『Abstract
Fe released into solution is isotopically lighter (enriched in
the lighter isotope) than hornblende starting material when dissolution
occurs in the presence of the siderophore desferrioxamine mesylate
(DFAM). In contrast, Fe released from goethite dissolving in the
presence of DFAM is isotopically unchanged. Furthermore, Δ56Fesolution-hornblende for Fe released to solution
in the presence of ligands varies with the affinity of the ligand
for Fe. The extent of isotopic fractionation of Fe released from
hornblende also increases when experiments are agitated continuously.
The Fe isotope fractionation observed during hornblende dissolution
with organic ligands is attributed predominantly to retention
of 56Fe in an altered surface layer, while the lack
of isotopic fractionation during goethite dissolution in DFAM
is consistent with the lack of an altered layer. When a siderophore-producing
soil bacterium is added to the system (without added organic ligands),
Fe released to solution from both hornblende and goethite differs
isotopically from Fe in the bulk mineral: Δ56Fesolution-starting material =−0.56±0.19 (hornblende)
and −1.44±0.16 (goethite). Increased isotopic fractionation is
attributed in this case to the fact that as bacterial respiration
depletes the system in oxygen and aqueous Fe is reduced, equilibration
between aqueous ferrous and ferric iron creates a pool of isotopically
heavy ferric iron that is assimilated by bacterial cells. Adsorption
of isotopically heavy ferrous iron (Fe(II) enriched in the heavier
isotope) or precipitation of isotopically heavy Fe minerals may
also contribute to observed fractionations.
To test whether these Fe isotope signatures are recorded in natural
systems, we also investigated extractions of samples of soils
from which the bacteria were isolated. These extractions show
variability in the isotopic signatures of exchangeable Fe and
Fe oxyhydroxide fractions from one soil sample to another, but
exchangeable Fe is observed to be lighter than Fe in soil Fe oxyhydroxides
and hornblende. This observation is consistent with isotopically
light Fe-organic complexes in soil pore water derived from the
Fe-silicate starting materials in the presence of growing microorganisms,
as documented in experiments reported here. The contributions
from phenomena including organic ligand-promoted nonstoichiometric
dissolution of Fe silicates, uptake of ferric iron by organisms,
adsorption of isotopically heavy ferrous iron, and precipitation
of iron minerals should create complex isotopic signatures in
soils. Better understanding of these processes and the timescales
over which they contribute to fractionation is needed.』
1. Introduction
2. Methods
2.1. Experimental setup
2.1.1. Medium and bacteria
2.1.2. Hornblende experiments
2.1.3. Goethite experiments
2.3. Analysis
2.3.1. Concentrations
2.3.2. Fe isotopes
2.4. Soil extractions
3. Results
3.1. Aqueous chemistry vs. Time
3.2. Isotopic signature of starting materials
3.3. Isotopic measurement of aqueous Fe
3.4. Soil extractions
4. Discussion
4.1. Siderophores and dissolution
4.2. Fe redox chemistry
4.3. Fe isotope fractionation
4.3.1. Abiotic hornblende experiments
4.3.1.1. Incongruent dissolution of bulk mineral
4.3.1.2. Precipitation/adsorption
4.3.1.3. Preferential dissolution from a leached layer
4.3.2. Goethite experiments
4.3.3. Weathering
5. Conclusions
Acknowledgments
References