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Applied and Environmental Microbiology, August 2005, p. 4822-4832, Vol. 71, No. 8
0099-2240/05/$08.00+0 doi:10.1128/AEM.71.8.4822-4832.2005
Copyright © 2005, American Society for Microbiology. All Rights Reserved.
Jeffrey J. Walker,2,3,
Stephen J. Mojzsis,1,2 and
Norman R. Pace2,3*
Department of Geological Sciences, University of Colorado, Boulder, Colorado 80309-0399,1 Department of Molecular, Cellular, and Developmental Biology, University of Colorado, Boulder, Colorado 80309-0347,3 Center for Astrobiology, University of Colorado, Boulder, Colorado 80309-03922
Received 8 December 2004/ Accepted 7 March 2005
Stromatolites, organosedimentary structures formed by microbial activity, are found throughout the geological record and are important markers of biological history. More conspicuous in the past, stromatolites occur today in a few shallow marine environments, including Hamelin Pool in Shark Bay, Western Australia. Hamelin Pool stromatolites often have been considered contemporary analogs to ancient stromatolites, yet little is known about the microbial communities that build them. We used DNA-based molecular phylogenetic methods that do not require cultivation to study the microbial diversity of an irregular stromatolite and of the surface and interior of a domal stromatolite. To identify the constituents of the stromatolite communities, small subunit rRNA genes were amplified by PCR from community genomic DNA with universal primers, cloned, sequenced, and compared to known rRNA genes. The communities were highly diverse and novel. The average sequence identity of Hamelin Pool sequences compared to the >200,000 known rRNA sequences was only
92%. Clone libraries were
90% bacterial and
10% archaeal, and eucaryotic rRNA genes were not detected in the libraries. The most abundant sequences were representative of novel proteobacteria (
28%), planctomycetes (
17%), and actinobacteria (
14%). Sequences representative of cyanobacteria, long considered to dominate these communities, comprised <5% of clones. Approximately 10% of the sequences were most closely related to those of
-proteobacterial anoxygenic phototrophs. These results provide a framework for understanding the kinds of organisms that build contemporary stromatolites, their ecology, and their relevance to stromatolites preserved in the geological record.
D.P. and J.J.W. contributed equally to this study.
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