WILLIAM P. INSKEEP, RICHARD E. MACUR, GREGORY HARRISON, BENJAMIN C. BOSTICK, and SCOTT FENDORF
Geochimica et Cosmochimica Acta, 2004 68:3141-3155
Acid-sulfate-chloride (pH~3) geothermal springs in Yellowstone National Park (YNP) often
contain Fe(II), As(III), and S(-II) at discharge, providing several electron donors for chemolithotrophic
metabolism. The microbial populations inhabiting these environments are inextricably linked with geochemical
processes controlling the behavior of As and Fe. Consequently, the objectives of the current study were
to (i) characterize Fe-rich microbial mats of an ASC thermal spring, (ii) evaluate the composition and structure
of As-rich hydrous ferric oxides (HFO) associated with these mats, and (iii) identify microorganisms that are
potentially responsible for mat formation via the oxidation of Fe(II) and or As(III). Aqueous and solid phase
mat samples obtained from a spring in Norris Basin, YNP (YNP Thermal Inventory NHSP35) were analyzed
using a complement of chemical, microscopic and spectroscopic techniques. In addition, molecular analysis
(16S rDNA) was used to identify potentially dominant microbial populations within different mat locations.
The biomineralization of As-rich HFO occurs in the presence of nearly equimolar aqueous As(III) and As(V)
(~12 uM), and ~48 uM Fe(II), forming sheaths external to microbial cell walls. These solid phases were
found to be poorly ordered nanocrystalline HFO containing mole ratios of As(V):Fe(III) of 0.62 +- 0.02. The
bonding environment of As(V) and Fe(III) is consistent with adsorption of arsenate on edge and corner
positions of Fe(III)-OH octahedra. Numerous archaeal and bacterial sequences were identified (with no closely
related cultured relatives), along with several 16S sequences that are closely related to Acidimicrobium,
Thiomonas, Metallosphaera and Marinithermus isolates. Several of these cultured relatives have been
implicated in Fe(II) and or As(III) oxidation in other low pH, high Fe, and high As environments (e.g.
acid-mine drainage). The unique composition and morphologies of the biomineralized phases may be useful
as modern-day analogs for identifying microbial life in past Fe-As rich environments.
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