Abstract
Ocean acidification impacts the iron (Fe) biogeochemistry both by its redox and its complexation reactions. This has a direct effect on the ecosystems due to Fe being an essential micronutrient. Polyphenols exudated by marine microorganisms can complex Fe(III), modifying the Fe(II) oxidation rates as well as promoting the reduction of Fe(III) to Fe(II) in seawater. The effect of the polyphenol gallic acid (GA; 3,4,5-trihydroxy benzoic acid) on the oxidation and reduction of Fe was studied. The Fe(II) oxidation rate constant decreased, increasing the permanence of Fe(II) in solutions at nM levels. At pH = 8.0 and in the absence of gallic acid, 69.3% of the initial Fe(II) was oxidized after 10 min. With 100 nM of gallic acid (ratio 4:1 GA:Fe), and after 30 min, 37.5% of the initial Fe(II) was oxidized. Fe(III) is reduced to Fe(II) by gallic acid in a process that depends on the pH and composition of solution, being faster as pH decreases. At pH > 7.00, the Fe(III) reduction rate constant in seawater was lower than in NaCl solutions, being the difference at pH 8.0 of 1.577 × 10–5 s–1. Moreover, the change of the Fe(III) rate constant with pH, within the studied range, was higher in seawater (slope = 0.91) than in NaCl solutions (slope = 0.46). The Fe(III) reduction rate constant increased with increasing ligand concentration, being the effect higher at pH 7.0 [k′ = 1.078 × 10–4 s–1; (GA) = 250 nM] compared with that at pH 8.0 [k′ = 3.407 × 10–5 s–1; (GA) = 250 nM]. Accordingly, gallic acid reduces Fe(III) to Fe(II) in seawater, making possible the presence of Fe(II) for longer periods and favoring its bioavailability.
Highlights
The increase in atmospheric carbon dioxide (CO2) by human activity and its uptake by seawater has resulted in substantive changes in ocean chemistry over the last few decades (Orr et al, 2005; Bates et al, 2014)
Given the importance that ocean acidification can have on Fe biogeochemistry and that the presence of organic matter, including polyphenols, is a major factor in the mechanisms affecting Fe bioavailability in marine ecosystems, we have investigated the role of gallic acid in the oxidation and reduction of Fe in seawater, as well as the different factors that can affect these redox processes
The permanence of Fe(II) in seawater increased by the presence of gallic acid (GA), a polyphenol produced by marine phytoplankton
Summary
The increase in atmospheric carbon dioxide (CO2) by human activity and its uptake by seawater has resulted in substantive changes in ocean chemistry over the last few decades (Orr et al, 2005; Bates et al, 2014). Ocean acidification results from the decrease in ocean pH and calcium carbonate saturation state (Caldeira and Wickett, 2003) and future projections of CO2 chemistry (Orr et al, 2005), raise potential consequences for marine organisms, ecosystems, and chemical. The organic and inorganic speciation of trace metal chemistry is affected by ocean acidification (Millero et al, 2009; Shi et al, 2010; Gledhill et al, 2015; Ivanina and Sokolova, 2015; Avendaño et al, 2016). Ocean acidification could change the type and amount of dissolved organic material released by phytoplankton (Spilling et al, 2016), as seawater pH/pCO2 conditions regulate inorganic carbon concentration mechanisms and photosynthetic carbon fixation rates in autotrophic microorganisms (Riebesell et al, 2007)
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