Using the L* concept to explore controls on the relationship between paired ligand and dissolved iron concentrations in the ocean
Boyd, PW and Tagliabue, A, Using the L* concept to explore controls on the relationship between paired ligand and dissolved iron concentrations in the ocean, Marine Chemistry, 173 pp. 52-66. ISSN 0304-4203 (2015) [Refereed Article]
Ligand (L) dynamics are inextricably linked to iron biogeochemistry, and their binding characteristics define much of the oceanic distributions of dissolved iron (DFe). Usually, L concentrations [L] are considered to be perennially in excess of [DFe] at any oceanic locale or point in time. Here we use the biogeochemical * concept to investigate whether distinct trends and patterns are evident for L* (the excess of [L] over [DFe]) across the two conventional ligand classes L1 and L2. The largest global datasets are available for L2* and point overwhelmingly to positive L2* values (but clearly establishing whether ligands in published studies are L2 versus L1 can be problematic). This trend is also apparent, for a more limited dataset, for L1*. Negative L2* values are mainly linked to high-iron waters (> 2 nmol L− 1). Datasets from process studies, such as mesoscale iron-enrichments and shipboard particle remineralisation time-series, provide insights into the main drivers of L* in surface and subsurface waters, respectively. Multiple studies reveal rapid (days) microbial responses to iron-enrichment, with L1* increasing from negative to positive values. Deeper in the water column, particle remineralisation releases L2 concurrently with DFe but at higher concentrations (i.e. + L2*). We propose that + L1* is driven by opportunism within marine bacteria, but the magnitude of L1* is constrained by the energetic demands of producing siderophores, for example in response to episodic iron-enrichment, such that L1 is produced in slight excess only. In contrast, during subsurface particle solubilisation, + L2* values are probably driven by concurrent release of a larger excess of organic compounds (linked to major elements like C, which can act as L2) relative to trace amounts of DFe.