine waters
Creators: name: Mahieu, Leo
Publisher: name: University of Liverpool Repository
Year: 2023

description: Iron (Fe) is an essential micronutrient for marine phytoplankton, but its poor solubility in oxic waters is responsible for limited primary production across large parts of the ocean. It has been shown that organic compounds are able to bind with Fe to keep it in the dissolved phase (DFe), thought to be the most bioavailable for phytoplankton. The compounds composing the fraction of the dissolved organic matter (DOM) able to bind with Fe are referred to as Fe-binding ligands. While &gt; 99% of DFe is bound to Fe-binding ligands, the knowledge on their identity and cycling is limited as they represent a very small and diverse fraction of the DOM. There is a one-thousand factor difference between the range of concentrations of Fe-binding ligands and the wider DOM pool (nanomolar and micromolar, respectively), the composition and structure of which is impacted by multiple biological and physical processes. One method to investigate Fe-binding properties of the DOM is to titrate Fe-binding ligands against a calibrated added ligand at different DFe concentrations. This method is called Competitive Ligand Exchange (CLE) using Cathodic Stripping Voltammetry (ACSV). The CLE-ACSV approach allows the estimation of the Fe-binding ligand concentration ([L]), and of their average binding strength (Kcond). This approach, however, suffers from technical and practical limitations. In this work, I address several limitations of the CLE-ACSV approach and present techniques to improve accessibility for new users and minimise the risk of user subjectivity within data selection. I then apply these to samples collected from the subtropical South Pacific to assess the role of Fe-binding ligands in the distribution and cycling of DFe in waters impacted by intense diazotrophic and hydrothermal activities. In Chapter 1, I introduce the general background of my work by reviewing the inorganic and organic aspects of DFe speciation. I present in Chapter 2 a description of the CLE-ACSV approach; the concept, the theory, the apparatus, and the technical limitations are discussed to provide general knowledge on the different aspects hampering the application of the CLE-ACSV. In Chapter 3, I address the limitation of the CLE-ACSV approach related to the pH buffering of the sample, a technical requirement of the current methods which are potentially impacting and hampering our understanding of the DFe speciation. In Chapter 4, limitations related to the interpretation and comparability of the CLE-CSV titrations are addressed. I present a procedure developed to limit the subjectivity of the analyst on the results produced, aiming to ease the comparability of the results between laboratories. Finally, in Chapter 5, I present the application of my CLE-ACSV development on natural samples collected in the Western Tropical South Pacific. Fe-binding ligand data are combined with electrochemical and fluorescence data of the humic fraction, a known contributor to the Fe-binding ligand pool, to interpret the composition and cycling of the fraction of the DOM implicated in DFe distribution in this region. I finally conclude this thesis by sharing some thoughts about how to move forward in this challenging but important research area that marine Fe speciation is.