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Palaeoecology A breath of fresh air

Nature volume 400, pages 503–505 (1999)Cite this article

Molecular fossils (biomarkers) are organic compounds that were derived from living organisms, and are found in sedimentary rocks rich in organic compounds. To be useful, biomarkers must retain some resemblance to the original biological molecules, even after the long history of decomposition and alteration that accompanies their burial and sedimentary rock formation. A biomarker must also be diagnostic — that is, it must be indicative of a certain group of organisms. In cyanobacteria, a class of bacteriohopanepolyols (BHP) serve to make the cell-wall rigid and are frequently methylated at the C-2 position (2-methyl-BHP). These 2-methyl-BHP compounds have not yet been found in quantitatively significant amounts in any other group of organisms, and, as far as we know, are uniquely diagnostic of cyanobacteria. Luckily, through burial and thermal maturation, 2-methyl-BHP is converted to easily recognizable, and still diagnostic, 2-methylhopanes. A constant concern with old sedimentary rocks of Precambrian age is that biomarker signatures have arisen from a contaminating source, such as remobilized organic compounds from nearby, younger sedimentary strata. Summons and colleagues2 have ruled out this possibility, leaving us with the fingerprint of cyanobacterial activity.

Other indirect evidence suggests that cyanobacteria may be much older than this. The isotopic composition of the organic carbon preserved in sedimentary rocks shows a decided shift to values that are very 13C-depleted around 2.7-2.8 Gyr ago3. This isotope trend is believed to show the incorporation of highly 13C-depleted biomass from methanotropic bacteria into the sedimentary carbon record. Methanotropic bacteria oxidize methane, and the shift to highly 13C-depleted values could, therefore, indicate the emergence of oxygen-producing cyanobacteria. Furthermore, geological and geochemical considerations point to the involvement of cyanobacteria in the formation of stromatolites (believed to be fossilized, layered microbial communities) in lakes 2.7 Gyr ago4. Older still, bacterial microfossils miraculously preserved in 3.47-Gyr-old sedimentary rocks bear a striking resemblance to modern cyanobacteria, although morphology alone is not a definitive indicator of bacterial affiliation5.

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Figure 1: Two of the most abundant modern open-ocean cyanobacteria.

ROLF SCHAUDER,WOODS HOLE MAR. BIOL. LABS BIODIVERSITY COURSE

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  1. Institute of Biology, Odense University, Campusvej 55, Odense M, 5230, Denmark

    D. E. Canfield

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Canfield, D. Palaeoecology A breath of fresh air. Nature 400, 503–505 (1999). https://doi.org/10.1038/22872

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