Biological rejuvenation of iron oxides in bioturbated marine sediments (original) (raw)
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A biological source of marine sedimentary iron oxides
The biogeochemical cycle of iron is intricately linked to numerous element cycles. Although reductive biological processes that bridge the iron cycle to other element cycles are established, little is known about microbial oxidative processes on iron cycling in sedimentary environments--resulting in the formation of iron oxides. Here, we show that a major source of sedimentary iron oxides originates from the metabolic activity of iron-oxidizing bacteria from the class Zetaproteobacteria, stimulated by burrowing animals in coastal sediments. Zetaproteobacteria were estimated to be a global total of 10 26 cells in coastal, bioturbated sediments and would equate to an annual production of approximately 7.9 x 10 15 grams of sedimentary iron oxides--twenty-five times larger than the annual flux of iron oxides by rivers. These data suggest that iron-oxidizing Zetaproteobacteria are keystone organisms in marine sedimentary environments given their low numerical abundance; yet exert a profo...
Planktonic marine iron oxidizers drive iron mineralization under low‐oxygen conditions
Geobiology, 2016
Observations of modern microbes have led to several hypotheses on how microbes precipitated the extensive iron formations in the geologic record, but we have yet to resolve the exact microbial contributions. An initial hypothesis was that cyanobacteria produced oxygen which oxidized iron abiotically; however, in modern environments such as microbial mats, where Fe(II) and O 2 coexist, we commonly find microaerophilic chemolithotrophic iron-oxidizing bacteria producing Fe(III) oxyhydroxides. This suggests that such iron oxidizers could have inhabited niches in ancient coastal oceans where Fe(II) and O 2 coexisted, and therefore contributed to banded iron formations (BIFs) and other ferruginous deposits. However, there is currently little evidence for planktonic marine iron oxidizers in modern analogs. Here, we demonstrate successful cultivation of planktonic microaerophilic iron-oxidizing Zetaproteobacteria from the Chesapeake Bay during seasonal stratification. Iron oxidizers were associated with low oxygen concentrations and active iron redox cycling in the oxic-anoxic transition zone (<3 lM O 2 , <0.2 lM H 2 S). While cyanobacteria were also detected in this transition zone, oxygen concentrations were too low to support significant rates of abiotic iron oxidation. Cyanobacteria may be providing oxygen for microaerophilic iron oxidation through a symbiotic relationship; at high Fe(II) levels, cyanobacteria would gain protection against Fe(II) toxicity. A Zetaproteobacteria isolate from this site oxidized iron at rates sufficient to account for deposition of geologic iron formations. In sum, our results suggest that once oxygenic photosynthesis evolved, microaerophilic chemolithotrophic iron oxidizers were likely important drivers of iron mineralization in ancient oceans.
Iron-oxidizing microbial ecosystems thrived in late Paleoproterozoic redox-stratified oceans
Earth and Planetary Science Letters, 2009
We conducted a geochemical and petrographic study of the 1.89 billion year old Gunflint and Biwabik iron formations, with the goal of determining the importance of microbial iron-oxidation in the formation of ironand microfossil-rich stromatolites. We used redox-sensitive tracers, such as iron isotopes and rare earth elements, to decipher whether these ancient microbial ecosystems harbored cyanobacteria or Fe-oxidizing bacteria as primary producers. Iron-rich stromatolites contain non-significant or positive Ce anomalies, which contrast with shallow water deposits having negative Ce anomalies. This trend in Ce anomalies indicates that the stromatolites formed in low oxygen conditions, which is the ideal setting for the proliferation of Fe-oxidizing bacterial ecosystems. The stromatolites yield a large range of δ 56 Fe values, from −0.66 to +0.82‰, but contain predominantly positive values indicating the prevalence of partial Feoxidation. Based on modern analogues, Fe-oxides precipitated in cyanobacterial mats are expected to record an isotopic signature of quantitative oxidation, which in marine settings will yield negative δ 56 Fe values. The stromatolite iron isotope data, therefore, provide evidence for the presence of Fe-oxidizing bacteria. The stromatolites can be traced for a distance of over 100 km in these iron formations, indicating that they record a pervasive rather than localized ecosystem. Their preservation in late Paleoproterozoic successions deposited along the margins of the Superior craton suggests that there was a global expansion of ironoxidizing bacterial communities at shallow-water redox boundaries in late Paleoproterozoic oceans.
Marine bacteria and biogeochemical cycling of iron in the oceans
Fems Microbiology Ecology, 1999
Prokaryotic microbes play a critical role in oceanic Fe cycling. They contain most of the biogenic Fe in offshore waters and are responsible for a large portion of the Fe uptake by the plankton community. In the subarctic North Pacific, surface populations of heterotrophic species assimilate more than 50% of the dissolved Fe and thus compete directly with phytoplankton for this limiting resource. In oligotrophic tropical and subtropical waters, photosynthetic bacteria become more important in Fe cycling as the number of unicellular cyanobacteria increases and the nitrogen-fixing Trichodesmium, which contains most of the biogenic Fe in the mixed layer, becomes abundant. Like their terrestrial counterparts, heterotrophic and phototrophic marine bacteria produce Fe-binding siderophores that are involved in Fe acquisition. Evidence exists that bacteria may modify Fe chemistry in the sea through the production of these ligands and thereby play a significant role in regulating production of eukaryotic phytoplankton. z
One sentence summary: This study gives insight into the bacterial community composition related to iron-cycling in suboxic marine sediments based on 16S rRNA gene pyrosequencing. ABSTRACT To gain insight into the bacterial communities involved in iron-(Fe) cycling under marine conditions, we analysed sediments with Fe-contents (0.5–1.5 wt %) from the suboxic zone at a marine site in the Skagerrak (SK) and a brackish site in the Bothnian Bay (BB) using 16S rRNA gene pyrosequencing. Several bacterial families, including Desulfobulbaceae, Desulfuromonadaceae and Pelobacteraceae and genera, including Desulfobacter and Geobacter, known to reduce Fe were detected and showed highest abundance near the Fe(III)/Fe(II) redox boundary. Additional genera with microorganisms capable of coupling fermentation to Fe-reduction, including Clostridium and Bacillus, were observed. Also, the Fe-oxidizing families Mariprofundaceae and Gallionellaceae occurred at the SK and BB sites, respectively, supporting Fe-cycling. In contrast, the sulphate (SO 4 2−) reducing bacteria Desulfococcus and Desulfobacterium were more abundant at greater depths concurring with a decrease in Fe-reducing activity. The communities revealed by pyrosequencing, thus, match the redox stratification indicated by the geochemistry, with the known Fe-reducers coinciding with the zone of Fe-reduction. Not the intensely studied model organisms, such as Geobacter spp., but rather versatile microorganisms, including sulphate reducers and possibly unknown groups appear to be important for Fe-reduction in these marine suboxic sediments.
Biosignatures link microorganisms to iron mineralization in a paleoaquifer
2012
Concretions, preferentially cemented masses within sediments and sedimentary rocks, are records of sediment diagenesis and tracers of pore water chemistry. For over a century, rinded spheroidal structures that exhibit an Fe(III) oxide-rich exterior and Fe-poor core have been described as oxidation products of Fe(II) carbonate concretions. However, mechanisms governing Fe(III) oxide precipitation within these structures remain an enigma. Here we present chemical and morphological evidence of microbial biosignatures in association with Fe(III) oxides in the Fe(III) oxide-rich rind of spheroidal concretions collected from the Jurassic Navajo Sandstone (southwest United States), implicating a microbial role in Fe biomineralization. The amount of total organic carbon in the exterior Fe(III) oxides exceeded measured values in the friable interior. The mean δ 13 C value of organic carbon from the Fe(III) oxidecemented exterior, δ 13 C of −20.55‰, is consistent with a biogenic signature from autotrophic bacteria. Scanning electron micrographs reveal microstructures consistent with bacterial size and morphology, including a twisted-stalk morphotype that resembled an Fe(II)-oxidizing microorganism, Gallionella sp. Nanoscale associations of Fe, O, C, and N with bacterial morphotypes demonstrate microorganisms associated with Fe(III) oxides. Together these results indicate that autotrophic microorganisms were present during Fe(III) oxide precipitation and present microbial catalysis as a mechanism of Fe(III) oxide concretion formation. Microbial biosignatures in rinded Fe(III) oxide-rich concretions within an exhumed, Quaternary aquifer has broad implications for detection of life within the geological record on Earth as well as other Fe-rich rocky planets such as Mars, where both Fe(II) carbonate and Fe(III) oxide-rich concretions have been identifi ed.
Microbial Diversity in Actively Forming Iron Oxides from Weathered Banded Iron Formation Systems
Microbes and Environments
The surface crust that caps highly weathered banded iron formations (BIFs) supports a unique ecosystem that is a post-mining restoration priority in iron ore areas. Geochemical evidence indicates that biological processes drive the dissolution of iron oxide minerals and contribute to the ongoing evolution of this duricrust. However, limited information is available on presentday biogeochemical processes in these systems, particularly those that contribute to the precipitation of iron oxides and, thus, the cementation and stabilization of duricrusts. Freshly formed iron precipitates in water bodies perched on cangas in Karijini National Park, Western Australia, were sampled for microscopic and molecular analyses to understand currently active microbial contributions to iron precipitation in these areas. Microscopy revealed sheaths and stalks associated with iron-oxidizing bacteria. The iron-oxidizing lineages Sphaerotilus, Sideroxydans, and Pedomicrobium were identified in various samples and Leptothrix was common in four out of five samples. The iron-reducing bacteria Anaeromyxobacter dehalogens and Geobacter lovleyi were identified in the same four samples, with various heterotrophs and diverse cyanobacteria. Given this arid, deeply weathered environment, the driver of contemporary iron cycling in Karijini National Park appears to be iron-reducing bacteria, which may exist in anaerobic niches through associations with aerobic heterotrophs. Overall oxidizing conditions and Leptothrix iron-oxidizers contribute to net iron oxide precipitation in our sampes, rather than a closed biogeochemical cycle, which would result in net iron oxide dissolution as has been suggested for canga caves in Brazil. Enhancements in microbial iron oxide dissolution and subsequent reprecipitation have potential as a surface-crust-ecosystem remediation strategy at mine sites.