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| Titel |
A novel methodology to investigate isotopic biosignatures |
| VerfasserIn |
T. J. Horner, R. B. Y. Lee, G. M. Henderson, R. E. M. Rickaby |
| Konferenz |
EGU General Assembly 2012
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 14 (2012) |
| Datensatznummer |
250068112
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| Zusammenfassung |
| An enduring goal of trace metal isotopic studies of Earth History is to find isotopic
‘fingerprints’ of life or of life’s individual physiochemical processes. Generally, such
signatures are sought by relating an isotopic effect observed in controlled laboratory
conditions or a well-characterized environment to a more complex system or the geological
record. However, such an approach is ultimately limited because life exerts numerous
isotopic fractionations on any one element so it is hard to dissect the resultant net
fractionation into its individual components. Further, different organisms, often with
the same apparent cellular function, can express different isotopic fractionation
factors.
We have used a novel method to investigate the isotopic fractionation associated
with a single physiological process—enzyme specific isotopic fractionation. We
selected Cd isotopes since only one biological use of Cd is known, CdCA (a Cd/Zn
carbonic anhydrase from the coastal diatom T. Weissflogii). Thus, our investigation
can also inform the long standing mystery as to why this generally toxic element
appears to have a nutrient-like dissolved isotopic and concentration profile in the
oceans.
We used the pET-15b plasmid to insert the CdCA gene into the E. coli genome. There is
no known biochemical function for Cd in E. coli, making it an ideal vector for studying
distinct physiological processes within a single organism. The uptake of Cd and
associated isotopic fractionation was determined for both normal cells and those
expressing CdCA. It was found that whole cells always exhibited a preference for the
light isotopes of Cd, regardless of the expression of CdCA; adsorption of Cd to
cell surfaces was not seen to cause isotopic fractionation. However, the cleaning
procedure employed exerted a strong control on the observed isotopic composition of
cells.
Using existing protein purification techniques, we measured the Cd isotopic
composition of different subcellular fractions of E. coli (e.g. membranes, cytosol, etc.),
including the catalytic metal atoms within CdCA. These experiments allow isotopic
exchange reactions to be observed in biological systems at an unparalleled resolution,
demonstrating that isotopic fractionation can occur, in vivo, on length scales as
small as a few Å. We will explore future applications of this technique using the
marine geochemistry of Cd as a case study. This experimental approach has great
promise for studying the individual isotopic biosignatures of other biochemical
reactions, in particular those which may have been active during early Earth History. |
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