Iron Isotope Biosignatures

Beard, Brian L.; Johnson, Clark M.; Cox, Lea; Sun, Henry J.; Nealson, Kenneth H.; Aguilar, Carmen

doi:10.1126/science.285.5435.1889

Cited by 361 publications

(279 citation statements)

References 6 publications

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“…= 0.9986 ± 0.0006; ref. 8) and the growing literature for other biologically active transition metals, whereby microorganisms exhibit a general preference for the light isotopes of a given element, e.g., Mo (12), Cu (13), Zn (14), Fe (15), and Ni (16). We rule out adsorption as the cause of the light isotopic fractionation observed for Cd in cultured cells, because a subset of cells plunged into the growth medium for a short time (∼10 s) resulted in no fractionation of Cd isotopes ( Fig.…”

Section: Resultsmentioning

confidence: 99%

Nonspecific uptake and homeostasis drive the oceanic cadmium cycle

Horner

Lee

Henderson

et al. 2013

Proc. Natl. Acad. Sci. U.S.A.

102

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The global marine distributions of Cd and phosphate are closely correlated, which has led to Cd being considered as a marine micronutrient, despite its toxicity to life. The explanation for this nutrient-like behavior is unknown because there is only one identified biochemical function for Cd, an unusual Cd/Zn carbonic anhydrase. Recent developments in Cd isotope mass spectrometry have revealed that Cd uptake by phytoplankton causes isotopic fractionation in the open ocean and in culture. Here we investigate the physiochemical pathways that fractionate Cd isotopes by performing subcellular Cd isotope analysis on genetically modified microorganisms. We find that expression of the Cd/Zn carbonic anhydrase makes no difference to the Cd isotope composition of whole cells. Instead, a large proportion of the Cd is partitioned into cell membranes with a similar direction and magnitude of Cd isotopic fractionation to that seen in surface seawater. This observation is well explained if Cd is mistakenly imported with other divalent metals and subsequently managed by binding within the cell to avoid toxicity. This process may apply to other divalent metals, whereby nonspecific uptake and subsequent homeostasis may contribute to elemental and isotopic distributions in seawater, even for elements commonly considered as micronutrients.biological fractionation | isotope geochemistry | metal homeostasis | subcellular analysis | trace metal I n addition to the macronutrients nitrate, phosphate, and silicate, marine phytoplankton require many essential metals to function correctly (1). The uptake and utilization of these micronutrients results in large vertical isotopic and concentration gradients for many transition metals in seawater (2). In the open ocean, Cd concentrations are as low as a few picomoles per kilogram (2, 3), with associated highly fractionated Cd isotopic compositions of up to several permil ( Fig. 1) (4). In culture, phytoplankton consume small quantities of Cd (5-7) and exhibit light Cd isotope compositions (8). Taken together, the available data suggest that the marine geochemistry of Cd is dominated by nutrient-like processes (i.e., required uptake). However, only one biochemical function for Cd is known: CdCA1, a Cd/Zn carbonic anhydrase from the marine diatom Thalassiosira weissflogii (9, 10). Although ubiquitous in natural waters (11), CdCA1 is absent in numerous phytoplankton including coccolithophores, cyanobacteria, archaea, and several species of diatom (11) (Table S1), and it is thus debateable whether the expression of this single enzyme can account for the nutrient-like distribution of Cd in the global ocean. Here we examine the isotopic fractionation of Cd associated with CdCA1 that has been expressed in vivo by the model microorganism, Escherichia coli. Because E. coli has no inherent use for Cd, our experimental design permits comparison of cultures both overexpressing and not expressing CdCA1 in otherwise identical experiments. Using a systematic experimental approach, we are able to identify ...

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Section: Resultsmentioning

confidence: 99%

Nonspecific uptake and homeostasis drive the oceanic cadmium cycle

Horner

Lee

Henderson

et al. 2013

Proc. Natl. Acad. Sci. U.S.A.

102

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“…In an experimental study involving Fe reduction by dissimilatory bacteria, Beard et al (1999) found significant Fe-isotope fractionation associated with this process. Subsequent experimental work using both reducing and oxidizing bacteria further confirmed large Fe-isotope fractionation associated with the metabolic processing of Fe by the bacteria (Beard et al 2003; E. Hutchens, The Natural History Museum, London 2005, personal communication).…”

Section: Geochemical Signatures Of Lifementioning

confidence: 99%

Searching for signatures of life on Mars: an Fe-isotope perspective

Anand

Russell

Blackhurst

et al. 2006

Phil. Trans. R. Soc. B

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Recent spacecraft and lander missions to Mars have reinforced previous interpretations that Mars was a wet and warm planet in the geological past. The role of liquid water in shaping many of the surface features on Mars has long been recognized. Since the presence of liquid water is essential for survival of life, conditions on early Mars might have been more favourable for the emergence and evolution of life. Until a sample return mission to Mars, one of the ways of studying the past environmental conditions on Mars is through chemical and isotopic studies of Martian meteorites. Over 35 individual meteorite samples, believed to have originated on Mars, are now available for labbased studies. Fe is a key element that is present in both primary and secondary minerals in the Martian meteorites. Fe-isotope ratios can be fractionated by low-temperature processes which includes biological activity. Experimental investigations of Fe reduction and oxidation by bacteria have produced large fractionation in Fe-isotope ratios. Hence, it is considered likely that if there is/were any form of life present on Mars then it might be possible to detect its signature by Fe-isotope studies of Martian meteorites. In the present study, we have analysed a number of Martian meteorites for their bulk-Fe-isotope composition. In addition, a set of terrestrial analogue material has also been analysed to compare the results and draw inferences. So far, our studies have not found any measurable Fe-isotopic fractionation in bulk Martian meteorites that can be ascribed to any lowtemperature process operative on Mars.

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“…Experimental data on aqueous Fe species, Fe-oxides and Fe-carbonates have been documented and demonstrate that the largest Fe isotope fractionations are produced during redox reactions in both biologically mediated (Brantley et al, 2001Anbar, 2004;Johnson et al, 2004;Beard et al, 1999Beard et al, ,2003Icopini et al, 2004;Croal et al, 2004) and abiotic systems (Anbar et al, 2000;Skulan et al, 2002Brantley et al, 2004Welch et al, 2003;Matthews et al, 2004;Teutsch et al, 2005;Jang et al, 2008, Handler et al, 2009McAnena, 2009, Beard et al, 2010. Smaller, but significant fractionations have been seen in abiotic non-redox reactions (Wiesli et al, 2004;Wiedehold et al, 2006;Dideriksen et al, 2008;Mikutta et al, 2009), including the ligand-exchange process involved in mackinawite (FeS m ) formation (Butler et al, 2005).…”

Section: Introductionmentioning

confidence: 99%