Coupling macro- and micronutrient biogeochemistry: distribution and speciation of iron and other bioactive trace metals required for phosphorus acquisition in the subtropical North Atlantic

• Main Author: Kunde, Korinna, Gerda Lydia
• Format: Thesis
• Language: English
• Published: University of Southampton 2021
• Subjects: Mid-Atlantic Ridge; North Atlantic
• Online Access: https://eprints.soton.ac.uk/450504/; https://eprints.soton.ac.uk/450504/1/KKunde_Final_PhD_thesis_July2021.pdf; https://eprints.soton.ac.uk/450504/2/KKunde_Permission_to_deposit_thesis.docx

To meet cellular phosphorus (P) demands in the phosphate-deplete subtropical North Atlantic, phytoplankton depend on the less readily bioavailable dissolved organic phosphorus (DOP), which is accessible via alkaline phosphatases – metalloenzymes that require co-factors of iron (Fe), zinc(Zn) or cobalt (Co). As the oceanic concentrations of these metals are vanishingly low and as their physicochemical speciation further constrains bioavailability, the coupled biogeochemistry of macronutrient P and metal micronutrients Fe, Zn and Co has the potential to exert a biological control on primary production and hence, impact upon the global carbon cycle. A summertime longitudinal transect along 22 °N between 60 °W to 30 °W in the subtropical North Atlantic gyre served as a natural laboratory for this thesis, to investigate the distribution, speciation and biological control of Fe (and Zn and Co) for DOP acquisition across strong biogeochemical gradients in metals, macronutrients and phytoplankton community. High-resolution surface sampling and full-depth water column profiling of the Fe distribution were conducted and size-fractionated into soluble (sFe <0.02 µm), colloidal (0.02 µm unfiltered, acid-leachable) Fe species, which revealed the pervasive role of cFe in driving the distribution of dissolved Fe (dFe <0.2 µm, i.e. dFe = cFe + sFe). While the largest local input across the basin was due to hydrothermal venting from the Mid-Atlantic ridge in the abyss, where dFe reached 27 nM with ~90 % cFe, the surface dFe inventory was strongly controlled by seasonal dust deposition. This resulted in a strong west-to-east decrease in dFe concentrations from 1.53 to 0.26nM, with the colloidal fraction decreasing concurrently (from 85 to 61 % of dFe). Particle scavenging and biological uptake were the major removal processes of dFe and cFe in the subsurface, drawing cFe down to 0 to 30 % of dFe in the deep chlorophyll-a maximum (DCM). Due the pivotal position of cFe between the particulate and the soluble (i.e. the truly ...