Influence of Physical Perturbation on Fe(II) Supply in Coastal Marine Sediments

Abstract

Iron (Fe) biogeochemistry in marine sediments is driven by redox transformations creating Fe(II) and Fe(III) gradients. As sediments are physically mixed by wave action or bioturbation, Fe gradients re-establish regularly. In order to identify the response of dissolved Fe(II) (Fe 2+ ) and Fe mineral phases toward mixing processes, we performed voltammetric microsensor measurements, sequential Fe extractions, and Mossbauer spectroscopy of 12 h light−dark cycle incubated marine coastal sediment. Fe 2+ decreased during 7 days of undisturbed incubation from approximately 400 to 60 μM. In the first 2−4 days of incubation, Fe 2+ accumulated up to 100 μM in the top 2 mm due to Fe(III) photoreduction. After physical perturbation at day 7, Fe 2+ was remobilized reaching concentrations of 320 μM in 30 mm depth, which decreased to below detection limit within 2 days afterward. Mossbauer spectroscopy showed that the relative abundance of metastable iron-sulfur mineral phases (FeS x ) increased during initial incubation and decreased together with pyrite (FeS 2 ) after perturbation. We show that Fe 2+ mobilization in marine sediments is stimulated by chemical changes caused by physical disturbances impacting the Fe redox distribution. Our study suggests that, in addition to microbial and abiotic Fe(III) reduction, including Fe(III) photoreduction, physical mixing processes induce chemical changes providing sediments and the inhabiting microbial community with Fe 2+ .

Seasonal Fluctuations in Iron Cycling in Thawing Permafrost Peatlands

“…Anoxic Na-acetate solution (1 M, pH 5), followed by extractions with 0.5 and 6 M HCl were used to successively dissolve Fe phases with increasing crystallinity. 31 Adsorbed Fe(II) 32 , 33 and Fe in amorphous Fe sulfides 34 were extracted by the Na-acetate (referred to adsorbed/amorphous Fe). 0.5 M HCl was chosen to extract poorly crystalline Fe(III) (oxyhydr)oxides and remaining reduced Fe(II) species such as FeCO 3 35 or FeS (referred to as poorly crystalline Fe) and 6 M HCl to extract more crystalline, remaining Fe fractions, such as more crystalline Fe(III) (oxyhydr)oxides, poorly reactive sheet silicate Fe or FeS species 15 , 31 (referred to as more crystalline Fe) from the Fe mineral coated sand (for the exact extraction procedure, see the SI ).…”

“…31 Adsorbed Fe(II) 32,33 and Fe in amorphous Fe sulfides 34 were extracted by the Na-acetate (referred to adsorbed/amorphous Fe). 0.5 M HCl was chosen to extract poorly crystalline Fe(III) (oxyhydr)oxides and remaining reduced Fe(II) species such as FeCO 3 35 or FeS (referred to as poorly crystalline Fe) and FeS species 15,31 (referred to as more crystalline Fe) from the Fe mineral coated sand (for the exact extraction procedure, see the SI). Total Fe is calculated as the sum of Fe extracted by 1 M Na-acetate and 0.5 and 6 M HCl.…”

Photochemistry of iron in aquatic environments

“…Only recently, Fe(III) photoreduction was found to form Fe 2+ aq in the porewater of freshwater and marine sediments. 136,137 In sediments, Fe concentrations usually are in the range of micromolar concentrations [138][139][140] and therefore Fe is generally not considered to be a limiting nutrient. However, gradients of Fe(II) establish at a millimeter to centimeter scale with increasing concentrations downwards due to chemical redox processes and microbially catalyzed reactions, including Fe(III) reduction in deeper, anoxic as well as abiotic Fe(II) oxidation by O2 in shallow, oxic sediment layers.…”

“…However, gradients of Fe(II) establish at a millimeter to centimeter scale with increasing concentrations downwards due to chemical redox processes and microbially catalyzed reactions, including Fe(III) reduction in deeper, anoxic as well as abiotic Fe(II) oxidation by O2 in shallow, oxic sediment layers. 136,137,140 Fe-metabolizing bacteria that use Fe for gaining electrons and energy, need Fe concentrations above the trace element level for growth. In the upper millimeters of light-influenced, oxic sediment layers, Fe(III) photoreduction produces substantial amounts (micromolar range) of Fe 2+ aq, where it is usually limited as substrate for growth.…”

Growth of microaerophilic Fe(II)‐oxidizing bacteria using Fe(II) produced by Fe(III) photoreduction

“…Microaerophilic Fe(II)‐oxidizing bacteria are commonly found at anoxic–oxic interfaces in the environment, such as in stratified water columns (Field et al, 2016), in wetland rhizospheres (Weiss et al, 2003), or in freshwater and marine sediments (Laufer et al, 2016; Otte et al, 2018). Despite absorption of light, especially of UV light, by attenuating substances in water (Piazena et al, 2002), many of those environments are illuminated by sunlight, and Fe(III) photoreduction can be an important Fe(II) source if photoactive organic complexing agents are not limited (Lueder, Jørgensen, et al, 2020; Lueder, Maisch, et al, 2020). Fe(III)‐complexing molecules such as low molecular weight organic acids (e.g., citrate) or humic substances are commonly found in natural environments (Jones, 1998; Mucha et al, 2005; Straub et al, 2005; Zhang & Yuan, 2017).…”