The Importance of Kinetics and Redox in the Biogeochemical Cycling of Iron in the Surface Ocean

Abstract

It is now well established that Iron (Fe) is a limiting element in many regions of the open ocean. Our current understanding of the key processes which control iron distribution in the open ocean have been largely based on thermodynamic measurements performed under the assumption of equilibrium conditions. Using this equilibrium approach, researchers have been able to detect and quantify organic complexing ligands in seawater and examine their role in increasing the overall solubility of iron. Our current knowledge about iron bioavailability to phytoplankton and bacteria is also based heavily on carefully controlled laboratory studies where it is assumed the chemical species are in equilibrium in line with the free ion association model and/or its successor the biotic ligand model. Similarly most field work on iron biogeochemistry generally consists of a single profile which is in essence a “snap-shot” in time of the system under investigation. However it is well known that the surface ocean is an extremely dynamic environment and it is unlikely if thermodynamic equilibrium between all the iron species present is ever truly achieved. In sunlit waters this is mostly due to the daily passage of the sun across the sky leading to photoredox processes which alter Fe speciation by cycling between redox states and between inorganic and organic species. Episodic deposition events, dry and wet, are also important perturbations to iron cycling as they bring in new iron to the system and alter the equilibrium between iron species and phases. Here we utilize new field data collected in the open ocean on the complexation kinetics of iron in the surface ocean to identify the important role of weak iron binding ligands (i.e., those that cannot maintain iron in solution indefinitely at seawater pH: αFeL < αFe′) in allowing transient increases in iron solubility in response to iron deposition events. Experiments with the thermal O2- source SOTS-1 also indicate the short term impact of this species on iron solubility also with relevance to the euphotic zone. This data highlights the roles of kinetics, redox, and weaker iron binding ligands in the biogeochemical cycling of iron in the ocean.

Toward a Regional Classification to Provide a More Inclusive Examination of the Ocean Biogeochemistry of Iron-Binding Ligands

“…The production of L 2 as well as L 1 ligands in remineralization experiments Bundy et al, 2016;Velasquez et al, 2016), clearly highlights a dynamic environment which is probably linked with bacterial colonization of sinking particles. Photochemistry is responsible for the production of weaker ligands and/or the degradation of iron binding ligands (e.g., Barbeau et al, 2001;Butler and Theisen, 2010;Gledhill and Buck, 2012), thus representing a loss pathway of strong ligands that can predominate in the upper few meters of surface waters (Figure 1) where they are likely to represent an important transient source of labile iron (Croot and Heller, 2012) to support phytoplankton growth.…”

The Effect of Metal Concentration on the Parameters Derived from Complexometric Titrations of Trace Elements in Seawater—A Model Study

“…A recent review by Laglera and Filella (2015), has thoroughly covered the subject of ligand exchange processes and their (lack of) investigation in the field of marine metal speciation studies and so this topic is not covered in depth here. Again, this issue is likely to have a particular impact on iron (Laglera and Filella, 2015) and relatively few studies have actually undertaken investigations into the kinetics of metal ligand formation and dissociation (Witter et al, 2000;Gerringa et al, 2007;Croot and Heller, 2012). Nevertheless recent field determinations of hydrothermal derived colloidal and soluble iron in the deep ocean suggest that there are processes, that could be phase changes or reactions, that likely take many years to reach equilibrium (Fitzsimmons et al, 2014(Fitzsimmons et al, , 2017.…”

Molecular determination of marine iron ligands by mass spectrometry

“…In nature, photochemical or biological reductive mechanisms or other strong ligands may be responsible for enhancing the dissociation of iron from hydroxamate siderophores [Croot and Heller, 2012;Miethke, 2013].…”

“…Therefore, determining the kinetics of iron exchange in seawater is essential for understanding biogeochemical iron cycling. A number of studies have addressed this question using competitive ligand exchange Witter and Luther, 1998;Witter et al, 2000;Gerringa et al, 2007;Croot and Heller, 2012;Laglera and Filella, 2014]. These studies have determined the timescales of iron association and dissociation from both model and natural ligands.…”