Abstract
Low bioavailability of iron due to poor solubility of iron(hydr)oxides limits the growth of microorganisms and plants in soils and aquatic environments. Previous studies described accelerated dissolution of iron(hydr)oxides under continuous illumination, but did not distinguish between photoreductive dissolution and non-reductive processes in which photogenerated Fe(II) catalyzes ligand-controlled dissolution. Here we show that short illuminations (5–15 min) accelerate the dissolution of iron(hydr)oxides by ligands during subsequent dark periods under anoxic conditions. Suspensions of lepidocrocite (Lp) and goethite (Gt) (1.13 mM) with 50 μM EDTA or DFOB were illuminated with UV-A light of comparable intensity to sunlight (pH 7.0, bicarbonate-CO2 buffered solutions). During illumination, the rate of Fe(II) production was highest with Gt-EDTA; followed by Lp-EDTA > Lp-DFOB > Lp > Gt-DFOB > Gt. Under anoxic conditions, photochemically produced Fe(II) increased dissolution rates during subsequent dark periods by factors of 10–40 and dissolved Fe(III) reached 50 μM with DFOB and EDTA. Under oxic conditions, dissolution rates increased by factors of 3–5 only during illumination. With DFOB dissolved Fe(III) reached 35 μM after 10 h of illumination, while with EDTA it peaked at 15 μM and then decreased to below 2 μM. The observations are explained and discussed based on a kinetic model. The results suggest that in anoxic bottom water of ponds and lakes, or in microenvironments of algal blooms, short illuminations can dramatically increase the bioavailability of iron by Fe(II)-catalyzed ligand-controlled dissolution. In oxic environments, photostable ligands such as DFOB can maintain Fe(III) in solution during extended illumination.
Highlights
Dissolution of Fe(III)(hydr)oxides is one of the key processes in the biogeochemical Fe cycle and may govern theavailability of Fe in the environment
In recent studies (Biswakarma et al, 2019, 2020; Kang et al, 2019), we examined the effect of added Fe(II) on the dissolution of Fe(III)(oxyhydr)oxides and found that micromolar concentrations of Fe(II) lead to up to a 60-fold acceleration of dissolution rates under anoxic conditions
The concentration of Fe(II)32þ in suspensions of either Lp and Gt increased linearly as a function of the total duration of illumination. This linear increase confirms that the low concentrations of Fe(II)32þ formed in our experiments did not lead to significant light absorption or interactions with Lp or Gt
Summary
Dissolution of Fe(III)(hydr)oxides is one of the key processes in the biogeochemical Fe cycle and may govern the (bio)availability of Fe in the environment. Suspended Fe(III)(hydr)oxide particles in surface waters are important sources of Fe to microorganisms, but under oxic conditions their low solubility and slow dissolution kinetics can lead to Fe deficiency. Laboratory and field studies have demonstrated that solar and UV light induces photoreductive dissolution of Fe(III)(hydr)oxides in the presence of ligands secreted by marine phytoplankton and algae, and by terrestrial microbes and plants (Litter et al, 1991; Goldberg et al, 1993; Barbeau et al, 2003; Kraemer et al, 2005). Our recent studies have shown that traces of Fe(II) can catalyze the overall non-reductive dissolution of a range of Fe(III)(hydr)oxides in the presence of ligands (Biswakarma et al, 2019, 2020; Kang et al, 2019). Short intermittent illuminations might be sufficient to lead to rapid dissolution, without much reduction of Fe(III) to Fe(II)
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