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Titel |
Would acetate (or its derivatives) be the most reliable guide to life on terraqueous globes? |
VerfasserIn |
Michael Russell, Javier Martín-Torres, Yuk Yung, Isik Kanik |
Konferenz |
EGU General Assembly 2010
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Medientyp |
Artikel
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Sprache |
Englisch
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Digitales Dokument |
PDF |
Erschienen |
In: GRA - Volume 12 (2010) |
Datensatznummer |
250036951
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Zusammenfassung |
At bottom life hydrogenates carbon dioxide. But so does serpentinization—to
methane—hence the problem of diagnosing its source (Mumma et al. 2009). However, this
abiotic process does not appear to produce acetate or acetic acid (CH3COOH) in
measurable quantities—only the acetogenic and sulfate-reducing bacteria do that. On the
early Earth it seems that the homoacetogens were the first to resolve the tension
between CO2 and H2 via the autotrophic acetyl coenzyme-A pathway. The acetyl co-A
pathway employs two separate redox controlled tributaries—one Ni-Fe-directed,
merely reduces CO2 to CO, while the other, initially molybdenum-directed, reduces
CO2 through to a methyl group. The CO and the -CH3 are then assembled on the
nickel-bearing acetyl coenzyme-A synthase. Such a complex dual delivery system from
contrasting redox conditions could not be prefigured by serpentinization but required a
chemiosmotic drive, as did the origin of life itself (Nitschke and Russell 2009).
Homoacetogens can compete successfully against the methanoarchaea for H2 and CO2
in the cold, as can the sulfate-reducing acetate-generating bacteria (Krumholz et
al. 1999). Thus we argue that acetate or acetic acid effluent (depending on pH)
from putative microbes on wet rocky planets would be a more reliable indicator of
life.
What are the difficulties? The most critical is that in ground-waters and oceans with pH
>5 acetate remains in solution and would therefore not be detectable remotely. Even were the
waters acidic enough to release volatile acetic acid, it would be prone to photo- and chemical
oxidation. However, apart from CO2 and CH4, the products are formic (HCOOH), glycolic
(HOCH2.COOH) and tartaric (HOOC.HCOH.HOCH.COOH) acids (Ogata et al. 1981).
Remote sensing in the ultraviolet to near-infrared might be used for detection of all these
acids, especially when their concentrations are enhanced in plumes. In situ techniques would
be required for acetate detection.
Krumholz, L.R. et al. (1999) Appl. Environ Microbiol. 65, 2300-2306.
Ogata, Y. et al. (1981) Can. J. Chem. 59, 14-18.
Mumma, M. et al. (2009) Science 323, 1041-1045.
Nitschke, W., Russell, M.J. (2009) J. Molec. Evol. 69, DOI:10.1007/s00239-009-9289-3 |
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